NetBSD/bin/sh/sh.1

.\"	$NetBSD: sh.1,v 1.229 2020/09/18 07:21:26 wiz Exp $
.\" Copyright (c) 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Kenneth Almquist.
.\"
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.\" modification, are permitted provided that the following conditions
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.\"	@(#)sh.1	8.6 (Berkeley) 5/4/95
.\"
.Dd September 18, 2020
.Dt SH 1
.\" everything except c o and s (keep them ordered)
.ds flags abCEeFfhIiLmnpquVvXx
.Os
.Sh NAME
.Nm sh
.Nd command interpreter (shell)
.Sh SYNOPSIS
.Nm
.Bk -words
.Op Fl \*[flags]
.Op Cm +\*[flags]
.Ek
.Bk -words
.Op Fl o Ar option_name
.Op Cm +o Ar option_name
.Ek
.Bk -words
.Op Ar command_file Op Ar argument ...
.Ek
.Nm
.Fl c
.Bk -words
.Op Fl s
.Op Fl \*[flags]
.Op Cm +\*[flags]
.Ek
.Bk -words
.Op Fl o Ar option_name
.Op Cm +o Ar option_name
.Ek
.Bk -words
.Ar command_string
.Op Ar command_name Op Ar argument ...
.Ek
.Nm
.Fl s
.Bk -words
.Op Fl \*[flags]
.Op Cm +\*[flags]
.Ek
.Bk -words
.Op Fl o Ar option_name
.Op Cm +o Ar option_name
.Ek
.Bk -words
.Op Ar argument ...
.Ek
.Sh DESCRIPTION
.Nm
is the standard command interpreter for the system.
The current version of
.Nm
is in the process of being changed to conform more closely to the
POSIX 1003.2 and 1003.2a specifications for the shell.
This version has many
features which make it appear similar in some respects to the Korn shell,
but it is not a Korn shell clone (see
.Xr ksh 1 ) .
This man page is not intended
to be a tutorial or a complete specification of the shell.
.Ss Overview
The shell is a command that reads lines from either a file or the
terminal, interprets them, and generally executes other commands.
A shell is the program that is running when a user logs into the system.
(Users can select which shell is executed for them at login with the
.Xr chsh 1
command).
The shell implements a language that has flow control
constructs, a macro facility that provides a variety of features in
addition to data storage, along with built in history and line editing
capabilities.
It incorporates many features to aid interactive use and
has the advantage that the interpretative language is common to both
interactive and non-interactive use (shell scripts).
That is, commands
can be typed directly to the running shell or can be put into a file and
the file can be executed directly by the shell.
.Ss Invocation
If no arguments are present and if the standard input,
and standard error output, of the shell
are connected to a terminal (or terminals, or if the
.Fl i
flag is set),
and the
.Fl c
option is not present, the shell is considered an interactive shell.
An interactive shell generally prompts before each command and handles
programming and command errors differently (as described below).
When first starting,
the shell inspects argument 0, and if it begins with a dash
.Sq - ,
the shell is also considered
a login shell.
This is normally done automatically by the system
when the user first logs in.
A login shell first reads commands
(as if by using the
.Dq \&.
command)
from the files
.Pa /etc/profile
and
.Pa .profile
in the user's home directory
.Pq \&$HOME ,
if they exist.
If the environment variable
.Ev ENV
is set on entry to a shell,
or is set in the
.Pa .profile
of a login shell,
and either the shell is interactive, or the
.Cm posix
option is not set,
the shell then performs parameter and arithmetic
expansion on the value of
.Ev ENV ,
(these are described later)
and if no errors occurred,
then reads commands from the file name that results.
Note that no error messages result from these
expansions, to verify that
.Ev ENV
is correct, as desired, use:
.Dl eval printf '%s\e\en' Dq \&${ENV}
Otherwise if
.Ev ENV
appears to contain a command substitution,
which is not performed here,
or if there were no expansions to expand, the value of
.Ev ENV
is used as the file name.
.Pp
Therefore, a user should place commands that are to be executed only at
login time in the
.Pa .profile
file, and commands that are executed for every shell inside the
.Ev ENV
file.
To set the
.Ev ENV
variable to some file, place the following line in your
.Pa .profile
of your home directory
.Pp
.Dl ENV=$HOME/.shinit; export ENV
.Pp
substituting for
.Dq .shinit
any filename you wish.
Since the
.Ev ENV
file can be read for every invocation of the shell, including shell scripts
and non-interactive shells, the following paradigm is useful for
restricting commands in the
.Ev ENV
file to interactive invocations.
Place commands within the
.Dq Ic case
and
.Dq Ic esac
below (these commands are described later):
.Bd -literal -offset indent
case $- in *i*)
        # commands for interactive use only
        ...
esac
.Ed
.Pp
If command line arguments besides the options have been specified, and
neither
.Fl c
nor
.Fl s
was given, then the shell treats the first argument
as the name of a file from which to read commands (a shell script).
This also becomes
.Li $0
and the remaining arguments are set as the
positional parameters of the shell
.Li ( $1 , $2 ,
etc).
Otherwise, if
.Fl c
was given, then the first argument, which must exist,
is taken to be a string of
.Nm
commands to execute.
Then if any additional arguments follow the command string,
those arguments become
.Li $0 , $1 ,
\&...
Otherwise, if additional arguments were given
(which implies that
.Fl s
was set)
those arguments become
.Li $1 , $2 ,
\&...
If
.Li $0
has not been set by the preceding processing, it
will be set to
.Va argv\^ Ns [ 0 ]
as passed to the shell, which will
usually be the name of the shell itself.
If
.Fl s
was given, or if neither
.Fl c
nor any additional (non-option) arguments were present,
the shell reads commands from its standard input.
.\"
.\"
.Ss Argument List Processing
.\"
Currently, all of the single letter options that can meaningfully
be set using the
.Ic set
built-in, have a corresponding name
that can be used as an argument to the
.Fl o
option.
The
.Ic set Fl o
name is provided next to the single letter option in
the description below.
Some options have only a long name, they are described after
the flag options, they are used with
.Fl o
or
.Cm +o
only, either on the command line, or with the
.Ic set
built-in command.
Other options described are for the command line only.
Specifying a dash
.Dq Cm \-
turns the option on, while using a plus
.Dq Cm +
disables the option.
The following options can be set from the command line and,
unless otherwise stated, with the
.Ic set
built-in (described later).
.\"
.\" strlen("quietprofile") == strlen("local_lineno"): pick the latter
.\" to give the indent as the _ in local_lineno, and the fi ligature in
.\" quietprofile combine to make "local_lineno' slightly wider when printed
.\" (in italics) in a variable width font.
.Bl -tag -width ".Fl L Em local_lineno" -offset indent
.\"
.It Fl a Em allexport
Automatically export any variable to which a value is assigned
while this flag is set, unless the variable has been marked as
not for export.
.It Fl b Em notify
Enable asynchronous notification of background job completion.
(Not implemented.)
.It Fl C Em noclobber
Don't overwrite existing files with
.Dq > .
.It Fl c
Read commands from the
.Ar command_string
operand instead of, or in addition to, from the standard input.
Special parameter
.Dv 0 \" $0	(comments like this for searching sources only)
will be set from the
.Ar command_name
operand if given, and the positional parameters
.Dv ( 1 , 2 ,
etc.)
set from the remaining argument operands, if any.
.Fl c
is only available at invocation, it cannot be
.Ic set ,
and there is no form using
.Dq Cm \&+ .
.It Fl E Em emacs
Enable the built-in emacs style
command line editor (disables
.Fl V
if it has been set).
(See the
.Sx Command Line Editing
section below.)
.It Fl e Em errexit
If not interactive, exit immediately if any untested command fails.
If interactive, and an untested command fails,
cease all processing of the current command and return to
prompt for a new command.
The exit status of a command is considered to be
explicitly tested if the command is used to control an
.Ic if ,
.Ic elif ,
.Ic while ,
or
.Ic until ,
or if the command is the left hand operand of an
.Dq &&
or
.Dq ||
operator,
or if it is a pipeline (or simple command) preceded by the
.Dq \&!
operator.
With pipelines, only the status of the entire pipeline
(indicated by the last command it contains)
is tested when
.Fl e
is set to determine if the shell should exit.
.It Fl F Em fork
Cause the shell to always use
.Xr fork 2
instead of attempting
.Xr vfork 2
when it needs to create a new process.
This should normally have no visible effect,
but can slow execution.
The
.Nm
can be compiled to always use
.Xr fork 2
in which case altering the
.Fl F
flag has no effect.
.It Fl f Em noglob
Disable pathname expansion.
.It Fl h Em trackall
Functions defined while this option is set will have paths bound to
commands to be executed by the function at the time of the definition.
When off when a function is defined,
the file system is searched for commands each time the function is invoked.
(Not implemented.)
.It Fl I Em ignoreeof
Ignore EOFs from input when interactive.
(After a large number of consecutive EOFs the shell will exit anyway.)
.It Fl i Em interactive
Force the shell to behave interactively.
.It Fl L Em local_lineno
When set, before a function is defined,
causes the variable
.Dv LINENO
when used within the function,
to refer to the line number defined such that
first line of the function is line 1.
When reset,
.Dv LINENO
in a function refers to the line number within the file
within which the definition of the function occurs.
This option defaults to
.Dq on
in this shell.
For more details see the section
.Sx LINENO
below.
.It Fl m Em monitor
Turn on job control (set automatically when interactive).
.It Fl n Em noexec
Read and parse commands, but do not execute them.
This is useful for checking the syntax of shell scripts.
If
.Fl n
becomes set in an interactive shell, it will automatically be
cleared just before the next time the command line prompt
.Pq Ev PS1
is written.
.It Fl p Em nopriv
Do not attempt to reset effective UID if it does not match UID.
This is not set by default to help avoid incorrect usage by setuid
root programs via
.Xr system 3
or
.Xr popen 3 .
.It Fl q Em quietprofile
If the
.Fl v
or
.Fl x
options have been set, do not apply them when reading
initialization files, these being
.Pa /etc/profile ,
.Pa .profile ,
and the file specified by the
.Ev ENV
environment variable.
.It Fl s Em stdin
Read commands from standard input (set automatically if
neither
.Fl c
nor file arguments are present).
If after processing a command_string with the
.Fl c
option, the shell has not exited, and the
.Fl s
option is set, it will continue reading more commands from standard input.
This option has no effect when set or reset after the shell has
already started reading from the command_file, or from standard input.
Note that the
.Fl s
flag being set does not cause the shell to be interactive.
.It Fl u Em nounset
Write a message to standard error when attempting to obtain a
value from a variable that is not set,
and if the shell is not interactive, exit immediately.
For interactive shells, instead return immediately to the command prompt
and read the next command.
Note that expansions (described later, see
.Sx Word Expansions
below) using the
.Sq \&+ ,
.Sq \&\- ,
.Sq \&= ,
or
.Sq \&?
operators test if the variable is set, before attempting to
obtain its value, and hence are unaffected by
.Fl u .
.It Fl V Em vi
Enable the built-in
.Xr vi 1
command line editor (disables
.Fl E
if it has been set).
(See the
.Sx Command Line Editing
section below.)
.It Fl v Em verbose
The shell writes its input to standard error as it is read.
Useful for debugging.
.It Fl X Em xlock
Cause output from the
.Ic xtrace
.Pq Fl x
option to be sent to standard error as it exists when the
.Fl X
option is enabled (regardless of its previous state.)
For example:
.Bd -literal -compact
        set -X 2>/tmp/trace-file
.Ed
will arrange for tracing output to be sent to the file named,
instead of wherever it was previously being sent,
until the X option is set again, or cleared.
.Pp
Each change (set or clear) to
.Fl X
is also performed upon
.Fl x ,
but not the converse.
.It Fl x Em xtrace
Write each command to standard error (preceded by the expanded value of
.Li $PS4 )
before it is executed.
Unless
.Fl X
is set,
.Dq "standard error"
means that which existed immediately before any redirections to
be applied to the command are performed.
Useful for debugging.
.It "\ \ " Em cdprint
Make an interactive shell always print the new directory name when
changed by the
.Ic cd
command.
In a non-interactive shell this option has no effect.
.It "\ \ " Em nolog
Prevent the entry of function definitions into the command history (see
.Ic fc
in the
.Sx Built-ins
section.)
(Not implemented.)
.It "\ \ " Em pipefail
If set when a pipeline is created,
the way the exit status of the pipeline is determined
is altered.
See
.Sx Pipelines
below for the details.
.It "\ \ " Em posix
Enables closer adherence to the POSIX shell standard.
This option will default set at shell startup if the
environment variable
.Ev POSIXLY_CORRECT
is present.
That can be overridden (set or reset) by the
.Fl o
option on the command line.
Currently this option controls whether (!posix) or not (posix)
the file given by the
.Ev ENV
variable is read at startup by a non-interactive shell.
It also controls whether file descriptors greater than 2
opened using the
.Ic exec
built-in command are passed on to utilities executed
.Dq ( yes
in posix mode),
whether a colon (:) terminates the user name in tilde (~) expansions
other than in assignment statements
.Dq ( no
in posix mode),
the format of the output of the
.Ic kill Fl l
command, where posix mode causes the names of the signals
be separated by either a single space or newline, and where otherwise
sufficient spaces are inserted to generate nice looking columns,
and whether the shell treats
an empty brace-list compound statement as a syntax error
(expected by POSIX) or permits it.
Such statements
.Dq "{\ }"
can be useful when defining dummy functions.
Lastly, in posix mode, only one
.Dq \&!
is permitted before a pipeline.
.It "\ \ " Em promptcmds
Allows command substitutions (as well as parameter
and arithmetic expansions, which are always performed)
upon the prompt strings
.Ev PS1 ,
.Ev PS2 ,
and
.Ev PS4
each time, before they are output.
This option should not be set until after the prompts
have been set (or verified) to avoid accidentally importing
unwanted command substitutions from the environment.
.It "\ \ " Em tabcomplete
Enables filename completion in the command line editor.
Typing a tab character will extend the current input word to match a
filename.
If more than one filename matches it is only extended to be the common prefix.
Typing a second tab character will list all the matching names.
One of the editing modes, either
.Fl E
or
.Fl V ,
must be enabled for this to work.
.El
.Ss Lexical Structure
The shell reads input in terms of lines from a file and breaks it up into
words at whitespace (blanks and tabs), and at certain sequences of
characters that are special to the shell called
.Dq operators .
There are two types of operators: control operators and redirection
operators (their meaning is discussed later).
The following is a list of operators:
.Bl -ohang -offset indent
.It "Control operators:"
.Dl &  &&  \&(  \&)  \&;  ;; ;& \&| || <newline>
.It "Redirection operators:"
.Dl <  >  >|  <<  >>  <&  >&  <<-  <>
.El
.Ss Quoting
Quoting is used to remove the special meaning of certain characters or
words to the shell, such as operators, whitespace, or keywords.
There are four types of quoting:
matched single quotes,
matched double quotes,
backslash,
and
dollar preceding matched single quotes (enhanced C style strings.)
.Ss Backslash
An unquoted backslash preserves the literal meaning of the following
character, with the exception of
.Aq newline .
An unquoted backslash preceding a
.Aq newline
is treated as a line continuation, the two characters are simply removed.
.Ss Single Quotes
Enclosing characters in single quotes preserves the literal meaning of all
the characters (except single quotes, making it impossible to put
single quotes in a single-quoted string).
.Ss Double Quotes
Enclosing characters within double quotes preserves the literal
meaning of all characters except dollar sign
.Pq Li \&$ ,
backquote
.Pq Li \&` ,
and backslash
.Pq Li \e .
The backslash inside double quotes is historically weird, and serves to
quote only the following characters (and these not in all contexts):
.Dl $  `  \*q  \e  <newline> ,
where a backslash newline is a line continuation as above.
Otherwise it remains literal.
.\"
.\"
.Ss Dollar Single Quotes ( Li \&$'...' )
.\"
.Bd -filled -offset indent
.Bf Em
Note: this form of quoting is still somewhat experimental,
and yet to be included in the POSIX standard.
This implementation is based upon the current proposals for
standardization, and is subject to change should the eventual
adopted text differ.
.Ef
.Ed
.Pp
Enclosing characters in a matched pair of single quotes, with the
first immediately preceded by an unquoted dollar sign
.Pq Li \&$
provides a quoting mechanism similar to single quotes, except
that within the sequence of characters, any backslash
.Pq Li \e ,
is an escape character, which causes the following character to
be treated specially.
Only a subset of the characters that can occur in the string
are defined after a backslash, others are reserved for future
definition, and currently generate a syntax error if used.
The escape sequences are modeled after the similar sequences
in strings in the C programming language, with some extensions.
.Pp
The following characters are treated literally when following
the escape character (backslash):
.Dl \e \&' \(dq
The sequence
.Dq Li \e\e
allows the escape character (backslash) to appear in the string literally.
.Dq Li \e'
allows a single quote character into the string, such an
escaped single quote does not terminate the quoted string.
.Dq Li \e\(dq
is for compatibility with C strings, the double quote has
no special meaning in a shell C-style string,
and does not need to be escaped, but may be.
.Pp
A newline following the escape character is treated as a line continuation,
like the same sequence in a double quoted string,
or when not quoted \(en
the two characters, the backslash escape and the newline,
are removed from the input string.
.Pp
The following characters, when escaped, are converted in a
manner similar to the way they would be in a string in the C language:
.Dl a b e f n r t v
An escaped
.Sq a
generates an alert (or
.Sq BEL )
character, that is, control-G, or 0x07.
In a similar way,
.Sq b
is backspace (0x08),
.Sq e
(an extension to C) is escape (0x1B),
.Sq f
is form feed (0x0C),
.Sq n
is newline (or line feed, 0x0A),
.Sq r
is return (0x0D),
.Sq t
is horizontal tab (0x09),
and
.Sq v
is vertical tab (0x13).
.Pp
In addition to those there are 5 forms that need additional
data, which is obtained from the subsequent characters.
An escape
.Pq Li \e
followed by one, two or three, octal digits
.Po So 0 Sc Ns \&.. Ns So 7 Sc Ns Pc
is processed to form an 8 bit character value.
If only one or two digits are present, the following
character must be something other than an octal digit.
It is safest to always use all 3 digits, with leading
zeros if needed.
If all three digits are present, the first must be one of
.So 0 Sc Ns \&.. Ns So 3 Sc .
.Pp
An escape followed by
.Sq x
(lower case only) can be followed by one or two
hexadecimal digits
.Po So 0 Sc Ns \&.. Ns So 9 Sc , So A Sc Ns \&.. Ns So F Sc , or So a Sc Ns \&.. Ns So f Sc . Pc
As with octal, if only one hex digit is present, the following
character must be something other than a hex digit,
so always giving 2 hex digits is best.
However, unlike octal, it is unspecified in the standard
how many hex digits can be consumed.
This
.Nm
takes at most two, but other shells will continue consuming
characters as long as they remain valid hex digits.
Consequently, users should ensure that the character
following the hex escape sequence is something other than
a hex digit.
One way to achieve this is to end the
.Li $'...'
string immediately
after the final hex digit, and then, immediately start
another, so
.Dl \&$'\ex33'$'4...'
always gives the character with value 0x33
.Pq Sq 3 ,
followed by the character
.Sq 4 ,
whereas
.Dl \&$'\ex334'
in some other shells would be the hex value 0x334 (10, or more, bits).
.Pp
There are two escape sequences beginning with
.Sq Li \eu
or
.Sq Li \eU .
The former is followed by from 1 to 4 hex digits, the latter by
from 1 to 8 hex digits.
Leading zeros can be used to pad the sequences to the maximum
permitted length, to avoid any possible ambiguity problem with
the following character, and because there are some shells that
insist on exactly 4 (or 8) hex digits.
These sequences are evaluated to form the value of a Unicode code
point, which is then encoded into UTF-8 form, and entered into the
string.
(The code point should be converted to the appropriate
code point value for the corresponding character in the character
set given by the current locale, or perhaps the locale in use
when the shell was started, but is not... currently.)
Not all values that are possible to write are valid, values that
specify (known) invalid Unicode code points will be rejected, or
simply produce
.Sq \&? .
.Pp
Lastly, as another addition to what is available in C, the escape
character (backslash), followed by
.Sq c
(lower case only) followed by one additional character, which must
be an alphabetic character (a letter), or one of the following:
.Dl \&@ \&[ \&\e \&] \&^ \&_ \&?
Other than
.Sq Li \ec?
the value obtained is the least significant 5 bits of the
ASCII value of the character following the
.Sq Li \ec
escape sequence.
That is what is commonly known as the
.Dq control
character obtained from the given character.
The escape sequence
.Sq Li \ec?
yields the ASCII DEL character (0x7F).
Note that to obtain the ASCII FS character (0x1C) this way,
.Pq "that is control-\e"
the trailing
.Sq Li \e
must be escaped itself, and so for this one case, the full
escape sequence is
.Dq Li \ec\e\e .
The sequence
.Dq Li \ec\e Ns Ar X\^
where
.Sq Ar X\^
is some character other than
.Sq Li \e
is reserved for future use, its meaning is unspecified.
In this
.Nm
an error is generated.
.Pp
If any of the preceding escape sequences generate the value
.Sq \e0
(a NUL character) that character, and all that follow in the same
.Li $'...'
string, are omitted from the resulting word.
.Pp
After the
.Li $'...'
string has had any included escape sequences
converted, it is treated as if it had been a single quoted string.
.\"
.\"
.Ss Reserved Words
.\"
Reserved words are words that have special meaning to the
shell and are recognized,
if completely unquoted,
at the beginning of a line,
after a control operator,
and where the syntax of a command specifically requires a reserved word.
The following are reserved words:
.Bl -column while while while while -offset indent
.It Ic \&! Ta Ic \&{ Ta Ic \&} Ta Ic case
.It Ic do Ta Ic done Ta Ic elif Ta Ic else
.It Ic esac Ta Ic fi Ta Ic for Ta Ic if
.It Ic in Ta Ic then Ta Ic until Ta Ic while
.El
.Pp
Their meanings are discussed later.
.\"
.\"
.Ss Aliases
.\"
An alias is a name and corresponding value set using the
.Ic alias
built-in command.
Whenever a reserved word (see above) may occur,
and after checking for reserved words, the shell
checks the word to see if it matches an alias.
If it does, it replaces it in the input stream with its value.
For example, if there is an alias called
.Dq lf
with the value
.Dq "ls -F" ,
then the input:
.Pp
.Dl lf foobar Aq return
.Pp
would become
.Pp
.Dl ls -F foobar Aq return
.Pp
Aliases provide a convenient way for naive users to create shorthands for
commands without having to learn how to create functions with arguments.
They can also be used to create lexically obscure code.
This use is strongly discouraged.
.\"
.Ss Commands
.\"
The shell interprets the words it reads according to a language, the
specification of which is outside the scope of this man page (refer to the
BNF in the POSIX 1003.2 document).
Essentially though, a line is read and if the first
word of the line (or after a control operator) is not a reserved word,
then the shell has recognized a simple command.
Otherwise, a complex
command or some other special construct may have been recognized.
.\"
.\"
.Ss Simple Commands
.\"
If a simple command has been recognized, the shell performs
the following actions:
.Bl -enum -offset indent
.It
Leading words of the form
.Dq Ar name Ns Li = Ns Ar value
are stripped off, the value is expanded, as described below,
and the results are assigned to the environment of the simple command.
Redirection operators and their arguments (as described below) are
stripped off and saved for processing in step 3 below.
.It
The remaining words are expanded as described in the
.Sx Word Expansions
section below.
The first remaining word is considered the command name and the
command is located.
Any remaining words are considered the arguments of the command.
If no command name resulted, then the
.Dq Ar name Ns Li = Ns Ar value
variable assignments recognized in item 1 affect the current shell.
.It
Redirections are performed, from first to last, in the order given,
as described in the next section.
.El
.\"
.\"
.Ss Redirections
.\"
Redirections are used to change where a command reads its input or sends
its output.
In general, redirections open, close, or duplicate an
existing reference to a file.
The overall format used for redirection is:
.Pp
.Dl Oo Ar n Oc Ns Va redir-op Ar file
.Pp
where
.Va redir-op
is one of the redirection operators mentioned previously.
A list of the possible redirections, and their meanings, follows.
.Pp
The
.Op Ar n
is an optional number, as in
.Sq Li 3
(not
.Sq Li [3] ) ,
that refers to a file descriptor.
If present it must occur unquoted, immediately before the redirection
operator, with no intervening white space, and becomes a
part of that operator.
If file descriptor
.Ar n
was open prior to the redirection, its previous use is closed.
.Pp
All redirections have a single word
.Ar file
argument following the operator
(white space is allowed between the redirection operator and
.Ar file ) ,
though it is sometimes expressed as
.Ar n2 .
That argument is expanded
(see
.Sx "Word Expansions"
below) using tilde expansion, parameter expansion,
arithmetic expansion, command substitution and quote removal
to produce the path name (or file descriptor) to be used.
No field splitting or pathname expansion takes place.
In the list below, where the
.Ar file
is given as
.Ar n2
the result of the expansions must be a number which
refers to a suitable open file descriptor.
.Bl -tag -width aaabsfiles -offset indent
.It Oo Ar n Oc Ns Ic > Ar file
Redirect standard output (or
.Ar n )
to
.Ar file .
.It Oo Ar n Oc Ns Ic >| Ar file
The same, but override the
.Fl C
option.
.It Oo Ar n Oc Ns Ic >> Ar file
Append standard output (or
.Ar n )
to
.Ar file .
.It Oo Ar n Oc Ns Ic < Ar file
Redirect standard input (or
.Ar n )
from
.Ar file .
.It Oo Ar n1 Oc Ns Ic <& Ar n2
Redirect standard input (or
.Ar n1 )
from a duplicate of file descriptor
.Ar n2 .
.It Oo Ar n Oc Ns Ic <& \(mi
Close standard input (or
.Ar n ) .
Note that the
.Sq \&\(mi
is minus sign (or hyphen) given literally or resulting
from the expansion of
.Ar file
(or
.Ar n2 )
for this format.
When given literally there is usually no space between
the redirection operator and the
.Sq \&\(mi ,
though that is just a convention.
.It Oo Ar n1 Oc Ns Ic >& Ar n2
Redirect standard output (or
.Ar n1 )
to be a duplicate of
.Ar n2 .
.It Oo Ar n Oc Ns Ic >& \(mi
Close standard output (or
.Ar n ) .
.It Oo Ar n Oc Ns Ic <> Ar file
Open
.Ar file
for reading and writing on standard input (or
.Ar n ) .
.El
.Pp
The following redirection is often called a
.Dq here-document .
.Bd -unfilled -offset indent
.Oo Ar n Oc Ns Ic << Ar delimiter
.Li \&... here-doc-text ...
.Ar delimiter
.Ed
.Pp
The
.Dq here-doc-text
starts immediately after the next unquoted newline character following
the here-document redirection operator.
If there is more than one here-document redirection on the same
line, then the text for the first (from left to right) is read
first, and subsequent here-doc-text for later here-document redirections
follows immediately after, until all such redirections have been
processed.
.Pp
All the text on successive lines up to the delimiter,
which must appear on a line by itself, with nothing other
than an immediately following newline, is
saved away and made available to the command on standard input, or file
descriptor n if it is specified.
If the delimiter as specified on the initial line is
quoted, then the here-doc-text is treated literally; otherwise, the text is
treated much like a double quoted string, except that
.Sq Li \(dq
characters have no special meaning, and are not escaped by
.Sq Li \&\e ,
and is subjected to parameter expansion, command substitution, and arithmetic
expansion as described in the
.Sx Word Expansions
section below.
If the operator is
.Ic <<-
instead of
.Ic << ,
then leading tabs in all lines in the here-doc-text, including before the
end delimiter, are stripped.
If the delimiter is not quoted, lines in here-doc-text that end with
an unquoted
.Li \e
are joined to the following line, the
.Li \e
and following
newline are simply removed while reading the here-document,
which thus guarantees
that neither of those lines can be the end delimiter.
.Pp
It is a syntax error for the end of the input file (or string) to be
reached before the delimiter is encountered.
.\"
.\"
.Ss Search and Execution
.\"
There are three types of commands: shell functions, built-in commands, and
normal programs \(em and the command is searched for (by name) in that order.
A command that contains a slash
.Sq \&/
in its name is always a normal program.
They each are executed in a different way.
.Pp
When a shell function is executed, all of the shell positional parameters
(note: excluding
.Li 0 , \" $0
which is a special, not positional, parameter, and remains unchanged)
are set to the arguments of the shell function.
The variables which are explicitly placed in the environment of
the command (by placing assignments to them before the function name) are
made local to the function and are set to the values given,
and exported for the benefit of programs executed with the function.
Then the command given in the function definition is executed.
The positional parameters, and local variables, are restored to
their original values when the command completes.
This all occurs within the current shell, and the function
can alter variables, or other settings, of the shell, but
not the positional parameters nor their related special parameters.
.Pp
Shell built-ins are executed internally to the shell, without spawning a
new process.
.Pp
Otherwise, if the command name doesn't match a function or built-in, the
command is searched for as a normal program in the file system (as
described in the next section).
When a normal program is executed, the shell runs the program,
passing the arguments and the environment to the program.
If the program is not a normal executable file, and if it does
not begin with the
.Dq magic number
whose ASCII representation is
.Dq Li "#!" ,
so
.Xr execve 2
returns
.Er ENOEXEC
then) the shell will interpret the program in a sub-shell.
The child shell will reinitialize itself in this case,
so that the effect will be as if a
new shell had been invoked to handle the ad-hoc shell script, except that
the location of hashed commands located in the parent shell will be
remembered by the child.
.Pp
Note that previous versions of this document and the source code itself
misleadingly and sporadically refer to a shell script without a magic
number as a
.Dq shell procedure .
.\"
.\"
.Ss Path Search
.\"
When locating a command,
command names containing a slash
.Pq Sq \&/
are simply executed without performing any searches.
.Pp
If there is no slash in the name,
the shell first looks to see if it is a special built-in command,
if not it looks to see if there is a shell function by that name.
If that fails it looks for an ordinary built-in command.
If a none of these searches located the command
the shell searches each entry in
.Ev PATH
in turn for the command.
The value of the
.Ev PATH
variable should be a series of entries separated by colons.
Each entry consists of a directory name.
The current directory may be indicated
implicitly by an empty directory name, or explicitly by a single period.
If a directory searched contains an executable file with the same
name as the command given,
the search terminates, and that program is executed.
.Ss Command Exit Status
Each command has an exit status that can influence the behavior
of other shell commands.
The paradigm is that a command exits
with zero in normal cases, or to indicate success,
and non-zero for failure,
error, or a false indication.
The man page for each command
should indicate the various exit codes and what they mean.
Additionally, the built-in commands return exit codes, as does
an executed shell function.
.Pp
If a command consists entirely of variable assignments then the
exit status of the command is that of the last command substitution
if any, otherwise 0.
.Pp
If redirections are present, and any fail to be correctly performed,
any command present is not executed, and an exit status of 2
is returned.
.Ss Complex Commands
Complex commands are combinations of simple commands with control
operators or reserved words, together creating a larger complex command.
Overall, a shell program is a:
.Bl -tag -width XpipelineX
.It list
Which is a sequence of one or more AND-OR lists.
.It "AND-OR list"
is a sequence of one or more pipelines.
.It pipeline
is a sequence of one or more commands.
.It command
is one of a simple command, a compound command, or a function definition.
.It "simple command"
has been explained above, and is the basic building block.
.It "compound command"
provides mechanisms to group lists to achieve different effects.
.It "function definition"
allows new simple commands to be created as groupings of existing commands.
.El
.Pp
Unless otherwise stated, the exit status of a list
is that of the last simple command executed by the list.
.\"
.\"
.Ss Pipelines
.\"
A pipeline is a sequence of one or more commands separated
by the control operator
.Sq Ic \(ba ,
and optionally preceded by the
.Dq Ic \&!
reserved word.
Note that
.Sq Ic \(ba
is an operator, and so is recognized anywhere it appears unquoted,
it does not require surrounding white space or other syntax elements.
On the other hand
.Dq Ic \&!
being a reserved word, must be separated from adjacent words by
white space (or other operators, perhaps redirects) and is only
recognized as the reserved word when it appears in a command word
position (such as at the beginning of a pipeline.)
.Pp
The standard output of all but
the last command in the sequence is connected to the standard input
of the next command.
The standard output of the last
command is inherited from the shell, as usual,
as is the standard input of the first command.
.Pp
The format for a pipeline is:
.Pp
.Dl [!] command1 Op Li \&| command2 No ...
.Pp
The standard output of command1 is connected to the standard input of
command2.
The standard input, standard output, or both of each command is
considered to be assigned by the pipeline before any redirection specified
by redirection operators that are part of the command are performed.
.Pp
If the pipeline is not in the background (discussed later), the shell
waits for all commands to complete.
.Pp
The commands in a pipeline can either be simple commands,
or one of the compound commands described below.
The simplest case of a pipeline is a single simple command.
.Pp
If the
.Ic pipefail
option was set when a pipeline was started,
the pipeline status is the status of
the last (lexically last, i.e.: rightmost) command in the
pipeline to exit with non-zero exit status, or zero, if,
and only if, all commands in the pipeline exited with a status of zero.
If the
.Ic pipefail
option was not set, which is the default state,
the pipeline status is the exit
status of the last (rightmost) command in the pipeline,
and the exit status of any other commands in the pipeline is ignored.
.Pp
If the reserved word
.Dq Ic \&!
precedes the pipeline, the exit status
becomes the logical NOT of the pipeline status as determined above.
That is, if the pipeline status is zero, the exit status is 1;
if the pipeline status is other than zero, the exit status is zero.
If there is no
.Dq Ic \&!
reserved word, the pipeline status becomes the exit status.
.Pp
Because pipeline assignment of standard input or standard output or both
takes place before redirection, it can be modified by redirection.
For example:
.Pp
.Dl $ command1 2>&1 \&| command2
.Pp
sends both the standard output and standard error of command1
to the standard input of command2.
.Pp
Note that unlike some other shells, each process in the pipeline is a
child of the invoking shell (unless it is a shell built-in, in which case
it executes in the current shell \(em but any effect it has on the
environment is wiped).
.Pp
A pipeline is a simple case of an AND-OR-list (described below.)
A
.Li \&;
or
.Aq newline
terminator causes the preceding pipeline, or more generally,
the preceding AND-OR-list to be executed sequentially;
that is, the shell executes the commands, and waits for them
to finish before proceeding to following commands.
An
.Li \&&
terminator causes asynchronous (background) execution
of the preceding AND-OR-list (see the next paragraph below).
The exit status of an asynchronous AND-OR-list is zero.
The actual status of the commands,
after they have completed,
can be obtained using the
.Ic wait
built-in command described later.
.\"
.\"
.Ss Background Commands \(em Ic \&&
.\"
If a command, pipeline, or AND-OR-list
is terminated by the control operator ampersand
.Pq Li \&& ,
the
shell executes the command asynchronously \(em that is, the shell does not
wait for the command to finish before executing the next command.
.Pp
The format for running a command in background is:
.Pp
.Dl command1 & Op Li command2 & No ...
.Pp
If the shell is not interactive, the standard input of an asynchronous
command is set to
.Pa /dev/null .
The process identifier of the most recent command started in the
background can be obtained from the value of the special parameter
.Dq Dv \&! \" $!
(see
.Sx Special Parameters )
provided it is accessed before the next asynchronous command is started.
.\"
.\"
.Ss Lists \(em Generally Speaking
.\"
A list is a sequence of one or more commands separated by newlines,
semicolons, or ampersands, and optionally terminated by one of these three
characters.
A shell program, which includes the commands given to an
interactive shell, is a list.
Each command in such a list is executed when it is fully parsed.
Another use of a list is as a complete-command,
which is parsed in its entirety, and then later the commands in
the list are executed only if there were no parsing errors.
.Pp
The commands in a list are executed in the order they are written.
If command is followed by an ampersand, the shell starts the
command and immediately proceeds to the next command; otherwise it waits
for the command to terminate before proceeding to the next one.
A newline is equivalent to a
.Sq Li \&;
when no other operator is present, and the command being input
could syntactically correctly be terminated at the point where
the newline is encountered, otherwise it is just whitespace.
.\"
.\"
.Ss AND-OR Lists (Short-Circuit List Operators)
.\"
.Dq Li \&&&
and
.Dq Li \&||
are AND-OR list operators.
After executing the commands that precede the
.Dq Li \&&&
the subsequent command is executed
if and only if the exit status of the preceding command(s) is zero.
.Dq Li \&||
is similar, but executes the subsequent command if and only if the exit status
of the preceding command is nonzero.
If a command is not executed, the exit status remains unchanged
and the following AND-OR list operator (if any) uses that status.
.Dq Li \&&&
and
.Dq Li \&||
both have the same priority.
Note that these operators are left-associative, so
.Dl true || echo bar && echo baz
writes
.Dq baz
and nothing else.
This is not the way it works in C.
.\"
.\"
.Ss Flow-Control Constructs \(em Ic if , while , until , for , case
.\"
These commands are instances of compound commands.
The syntax of the
.Ic if
command is
.Bd -literal -offset indent
.Ic if Ar list
.Ic then Ar list
.Ic [ elif Ar list
.Ic then  Ar list ] No ...
.Ic [ else Ar list ]
.Ic fi
.Ed
.Pp
The first list is executed, and if the exit status of that list is zero,
the list following the
.Ic then
is executed.
Otherwise the list after an
.Ic elif
(if any) is executed and the process repeats.
When no more
.Ic elif
reserved words, and accompanying lists, appear,
the list after the
.Ic else
reserved word, if any, is executed.
.Pp
The syntax of the
.Ic while
command is
.Bd -literal -offset indent
.Ic while Ar list
.Ic do Ar list
.Ic done
.Ed
.Pp
The two lists are executed repeatedly while the exit status of the
first list is zero.
The
.Ic until
command is similar, but has the word
.Ic until
in place of
.Ic while ,
which causes it to repeat until the exit status of the first list is zero.
.Pp
The syntax of the
.Ic for
command is
.Bd -literal -offset indent
.Ic for Ar variable Op Ic in Ar word No ...
.Ic do Ar list
.Ic done
.Ed
.Pp
The words are expanded, or
.Li \*q$@\*q
if
.Ic in
(and the following words) is not present,
and then the list is executed repeatedly with the
variable set to each word in turn.
If
.Ic in
appears after the variable, but no words are
present, the list is not executed, and the exit status is zero.
.Ic do
and
.Ic done
may be replaced with
.Sq Ic \&{
and
.Sq Ic \&} ,
but doing so is non-standard and not recommended.
.Pp
The syntax of the
.Ic break
and
.Ic continue
commands is
.Bd -literal -offset indent
.Ic break Op Ar num
.Ic continue Op Ar num
.Ed
.Pp
.Ic break
terminates the
.Ar num
innermost
.Ic for , while ,
or
.Ic until
loops.
.Ic continue
breaks execution of the
.Ar num\^ Ns -1
innermost
.Ic for , while ,
or
.Ic until
loops, and then continues with the next iteration of the enclosing loop.
These are implemented as special built-in commands.
The parameter
.Ar num ,
if given, must be an unsigned positive integer (greater than zero).
If not given, 1 is used.
.Pp
The syntax of the
.Ic case
command is
.Bd -literal -offset indent
.Ic case Ar word Ic in
.Oo Ic \&( Oc  Ar pattern Ns Ic \&) Oo Ar list Oc Ic \&;&
.Oo Ic \&( Oc  Ar pattern Ns Ic \&) Oo Ar list Oc Ic \&;;
.No \&...
.Ic esac
.Ed
.Pp
The pattern can actually be one or more patterns (see
.Sx Shell Patterns
described later), separated by
.Dq \(or
characters.
.Pp
.Ar word
is expanded and matched against each
.Ar pattern
in turn,
from first to last,
with each pattern being expanded just before the match is attempted.
When a match is found, pattern comparisons cease, and the associated
.Ar list ,
if given,
is evaluated.
If the list is terminated with
.Dq Ic \&;&
execution then falls through to the following list, if any,
without evaluating its pattern, or attempting a match.
When a list terminated with
.Dq Ic \&;;
has been executed, or when
.Ic esac
is reached, execution of the
.Ic case
statement is complete.
The exit status is that of the last command executed
from the last list evaluated, if any, or zero otherwise.
.\"
.\"
.Ss Grouping Commands Together
.\"
Commands may be grouped by writing either
.Dl Ic \&( Ns Ar list Ns Ic \&)
or
.Dl Ic \&{ Ar list Ns Ic \&; \&}
These also form compound commands.
.Pp
Note that while parentheses are operators, and do not require
any extra syntax, braces are reserved words, so the opening brace
must be followed by white space (or some other operator), and the
closing brace must occur in a position where a new command word might
otherwise appear.
.Pp
The first of these executes the commands in a sub-shell.
Built-in commands grouped into a
.Li \&( Ns Ar list Ns \&)
will not affect the current shell.
The second form does not fork another shell so is slightly more efficient,
and allows for commands which do affect the current shell.
Grouping commands together this way allows you to redirect
their output as though they were one program:
.Bd -literal -offset indent
{ echo -n \*qhello \*q ; echo \*qworld\*q ; } > greeting
.Ed
.Pp
Note that
.Dq Ic }
must follow a control operator (here,
.Dq Ic \&; )
so that it is recognized as a reserved word and not as another command argument.
.\"
.\"
.Ss Functions
.\"
The syntax of a function definition is
.Pp
.Dl Ar name Ns Ic \&() Ar command Op Ar redirect No ...
.Pp
A function definition is an executable statement; when executed it
installs a function named
.Ar name
and returns an exit status of zero.
To be portable, and standards compliant, the name must use the same
syntax as a variable name, (see
.Sx "Variables and Parameters"
below).
As an extension, this shell allows almost all characters in
.Ar name
.Po
the exception is slash
.Pq Sq \&/
as there is no way to invoke a function with a name containing
a slash
.Pc .
Including quoting, whitespace, and operator characters requires
that the word be quoted.
The
.Ar name
is subject to quote removal, but no other expansions.
Because of implementation issues,
unquoted dollar signs
.Pq Sq \&$
and backquotes
.Pq Sq \&`
are prohibited,
but can be included in a function name by use of quoting.
.Pp
The command is normally a list enclosed between
.Dq {
and
.Dq } .
The standard syntax also allows the command to be any of the other
compound commands, including a sub-shell, all of which are supported.
As an extension, this shell also allows a simple command
(or even another function definition) to be
used, though users should be aware this is non-standard syntax.
This means that
.Dl l() ls \*q$@\*q
works to make
.Dq l
an alternative name for the
.Ic ls
command.
.Pp
If the optional redirect, (see
.Sx Redirections ) ,
which may be of any of the normal forms,
is given, it is applied each time the
function is called.
This means that a simple
.Dq Hello World
function might be written (in the extended syntax) as:
.Bd -literal -offset indent
hello() cat <<EOF
Hello World!
EOF
.Ed
.Pp
To be correctly standards conforming this should be re-written as:
.Bd -literal -offset indent
hello() { cat; } <<EOF
Hello World!
EOF
.Ed
.Pp
Note the distinction between those forms, and
.Bd -literal -offset indent
hello() { cat <<EOF
Hello World!
EOF
\&}
.Ed
.Pp
which reads and processes the here-document
each time the shell executes the function, and which applies
that input only to the cat command, not to any other commands
that might appear in the function.
.Pp
Variables may be declared to be local to a function by using the
.Ic local
command.
This should usually appear as the first statement of a function,
though
.Ic local
is an executable command which can be used anywhere in a function.
See
.Sx Built-ins
below for its definition.
.Pp
The function completes after having executed
.Ar command
with exit status set to the status returned by
.Ar command .
If
.Ar command
is a compound-command
it can use the
.Ic return
command (see
.Sx Built-ins
below)
to finish before completing all of
.Ar command .
.Ss Variables and Parameters
The shell maintains a set of parameters.
A parameter denoted by a name is called a variable.
When starting up, the shell turns all the environment
variables into shell variables, and exports them.
New variables can be set using the form
.Pp
.Dl Ar name Ns Li = Ns Ar value
.Pp
Variables set by the user must have a name consisting solely of
alphabetics, numerics, and underscores \(em the first of which must not be
numeric.
A parameter can also be denoted by a number or a special
character as explained below.
.Ss Positional Parameters
A positional parameter is a parameter denoted by a number (n > 0).
The shell sets these initially to the values of its command line arguments
that follow the name of the shell script.
The
.Ic set
built-in can also be used to set or reset them, and
.Ic shift
can be used to manipulate the list.
.Pp
To refer to the 10th (and later) positional parameters,
the form
.Li \&${ Ns Ar n Ns Li \&}
must be used.
Without the braces, a digit following
.Dq $
can only refer to one of the first 9 positional parameters,
or the special parameter
.Dv 0 . \" $0
The word
.Dq Li $10
is treated identically to
.Dq Li ${1}0 .
.\"
.\"
.Ss Special Parameters
.\"
A special parameter is a parameter denoted by one of the following special
characters.
The value of the parameter is listed next to its character.
.Bl -tag -width thinhyphena
.It Dv *
Expands to the positional parameters, starting from one.
When the
expansion occurs within a double-quoted string it expands to a single
field with the value of each parameter separated by the first character of
the
.Ev IFS
variable, or by a
.Aq space
if
.Ev IFS
is unset.
.It Dv @ \" $@
Expands to the positional parameters, starting from one.
When the expansion occurs within double quotes, each positional
parameter expands as a separate argument.
If there are no positional parameters, the
expansion of @ generates zero arguments, even when
.Dv $@
is double-quoted.
What this basically means, for example, is
if
.Li $1
is
.Dq abc
and
.Li $2
is
.Dq def\ ghi ,
then
.Li \*q$@\*q
expands to
the two arguments:
.Pp
.Sm off
.Dl \*q abc \*q \  \*q def\ ghi \*q
.Sm on
.It Dv #
Expands to the number of positional parameters.
.It Dv \&?
Expands to the exit status of the most recent pipeline.
.It Dv \- No (hyphen, or minus)
Expands to the current option flags (the single-letter
option names concatenated into a string) as specified on
invocation, by the set built-in command, or implicitly
by the shell.
.It Dv $
Expands to the process ID of the invoked shell.
A sub-shell retains the same value of
.Dv $
as its parent.
.It Dv \&!
Expands to the process ID of the most recent background
command executed from the current shell.
For a pipeline, the process ID is that of the last command in the pipeline.
If no background commands have yet been started by the shell, then
.Dq Dv \&!
will be unset.
Once set, the value of
.Dq Dv \&!
will be retained until another background command is started.
.It Dv 0 No (zero) \" $0
Expands to the name of the shell or shell script.
.El
.\"
.\"
.Ss Word Expansions
.\"
This section describes the various expansions that are performed on words.
Not all expansions are performed on every word, as explained later.
.Pp
Tilde expansions, parameter expansions, command substitutions, arithmetic
expansions, and quote removals that occur within a single word expand to a
single field.
It is only field splitting or pathname expansion that can
create multiple fields from a single word.
The single exception to this
rule is the expansion of the special parameter
.Dv @ \" $@
within double quotes, as was described above.
.Pp
The order of word expansion is:
.Bl -enum
.It
Tilde Expansion, Parameter Expansion, Command Substitution,
Arithmetic Expansion (these all occur at the same time, and the
result of any of these expansions is not rescanned for further
instances of the expansion, or any of the others).
.It
Unless the
.Ev IFS
variable has an empty value,
Field Splitting is performed on fields
generated by step (1) except for Tilde Expansion.
.It
Pathname Expansion (unless set
.Fl f
is in effect).
.It
Quote Removal.
.El
.Pp
The $ character is used to introduce parameter expansion, command
substitution, or arithmetic evaluation.
.Ss Tilde Expansion (substituting a user's home directory)
A word beginning with an unquoted tilde character (~) is
subjected to tilde expansion.
Provided all of the subsequent characters in the word are unquoted
up to an unquoted slash (/)
or when in an assignment or not in posix mode, an unquoted colon (:),
or if neither of those appear, the end of the word,
they are treated as a user name
and are replaced with the pathname of the named user's home directory.
If the user name is missing (as in
.Pa ~/foobar ) ,
the tilde is replaced with the value of the
.Dv HOME
variable (the current user's home directory).
.Pp
In variable assignments,
an unquoted tilde immediately after the assignment operator (=), and
each unquoted tilde immediately after an unquoted colon in the value
to be assigned is also subject to tilde expansion as just stated.
.\"
.\"
.Ss Parameter Expansion
.\"
The format for parameter expansion is as follows:
.Pp
.Dl ${ Ns Ar expression Ns Li }
.Pp
where
.Ar expression
consists of all characters until the matching
.Sq Li } .
Any
.Sq Li }
escaped by a backslash or within a quoted string, and characters in
embedded arithmetic expansions, command substitutions, and variable
expansions, are not examined in determining the matching
.Sq Li } .
.Pp
The simplest form for parameter expansion is:
.Pp
.Dl ${ Ns Ar parameter Ns Li }
.Pp
The value, if any, of
.Ar parameter
is substituted.
.Pp
The parameter name or symbol can be enclosed in braces,
which are optional in this simple case,
except for positional parameters with more than one digit or
when parameter is followed by a character that could be interpreted as
part of the name.
If a parameter expansion occurs inside double quotes:
.Bl -enum
.It
pathname expansion is not performed on the results of the expansion;
.It
field splitting is not performed on the results of the
expansion.
Note that the special rules for
.Dv @  \" $@
can result in multiple fields being produced, but this is
not because of field-splitting.
If unquoted, each
field produced by
.Dv $@ \" $@
is subject to field splitting.
.El
.Pp
In addition, a parameter expansion where braces are used,
can be modified by using one of the following formats.
If the
.Sq Ic \&:
is omitted in the following modifiers, then the test in the expansion
applies only to unset parameters, not null ones.
.Bl -tag -width aaparameterwordaaaaa
.It Li ${ Ns Ar parameter Ns Ic :- Ns Ar word Ns Li }
.Sy Use Default Values.
If
.Ar parameter
is unset or null, the expansion of
.Ar word
is substituted; otherwise, the value of
.Ar parameter
is substituted.
.It Li ${ Ns Ar parameter Ns Ic := Ns Ar word Ns Li }
.Sy Assign Default Values.
If
.Ar parameter
is unset or null, the expansion of
.Ar word
is assigned to
.Ar parameter .
In all cases, the final value of
.Ar parameter
is substituted.
Only variables, not positional parameters or special
parameters, can be assigned in this way.
.It Li ${ Ns Ar parameter Ns Ic :? Ns Oo Ar word\^ Oc Ns Li }
.Sy Indicate Error if Null or Unset.
If
.Ar parameter
is unset or null, the expansion of
.Ar word
(or a message indicating it is unset if
.Ar word
is omitted)
is written to standard error and a non-interactive shell exits with
a nonzero exit status.
An interactive shell will not exit, but any associated command(s) will
not be executed.
If the
.Ar parameter
is set, its value is substituted.
.It Li ${ Ns Ar parameter Ns Ic :+ Ns Ar word Ns Li }
.Sy Use Alternative Value.
If
.Ar parameter
is unset or null, null is substituted;
otherwise, the expansion of
.Ar word
is substituted.
The value of
.Ar parameter
.Em is not used
in this expansion.
.It Li ${ Ns Ic # Ns Ar parameter Ns Li }
.Sy String Length.
The length in characters of the value of
.Ar parameter .
.El
.Pp
The following four varieties of parameter expansion provide for substring
processing.
In each case, pattern matching notation (see
.Sx Shell Patterns ) ,
rather than regular expression notation, is used to evaluate the patterns.
If parameter is
.Dv *
or
.Dv @ , \" $@
the result of the expansion is unspecified.
Enclosing the full parameter expansion string in double quotes does not
cause the following four varieties of pattern characters to be quoted,
whereas quoting characters within the braces has this effect.
.Bl -tag -width aaparameterwordaaaaa
.It Li ${ Ns Ar parameter Ns Ic % Ns Ar word Ns Li }
.Sy Remove Smallest Suffix Pattern.
The
.Ar word
is expanded to produce a pattern.
The parameter expansion then results in
.Ar parameter ,
with the
smallest portion of the suffix matched by the pattern deleted.
If the
.Ar word
is to start with a
.Sq Li \&%
character, it must be quoted.
.It Li ${ Ns Ar parameter Ns Ic %% Ns Ar word Ns Li }
.Sy Remove Largest Suffix Pattern.
The
.Ar word
is expanded to produce a pattern.
The parameter expansion then results in
.Ar parameter ,
with the largest
portion of the suffix matched by the pattern deleted.
The
.Dq Ic %%
pattern operator only produces different results from the
.Dq Ic \&%
operator when the pattern contains at least one unquoted
.Sq Li \&* .
.It Li ${ Ns Ar parameter Ns Ic \&# Ns Ar word Ns Li }
.Sy Remove Smallest Prefix Pattern.
The
.Ar word
is expanded to produce a pattern.
The parameter expansion then results in
.Ar parameter ,
with the
smallest portion of the prefix matched by the pattern deleted.
If the
.Ar word
is to start with a
.Sq Li \&#
character, it must be quoted.
.It Li ${ Ns Ar parameter Ns Ic \&## Ns Ar word Ns Li }
.Sy Remove Largest Prefix Pattern.
The
.Ar word
is expanded to produce a pattern.
The parameter expansion then results in
.Ar parameter ,
with the largest
portion of the prefix matched by the pattern deleted.
This has the same relationship with the
.Dq Ic \&#
pattern operator as
.Dq Ic %%
has with
.Dq Ic \&% .
.El
.\"
.\"
.Ss Command Substitution
.\"
Command substitution allows the output of a command to be substituted in
place of the command (and surrounding syntax).
Command substitution occurs when a word contains
a command list enclosed as follows:
.Pp
.Dl Ic $( Ns Ar list Ns Ic \&)
.Pp
or the older
.Pq Dq backquoted
version, which is best avoided:
.Pp
.Dl Ic \&` Ns Ar list Ns Ns Ic \&`
.Pp
See the section
.Sx Complex Commands
above for the definition of
.Ic list .
.Pp
The shell expands the command substitution by executing the
.Ar list
in a sub-shell environment and replacing the command substitution with the
standard output of the
.Ar list
after removing any sequence of one or more
.Ao newline Ac Ns s
from the end of the substitution.
(Embedded
.Ao newline Ac Ns s
before
the end of the output are not removed; however, during field splitting,
they may be used to separate fields
.Pq "as spaces usually are"
depending on the value of
.Ev IFS
and any quoting that is in effect.)
.Pp
Note that if a command substitution includes commands
to be run in the background,
the sub-shell running those commands
will only wait for them to complete if an appropriate
.Ic wait
command is included in the command list.
However, the shell in which the result of the command substitution
will be used will wait for both the sub-shell to exit and for the
file descriptor that was initially standard output for the command
substitution sub-shell to be closed.
In some circumstances this might not happen until all processes
started by the command substitution have finished.
.\" While the exit of the sub-shell closes its standard output,
.\" any background process left running may still
.\" have that file descriptor open.
.\" This includes yet another sub-shell which might have been
.\" (almost invisibly) created to wait for some other command to complete,
.\" and even where the background command has had its
.\" standard output redirected or closed,
.\" the waiting sub-shell may still have it open.
.\" Thus there is no guarantee that the result of a command substitution
.\" will be available in the shell which is to use it before all of
.\" the commands started by the command substitution have completed,
.\" though careful coding can often avoid delays beyond the termination
.\" of the command substitution sub-shell.
.\" .Pp
.\" For example, assuming a script were to contain the following
.\" code (which could be done better other ways, attempting
.\" this kind of trickery is not recommended):
.\" .Bd -literal -offset indent
.\" if [ "$( printf "Ready? " >&2; read ans; printf "${ans}";
.\"    { sleep 120 >/dev/null && kill $$ >/dev/null 2>&1; }& )" = go ]
.\" then
.\" 	printf Working...
.\" 	# more code
.\" fi
.\" .Ed
.\" .Pp
.\" the
.\" .Dq Working...
.\" output will not be printed, and code that follows will never be executed.
.\" Nor will anything later in the script
.\" .Po
.\" unless
.\" .Dv SIGTERM
.\" is trapped or ignored
.\" .Pc .
.\" .Pp
.\" The intent is to prompt the user, wait for the user to
.\" answer, then if the answer was
.\" .Dq go
.\" do the appropriate work, but set a 2 minute time limit
.\" on completing that work.
.\" If the work is not done by then, kill the shell doing it.
.\" .Pp
.\" It will usually not work as written, as while the 2 minute
.\" .Sq sleep
.\" has its standard output redirected, as does the
.\" .Sq kill
.\" that follows (which also redirects standard error,
.\" so the user would not see an error if the work were
.\" completed and there was no parent process left to kill);
.\" the sub-shell waiting for the
.\" .Sq sleep
.\" to finish successfully, so it can start the
.\" .Sq kill ,
.\" does not.
.\" It waits, with standard output still open,
.\" for the 2 minutes until the sleep is done,
.\" even though the kill is not going to need that file descriptor,
.\" and there is nothing else which could.
.\" The command substitution does not complete until after
.\" the kill has executed and the background sub-shell
.\" finishes \(en at which time the shell running it is
.\" presumably dead.
.\" .Pp
.\" Rewriting the background sub-shell part of that as
.\" .Dl "{ sleep 120 && kill $$ 2>&1; } >/dev/null &"
.\" would allow this
.\" .Nm
.\" to perform as expected, but that is not guaranteed,
.\" not all shells would behave as planned.
.\" It is advised to avoid starting background processes
.\" from within a command substitution.
.\"
.\"
.Ss Arithmetic Expansion
.\"
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value.
The format for arithmetic expansion is as follows:
.Pp
.Dl Ic $(( Ns Ar expression Ns Ic \&))
.Pp
The expression in an arithmetic expansion is treated as if it were in
double quotes, except that a double quote character inside the expression
is just a normal character (it quotes nothing.)
The shell expands all tokens in the expression for parameter expansion,
command substitution, and quote removal (the only quoting character is
the backslash
.Sq \&\e ,
and only when followed by another
.Sq \&\e ,
a dollar sign
.Sq \&$ ,
a backquote
.Sq \&`
or a newline.)
.Pp
Next, the shell evaluates the expanded result as an arithmetic expression
and substitutes the calculated value of that expression.
.Pp
Arithmetic expressions use a syntax similar to that
of the C language, and are evaluated using the
.Ql intmax_t
data type (this is an extension to POSIX, which requires only
.Ql long
arithmetic.)
Shell variables may be referenced by name inside an arithmetic
expression, without needing a
.Dq \&$
sign.
Variables that are not set, or which have an empty (null string) value,
used this way evaluate as zero (that is,
.Dq x
in arithmetic, as an R-Value, is evaluated as
.Dq ${x:-0} )
unless the
.Nm
.Fl u
flag is set, in which case a reference to an unset variable is an error.
Note that unset variables used in the ${var} form expand to a null
string, which might result in syntax errors.
Referencing the value of a variable which is not numeric is an error.
.Pp
All of the C expression operators applicable to integers are supported,
and operate as they would in a C expression.
Use white space, or parentheses, to disambiguate confusing syntax,
otherwise, as in C, the longest sequence of consecutive characters
which make a valid token (operator, variable name, or number) is taken
to be that token, even if the token designated cannot be used
and a different interpretation could produce a successful parse.
This means, as an example, that
.Dq a+++++b
is parsed as the gibberish sequence
.Dq "a ++ ++ + b" ,
rather than as the valid alternative
.Dq "a ++ + ++ b" .
Similarly, separate the
.Sq \&,
operator from numbers with white space to avoid the possibility
of confusion with the decimal indicator in some locales (though
fractional, or floating-point, numbers are not supported in this
implementation.)
.Pp
It should not be necessary to state that the C operators which
operate on, or produce, pointer types, are not supported.
Those include unary
.Dq \&*
and
.Dq \&&
and the struct and array referencing binary operators:
.Dq \&. ,
.Dq \&->
and
.Dq \&[ .
.\"
.\"
.Ss White Space Splitting (Field Splitting)
.\"
After parameter expansion, command substitution, and
arithmetic expansion the shell scans the results of
expansions and substitutions that did not occur in double quotes,
and
.Dq Li $@
even if it did,
for field splitting and multiple fields can result.
.Pp
The shell treats each character of the
.Ev IFS
as a delimiter and uses the delimiters to split the results of parameter
expansion and command substitution into fields.
.Pp
Non-whitespace characters in
.Ev IFS
are treated strictly as parameter separators.
So adjacent non-whitespace
.Ev IFS
characters will produce empty parameters.
On the other hand, any sequence of whitespace
characters that occur in
.Ev IFS
(known as
.Ev IFS
whitespace)
can occur, leading and trailing
.Ev IFS
whitespace, and any
.Ev IFS
whitespace surrounding a non whitespace
.Ev IFS
delimiter, is removed.
Any sequence of
.Ev IFS
whitespace characters without a non-whitespace
.Ev IFS
delimiter acts as a single field separator.
.Pp
If
.Ev IFS
is unset it is assumed to contain space, tab, and newline,
all of which are
.Ev IFS
whitespace characters.
If
.Ev IFS
is set to a null string, there are no delimiters,
and no field splitting occurs.
.Ss Pathname Expansion (File Name Generation)
Unless the
.Fl f
flag is set, file name generation is performed after word splitting is
complete.
Each word is viewed as a series of patterns, separated by slashes.
The process of expansion replaces the word with the names of all
existing files whose names can be formed by replacing each pattern with a
string that matches the specified pattern.
There are two restrictions on
this: first, a pattern cannot match a string containing a slash, and
second, a pattern cannot match a string starting with a period unless the
first character of the pattern is a period.
The next section describes the
patterns used for both Pathname Expansion and the
.Ic case
command.
.Ss Shell Patterns
A pattern consists of normal characters, which match themselves,
and meta-characters.
The meta-characters are
.Dq \&! ,
.Dq * ,
.Dq \&? ,
and
.Dq \&[ .
These characters lose their special meanings if they are quoted.
When command or variable substitution is performed
and the dollar sign or backquotes are not double-quoted,
the value of the variable or the output of
the command is scanned for these characters and they are turned into
meta-characters.
.Pp
An asterisk
.Pq Dq *
matches any string of characters.
A question mark
.Pq Dq \&?
matches any single character.
A left bracket
.Pq Dq \&[
introduces a character class.
The end of the character class is indicated by a right bracket
.Pq Dq \&] ;
if this
.Dq \&]
is missing then the
.Dq \&[
matches a
.Dq \&[
rather than introducing a character class.
A character class matches any of the characters between the square brackets.
A named class of characters (see
.Xr wctype 3 )
may be specified by surrounding the name with
.Pq Dq [:
and
.Pq Dq :] .
For example,
.Pq Dq [[:alpha:]]
is a shell pattern that matches a single letter.
A range of characters may be specified using a minus sign
.Pq Dq \(mi .
The character class may be complemented
by making an exclamation mark
.Pq Dq \&!
the first character of the character class.
.Pp
To include a
.Dq \&]
in a character class, make it the first character listed (after the
.Dq \&! ,
if any).
To include a
.Dq \(mi ,
make it the first (after !) or last character listed.
If both
.Dq \&]
and
.Dq \(mi
are to be included, the
.Dq \&]
must be first (after !)
and the
.Dq \(mi
last, in the character class.
.\"
.\"
.Ss Built-ins
.\"
This section lists the built-in commands which are built-in because they
need to perform some operation that can't be performed by a separate
process.
Or just because they traditionally are.
In addition to these, there are several other commands that may
be built in for efficiency (e.g.
.Xr printf 1 ,
.Xr echo 1 ,
.Xr test 1 ,
etc).
.Bl -tag -width 5n
.It Ic \&: Op Ar arg ...
A null command that returns a 0 (true) exit value.
Any arguments or redirects are evaluated, then ignored.
.\"
.It Ic \&. Ar file
The dot command reads and executes the commands from the specified
.Ar file
in the current shell environment.
The file does not need to be executable and is looked up from the directories
listed in the
.Ev PATH
variable if its name does not contain a directory separator
.Pq Sq / .
The
.Ic return
command (see below)
can be used for a premature return from the sourced file.
.Pp
The POSIX standard has been unclear on how loop control keywords (break
and continue) behave across a dot command boundary.
This implementation allows them to control loops surrounding the dot command,
but obviously such behavior should not be relied on.
It is now permitted by the standard, but not required.
.\"
.It Ic alias Op Ar name Ns Op Li = Ns Ar string ...
If
.Ar name Ns Li = Ns Ar string
is specified, the shell defines the alias
.Ar name
with value
.Ar string .
If just
.Ar name
is specified, the value of the alias
.Ar name
is printed.
With no arguments, the
.Ic alias
built-in prints the
names and values of all defined aliases (see
.Ic unalias ) .
.\"
.It Ic bg Op Ar job ...
Continue the specified jobs (or the current job if no
jobs are given) in the background.
.\"
.It Ic command Oo Fl pVv Oc Ar command Op Ar arg ...
Execute the specified command but ignore shell functions when searching
for it.
(This is useful when you
have a shell function with the same name as a command.)
.Bl -tag -width 5n
.It Fl p
search for command using a
.Ev PATH
that guarantees to find all the standard utilities.
.It Fl V
Do not execute the command but
search for the command and print the resolution of the
command search.
This is the same as the
.Ic type
built-in.
.It Fl v
Do not execute the command but
search for the command and print the absolute pathname
of utilities, the name for built-ins or the expansion of aliases.
.El
.\"
.It Ic cd Oo Fl P Oc Op Ar directory Op Ar replace
Switch to the specified directory (default
.Ev $HOME ) .
If
.Ar replace
is specified, then the new directory name is generated by replacing
the first occurrence of the string
.Ar directory
in the current directory name with
.Ar replace .
Otherwise if
.Ar directory
is
.Sq Li - ,
then the current working directory is changed to the previous current
working directory as set in
.Ev OLDPWD .
Otherwise if an entry for
.Ev CDPATH
appears in the environment of the
.Ic cd
command or the shell variable
.Ev CDPATH
is set and the directory name does not begin with a slash,
and its first (or only) component isn't dot or dot dot,
then the directories listed in
.Ev CDPATH
will be searched for the specified directory.
The format of
.Ev CDPATH
is the same as that of
.Ev PATH .
.Pp
The
.Fl P
option instructs the shell to update
.Ev PWD
with the specified physical directory path and change to that directory.
This is the default.
.Pp
When the directory changes, the variable
.Ev OLDPWD
is set to the working directory before the change.
.Pp
Some shells also support a
.Fl L
option, which instructs the shell to update
.Ev PWD
with the logical path and to change the current directory
accordingly.
This is not supported.
.Pp
In an interactive shell, the
.Ic cd
command will print out the name of the
directory that it actually switched to if this is different from the name
that the user gave,
or always if the
.Cm cdprint
option is set.
The destination may be different either because the
.Ev CDPATH
mechanism was used
.\" or because a symbolic link was crossed.
or if the
.Ar replace
argument was used.
.\"
.It Ic eval Ar string ...
Concatenate all the arguments with spaces.
Then re-parse and execute the command.
.\"
.It Ic exec Op Ar command Op Ar arg ...
Unless
.Ar command
is omitted, the shell process is replaced with the
specified program (which must be a real program, not a shell built-in or
function).
Any redirections on the
.Ic exec
command are marked as permanent, so that they are not undone when the
.Ic exec
command finishes.
When the
.Cm posix
option is not set,
file descriptors created via such redirections are marked close-on-exec
(see
.Xr open 2
.Dv O_CLOEXEC
or
.Xr fcntl 2
.Dv F_SETFD /
.Dv FD_CLOEXEC ) ,
unless the descriptors refer to the standard input,
output, or error (file descriptors 0, 1, 2).
Traditionally Bourne-like shells
(except
.Xr ksh 1 ) ,
made those file descriptors available to exec'ed processes.
To be assured the close-on-exec setting is off,
redirect the descriptor to (or from) itself,
either when invoking a command for which the descriptor is wanted open,
or by using
.Ic exec
(perhaps the same
.Ic exec
as opened it, after the open)
to leave the descriptor open in the shell
and pass it to all commands invoked subsequently.
Alternatively, see the
.Ic fdflags
command below, which can set, or clear, this, and other,
file descriptor flags.
.\"
.It Ic exit Op Ar exitstatus
Terminate the shell process.
If
.Ar exitstatus
is given it is used as the exit status of the shell; otherwise the
exit status of the preceding command (the current value of $?) is used.
.\"
.It Ic export Oo Fl nx Oc Ar name Ns Oo =value Oc ...
.It Ic export Oo Fl x Oc Oo Fl p Oo Ar name ... Oc Oc
.It Ic export Fl q Oo Fl x Oc Ar name ...
With no options,
but one or more names,
the specified names are exported so that they will appear in the
environment of subsequent commands.
With
.Fl n
the specified names are un-exported.
Variables can also be un-exported using the
.Ic unset
built in command.
With
.Fl x
(exclude) the specified names are marked not to be exported,
and any that had been exported, will be un-exported.
Later attempts to export the variable will be refused.
Note this does not prevent explicitly exporting a variable
to a single command, script or function by preceding that
command invocation by a variable assignment to that variable,
provided the variable is not also read-only.
That is
.Bd -literal -offset indent
export -x FOO # FOO will now not be exported by default
export FOO    # this command will fail (non-fatally)
FOO=some_value my_command
.Ed
.Pp
still passes the value
.Pq Li FOO=some_value
to
.Li my_command
through the environment.
.Pp
The shell allows the value of a variable to be set at the
same time it is exported (or unexported, etc) by writing
.Pp
.Dl export [-nx] name=value
.Pp
Note that in such a usage, the
.Dq name=value
argument often needs to be quoted,
more often than is required of an assignment statement,
as, like with any other command, the command name and
arguments are all subject to the various expansions,
including filename expansion and field splitting,
before the
.Ic export
command is invoked.
With the default value for
.Dv IFS :
.Bd -unfilled -compact -offset indent
X='a b c'
export Y=$X
.Ed
the command invoked would be
.Dl "export Y=a b c"
exporting
.Dv Y ,
with the value
.Dq a
and also exporting the variables named
.Dq b
and
.Dq c ,
which is probably not as intended.
.Pp
With no arguments the export command lists the names of all
set exported variables,
or if
.Fl x
was given, all set variables marked not for export.
With the
.Fl p
option specified, the output will be formatted suitably for
non-interactive use, and unset variables are included.
When
.Fl p
is given, variable names, but not values, may also be
given, in which case output is limited to the variables named.
.Pp
With
.Fl q
and a list of variable names, the
.Ic export
command will exit with status 0 if all the named
variables have been marked for export, or 1 if
any are not so marked.
If
.Fl x
is also given,
the test is instead for variables marked not to be exported.
.Pp
Other than with
.Fl q ,
the
.Ic export
built-in exits with status 0,
unless an attempt is made to export a variable which has
been marked as unavailable for export,
in which cases it exits with status 1.
In all cases if
an invalid option, or option combination, is given,
or an invalid variable name is present,
.Ic export
will write a message to the standard error output,
and exit with a non-zero status.
A non-interactive shell will terminate.
.Pp
Note that there is no restriction upon exporting,
or un-exporting, read-only variables.
The no-export flag can be reset by unsetting the variable
and creating it again \(en provided the variable is not also read-only.
.\"
.It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last
.It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last
.It Ic fc Fl s Oo Ar old=new Oc Op Ar first
The
.Ic fc
built-in lists, or edits and re-executes, commands previously entered
to an interactive shell.
.Bl -tag -width 5n
.It Fl e Ar editor
Use the editor named by
.Ar editor
to edit the commands.
The
.Ar editor
string is a command name, subject to search via the
.Ev PATH
variable.
The value in the
.Ev FCEDIT
variable is used as a default when
.Fl e
is not specified.
If
.Ev FCEDIT
is null or unset, the value of the
.Ev EDITOR
variable is used.
If
.Ev EDITOR
is null or unset,
.Xr ed 1
is used as the editor.
.It Fl l No (ell)
List the commands rather than invoking an editor on them.
The commands are written in the sequence indicated by
the first and last operands, as affected by
.Fl r ,
with each command preceded by the command number.
.It Fl n
Suppress command numbers when listing with
.Fl l .
.It Fl r
Reverse the order of the commands listed (with
.Fl l )
or edited (with neither
.Fl l
nor
.Fl s ) .
.It Fl s
Re-execute the command without invoking an editor.
.It Ar first
.It Ar last
Select the commands to list or edit.
The number of previous commands that
can be accessed are determined by the value of the
.Ev HISTSIZE
variable.
The value of
.Ar first
or
.Ar last
or both are one of the following:
.Bl -tag -width 5n
.It Oo Cm + Oc Ns Ar number
A positive number representing a command number; command numbers can be
displayed with the
.Fl l
option.
.It Cm \- Ns Ar number
A negative decimal number representing the command that was executed
number of commands previously.
For example, \-1 is the immediately previous command.
.El
.It Ar string
A string indicating the most recently entered command that begins with
that string.
If the
.Ar old Ns Li = Ns Ar new
operand is not also specified with
.Fl s ,
the string form of the first operand cannot contain an embedded equal sign.
.El
.Pp
The following environment variables affect the execution of
.Ic fc :
.Bl -tag -width HISTSIZE
.It Ev FCEDIT
Name of the editor to use.
.It Ev HISTSIZE
The number of previous commands that are accessible.
.El
.\"
.It Ic fg Op Ar job
Move the specified job or the current job to the foreground.
A foreground job can interact with the user via standard input,
and receive signals from the terminal.
.\"
.It Ic fdflags Oo Fl v Oc Op Ar fd ...
.It Ic fdflags Oo Fl v Oc Fl s Ar flags fd Op ...
Get or set file descriptor flags.
The
.Fl v
argument enables verbose printing, printing flags that are also off, and
the flags of the file descriptor being set after setting.
The
.Fl s
flag interprets the
.Ar flags
argument as a comma separated list of file descriptor flags, each preceded
with a
.Dq \(pl
or a
.Dq \(mi
indicating to set or clear the respective flag.
Valid flags are:
.Cm append ,
.Cm async ,
.Cm sync ,
.Cm nonblock ,
.Cm fsync ,
.Cm dsync ,
.Cm rsync ,
.Cm direct ,
.Cm nosigpipe ,
and
.Cm cloexec .
Unique abbreviations of these names, of at least 2 characters,
may be used on input.
See
.Xr fcntl 2
and
.Xr open 2
for more information.
.\"
.It Ic getopts Ar optstring var
The POSIX
.Ic getopts
command, not to be confused with the
Bell Labs\[en]derived
.Xr getopt 1 .
.Pp
The first argument should be a series of letters, each of which may be
optionally followed by a colon to indicate that the option requires an
argument.
The variable specified is set to the parsed option.
.Pp
The
.Ic getopts
command deprecates the older
.Xr getopt 1
utility due to its handling of arguments containing whitespace.
.Pp
The
.Ic getopts
built-in may be used to obtain options and their arguments
from a list of parameters.
When invoked,
.Ic getopts
places the value of the next option from the option string in the list in
the shell variable specified by
.Ar var
and its index in the shell variable
.Ev OPTIND .
When the shell is invoked,
.Ev OPTIND
is initialized to 1.
For each option that requires an argument, the
.Ic getopts
built-in will place it in the shell variable
.Ev OPTARG .
If an option is not allowed for in the
.Ar optstring ,
then
.Ev OPTARG
will be unset.
.Pp
.Ar optstring
is a string of recognized option letters (see
.Xr getopt 3 ) .
If a letter is followed by a colon, the option is expected to have an
argument which may or may not be separated from it by whitespace.
If an option character is not found where expected,
.Ic getopts
will set the variable
.Ar var
to a
.Sq Li \&? ;
.Ic getopts
will then unset
.Ev OPTARG
and write output to standard error.
By specifying a colon as the first character of
.Ar optstring
all errors will be ignored.
.Pp
A nonzero value is returned when the last option is reached.
If there are no remaining arguments,
.Ic getopts
will set
.Ar var
to the special option,
.Dq Li \-\- ,
otherwise, it will set
.Ar var
to
.Sq Li \&? .
.Pp
The following code fragment shows how one might process the arguments
for a command that can take the options
.Fl a
and
.Fl b ,
and the option
.Fl c ,
which requires an argument.
.Bd -literal -offset indent
while getopts abc: f
do
	case $f in
	a | b)	flag=$f;;
	c)	carg=$OPTARG;;
	\e?)	echo $USAGE; exit 1;;
	esac
done
shift $((OPTIND - 1))
.Ed
.Pp
This code will accept any of the following as equivalent:
.Bd -literal -offset indent
cmd \-acarg file file
cmd \-a \-c arg file file
cmd \-carg -a file file
cmd \-a \-carg \-\- file file
.Ed
.\"
.It Ic hash Oo Fl rv Oc Op Ar command ...
The shell maintains a hash table which remembers the
locations of commands.
With no arguments whatsoever,
the
.Ic hash
command prints out the contents of this table.
Entries which have not been looked at since the last
.Ic cd
command are marked with an asterisk; it is possible for these entries
to be invalid.
.Pp
With arguments, the
.Ic hash
command removes the specified commands from the hash table (unless
they are functions) and then locates them.
With the
.Fl v
option, hash prints the locations of the commands as it finds them.
The
.Fl r
option causes the hash command to delete all the entries in the hash table
except for functions.
.\"
.It Ic inputrc Ar file
Read the
.Ar file
to set key bindings as defined by
.Xr editrc 5 .
.\"
.It Ic jobid Oo Fl g Ns \&| Ns Fl j Ns \&| Ns Fl p Oc  Op Ar job
With no flags, print the process identifiers of the processes in the job.
If the
.Ar job
argument is omitted, the current job is used.
Any of the ways to select a job may be used for
.Ar job ,
including the
.Sq Li \&%
forms, or the process id of the job leader
.Po
.Dq Li \&$!
if the job was created in the background.
.Pc
.Pp
If one of the flags is given, then instead of the list of
process identifiers, the
.Ic jobid
command prints:
.Bl -tag -width ".Fl g"
.It Fl g
the process group, if one was created for this job,
or nothing otherwise (the job is in the same process
group as the shell.)
.It Fl j
the job identifier (using
.Dq Li \&% Ns Ar n
notation, where
.Ar n
is a number) is printed.
.It Fl p
only the process id of the process group leader is printed.
.El
.Pp
These flags are mutually exclusive.
.Pp
.Ic jobid
exits with status 2 if there is an argument error,
status 1, if with
.Fl g
the job had no separate process group,
or with
.Fl p
there is no process group leader (should not happen),
and otherwise exits with status 0.
.\"
.It Ic jobs Oo Fl l Ns \&| Ns Fl p Oc Op Ar job ...
Without
.Ar job
arguments,
this command lists out all the background processes
which are children of the current shell process.
With
.Ar job
arguments, the listed jobs are shown instead.
Without flags, the output contains the job
identifier (see
.Sx Job Control
below), an indicator character if the job is the current or previous job,
the current status of the job (running, suspended, or terminated successfully,
unsuccessfully, or by a signal)
and a (usually abbreviated) command string.
.Pp
With the
.Fl l
flag the output is in a longer form, with the process identifiers
of each process (run from the top level, as in a pipeline), and the
status of each process, rather than the job status.
.Pp
With the
.Fl p
flag, the output contains only the process identifier of the lead
process.
.Pp
In an interactive shell, each job shown as completed in the output
from the jobs command is implicitly waited for, and is removed from
the jobs table, never to be seen again.
In an interactive shell, when a background job terminates, the
.Ic jobs
command (with that job as an argument) is implicitly run just
before outputting the next PS1 command prompt, after the job
terminated.
This indicates that the job finished, shows its status,
and cleans up the job table entry for that job.
Non-interactive shells need to execute
.Ic wait
commands to clean up terminated background jobs.
.\"
.It Ic local Oo Fl INx Oc Oo Ar variable | \- Oc ...
Define local variables for a function.
Local variables have their attributes, and values,
as they were before the
.Ic local
declaration, restored when the function terminates.
.Pp
With the
.Fl N
flag, variables made local, are unset initially inside
the function.
Unless the
.Fl x
flag is also given, such variables are also unexported.
The
.Fl I
flag, which is the default in this shell, causes
the initial value and exported attribute
of local variables
to be inherited from the variable
with the same name in the surrounding
scope, if there is one.
If there is not, the variable is initially unset,
and not exported.
The
.Fl N
and
.Fl I
flags are mutually exclusive, if both are given, the last specified applies.
The read-only and unexportable attributes are always
inherited, if a variable with the same name already exists.
.Pp
The
.Fl x
flag (lower case) causes the local variable to be exported,
while the function runs, unless it has the unexportable attribute.
This can also be accomplished by using the
.Ic export
command, giving the same
.Ar variable
names, after the
.Ic local
command.
.Pp
Making an existing read-only variable local is possible,
but pointless.
If an attempt is made to assign an initial value to such
a variable, the
.Ic local
command fails, as does any later attempted assignment.
If the
.Ic readonly
command is applied to a variable that has been declared local,
the variable cannot be (further) modified within the function,
or any other functions it calls, however when the function returns,
the previous status (and value) of the variable is returned.
.Pp
Values may be given to local variables on the
.Ic local
command line in a similar fashion as used for
.Ic export
and
.Ic readonly .
These values are assigned immediately after the initialization
described above.
Note that any variable references on the command line will have
been expanded before
.Ic local
is executed, so expressions like
.Bd -unfilled -offset indent
local -N X="${X}"
.Ed
.Pp
are well defined, first $X is expanded, and then the command run is
.Dl "local -N X='old-value-of-X'"
See the description of the
.Ic export
built-in command for notes on why quoting the value is required.
.Pp
After arranging to preserve the old value and attributes, of
.Dv X
.Dq ( old-value-of X )
.Ic local
unsets
.Dv X ,
unexports it, and then assigns the
.Dq old-value-of-X
to
.Ev X .
.Pp
The shell uses dynamic scoping, so that if you make the variable
.Dv x
local to
function
.Dv f ,
which then calls function
.Dv g ,
references to the variable
.Dv x
made inside
.Dv g
will refer to the variable
.Dv x
declared inside
.Dv f ,
not to the global variable named
.Dv x .
.Pp
Another way to view this, is as if the shell just has one flat, global,
namespace, in which all variables exist.
The
.Ic local
command conceptually copies the variable(s) named to unnamed temporary
variables, and when the function ends, copies them back again.
All references to the variables reference the same global variables,
but while the function is active, after the
.Ic local
command has run, the values and attributes of the variables might
be altered, and later, when the function completes, be restored.
.Pp
Note that the positional parameters
.Dv 1 ,  \" $1
.Dv 2 ,  \" $2
\&... (see
.Sx Positional Parameters ) ,
and the special parameters
.Dv \&# , \" $#
.Dv \&*   \" $*
and
.Dv \&@   \" $@
(see
.Sx Special Parameters ) ,
are always made local in all functions, and are reset inside the
function to represent the options and arguments passed to the function.
Note that
.Li $0
however retains the value it had outside the function,
as do all the other special parameters.
.Pp
The only special parameter that can optionally be made local is
.Dq Li \- .
Making
.Dq Li \-
local causes any shell options that are changed via the set command inside the
function to be restored to their original values when the function
returns.
If
.Fl X
option is altered after
.Dq Li \-
has been made local, then when the function returns, the previous
destination for
.Cm xtrace
output (as of the time of the
.Ic local
command) will also be restored.
If any of the shell's magic variables
(those which return a value which may vary without
the variable being explicitly altered,
e.g.:
.Dv SECONDS
or
.Dv HOSTNAME )
are made local in a function,
they will lose their special properties when set
within the function, including by the
.Ic local
command itself
(if not to be set in the function, there is little point
in making a variable local)
but those properties will be restored when the function returns.
.Pp
It is an error to use
.Ic local
outside the scope of a function definition.
When used inside a function, it exits with status 0,
unless an undefined option is used, or an attempt is made to
assign a value to a read-only variable.
.Pp
Note that either
.Fl I
or
.Fl N
should always be used, or variables made local should always
be given a value, or explicitly unset, as the default behavior
(inheriting the earlier value, or starting unset after
.Ic local )
differs amongst shell implementations.
Using
.Dq Li local \&\-
is an extension not implemented by most shells.
.Pp
See the section
.Sx LINENO
below for details of the effects of making the variable
.Dv LINENO
local.
.\"
.It Ic pwd Op Fl \&LP
Print the current directory.
If
.Fl L
is specified the cached value (initially set from
.Ev PWD )
is checked to see if it refers to the current directory; if it does
the value is printed.
Otherwise the current directory name is found using
.Xr getcwd 3 .
The environment variable
.Ev PWD
is set to the printed value.
.Pp
The default is
.Ic pwd
.Fl L ,
but note that the built-in
.Ic cd
command doesn't support the
.Fl L
option and will cache (almost) the absolute path.
If
.Ic cd
is changed (as unlikely as that is),
.Ic pwd
may be changed to default to
.Ic pwd
.Fl P .
.Pp
If the current directory is renamed and replaced by a symlink to the
same directory, or the initial
.Ev PWD
value followed a symbolic link, then the cached value may not
be the absolute path.
.Pp
The built-in command may differ from the program of the same name because
the program will use
.Ev PWD
and the built-in uses a separately cached value.
.\"
.It Ic read Oo Fl p Ar prompt Oc Oo Fl r Oc Ar variable Op Ar ...
The
.Ar prompt
is printed if the
.Fl p
option is specified and the standard input is a terminal.
Then a line is read from the standard input.
The trailing newline is deleted from the
line and the line is split as described in the field splitting section of the
.Sx Word Expansions
section above, and the pieces are assigned to the variables in order.
If there are more pieces than variables, the remaining pieces
(along with the characters in
.Ev IFS
that separated them) are assigned to the last variable.
If there are more variables than pieces,
the remaining variables are assigned the null string.
The
.Ic read
built-in will indicate success unless EOF is encountered on input, in
which case failure is returned.
.Pp
By default, unless the
.Fl r
option is specified, the backslash
.Dq \e
acts as an escape character, causing the following character to be treated
literally.
If a backslash is followed by a newline, the backslash and the
newline will be deleted.
.\"
.It Ic readonly Ar name Ns Oo =value Oc ...
.It Ic readonly Oo Fl p Oo Ar name ... Oc Oc
.It Ic readonly Fl q Ar name ...
With no options,
the specified names are marked as read only, so that they cannot be
subsequently modified or unset.
The shell allows the value of a variable
to be set at the same time it is marked read only by writing
.Pp
.Dl readonly name=value
.Pp
where the value often needs to be quoted, as explained for the
.Ic export
command.
.Pp
With no arguments the
.Ic readonly
command lists the names of all set read only variables.
With the
.Fl p
option specified,
the output will be formatted suitably for non-interactive use,
and unset variables are included.
When the
.Fl p
option is given,
a list of variable names (without values) may also be specified,
in which case output is limited to the named variables.
.Pp
With the
.Fl q
option, the
.Ic readonly
command tests the read-only status of the variables listed
and exits with status 0 if all named variables are read-only,
or with status 1 if any are not read-only.
.Pp
Other than as specified for
.Fl q
the
.Ic readonly
command normally exits with status 0.
In all cases, if an unknown option, or an invalid option combination,
or an invalid variable name, is given;
or a variable which was already read-only is attempted to be set;
the exit status will not be zero, a diagnostic
message will be written to the standard error output,
and a non-interactive shell will terminate.
.\"
.It Ic return Op Ar n
Stop executing the current function or a dot command with return value of
.Ar n
or the value of the last executed command, if not specified.
For portability,
.Ar n
should be in the range from 0 to 255.
.Pp
The POSIX standard says that the results of
.Ic return
outside a function or a dot command are unspecified.
This implementation treats such a return as a no-op with a return value of 0
(success, true).
Use the
.Ic exit
command instead, if you want to return from a script or exit
your shell.
.\"
.It Ic set Oo { Fl options | Cm +options | Cm \-- } Oc Ar arg ...
The
.Ic set
command performs four different functions.
.Pp
With no arguments, it lists the values of all shell variables.
.Pp
With a single option of either
.Dq Fl o
or
.Dq Cm +o
.Ic set
outputs the current values of the options.
In the
.Fl o
form, all options are listed, with their current values.
In the
.Cm +o
form, the shell outputs a string that can later be used
as a command to reset all options to their current values.
.Pp
If options are given, it sets the specified option
flags, or clears them as described in the
.Sx Argument List Processing
section.
In addition to the options listed there,
when the
.Dq "option name"
given to
.Ic set Fl o
is
.Cm default
all of the options are reset to the values they had immediately
after
.Nm
initialization, before any startup scripts, or other input, had been processed.
While this may be of use to users or scripts, its primary purpose
is for use in the output of
.Dq Ic set Cm +o ,
to avoid that command needing to list every available option.
There is no
.Cm +o default .
.Pp
The fourth use of the
.Ic set
command is to set the values of the shell's
positional parameters to the specified arguments.
To change the positional
parameters without changing any options, use
.Dq -\|-
as the first argument to
.Ic set .
If no following arguments are present, the
.Ic set
command
will clear all the positional parameters (equivalent to executing
.Dq Li shift $# . )
Otherwise the following arguments become
.Li \&$1 ,
.Li \&$2 ,
\&...,
and
.Li \&$#
is set to the number of arguments present.
.\"
.It Ic setvar Ar variable Ar value
Assigns
.Ar value
to
.Ar variable .
(In general it is better to write
.Li variable=value
rather than using
.Ic setvar .
.Ic setvar
is intended to be used in
functions that assign values to variables whose names are passed as
parameters.)
.\"
.It Ic shift Op Ar n
Shift the positional parameters
.Ar n
times.
If
.Ar n
is omitted, 1 is assumed.
Each
.Ic shift
sets the value of
.Li $1
to the previous value of
.Li $2 ,
the value of
.Li $2
to the previous value of
.Li $3 ,
and so on, decreasing
the value of
.Li $#
by one.
The shift count must be less than or equal to the number of
positional parameters (
.Dq Li $# )
before the shift.
.\"
.It Ic specialvar Ar variable ...
For each
.Ar variable
name given,
if the variable named is one which,
in this
.Nm ,
could be treated as a special variable,
then cause that
.Ar variable
to be made special, undoing any effects of an earlier
.Ic unset
or assignment to the variable.
If all
.Ar variable Ns s
given are recognized special variables in this
.Nm
the
.Ic specialvar
command will exit with status 0, otherwise 1.
Invalid usage will result in an exit status of 2.
.Pp
Note that all variables capable of being special are created
that way, this command is not required to cause that to happen.
However should such a variable be imported from the environment,
that will cause (for those special variables so designated)
the special effects for that variable to be lost.
Consequently, as the contents of the environment cannot be controlled,
any script which desires to make use of the properties
of most of the special variables should use this command,
naming the variables required,
to ensure that their special properties are available.
.\"
.It Ic times
Prints two lines to standard output.
Each line contains two accumulated time values, expressed
in minutes and seconds (including fractions of a second.)
The first value gives the user time consumed, the second the system time.
.Pp
The first output line gives the CPU and system times consumed by the
shell itself.
The second line gives the accumulated times for children of this
shell (and their descendants) which have exited, and then been
successfully waited for by the relevant parent.
See
.Xr times 3
for more information.
.Pp
.Ic times
has no parameters, and exits with an exit status of 0 unless
an attempt is made to give it an option.
.\"
.It Ic trap Ar action signal ...
.It Ic trap \-
.It Ic trap Op Fl l
.It Ic trap Fl p Oo Ar signal ... Oc
.It Ic trap Fl P Ar signal ...
.It Ic trap Ar N signal ...
.Pp
Cause the shell to parse and execute action when any of the specified
signals are received.
The signals are specified by signal number or as the name of the signal.
If
.Ar signal
is
.Li 0 \" $0
or its equivalent,
.Li EXIT ,
the action is executed when the shell exits.
The
.Ar action
may be a null (empty) string,
which causes the specified signals to be ignored.
With
.Ar action
set to
.Sq Li -
the specified signals are set to their default actions.
If the first
.Ar signal
is specified in its numeric form, then
.Ar action
can be omitted to achieve the same effect.
This archaic,
but still standard,
form should not be relied upon, use the explicit
.Sq Li -
action.
If no signals are specified with an action of
.Sq Li - ,
all signals are reset.
.Pp
When the shell forks off a sub-shell, it resets trapped (but not ignored)
signals to the default action.
On non-interactive shells, the
.Ic trap
command has no effect on signals that were
ignored on entry to the shell.
On interactive shells, the
.Ic trap
command will catch or reset signals ignored on entry.
.Pp
Issuing
.Ic trap
with option
.Fl l
will print a list of valid signal names.
.Ic trap
without any arguments causes it to write a list of signals and their
associated non-default actions to the standard output in a format
that is suitable as an input to the shell that achieves the same
trapping results.
With the
.Fl p
flag, trap prints the same information for the signals specified,
or if none are given, for all signals, including those where the
action is the default.
The
.Fl P
flag is similar, but prints only the action(s) associated with the
named signals, at least (and usually only) one of which must be given.
Nothing is printed if the action is the default,
an empty line is printed for ignored signals.
These variants of the trap command may be executed in a sub-shell
.Pq "such as in a command substitution" ,
provided they appear as the initial sequence of commands in that sub-shell,
in which case the state of traps from the parent of that
sub-shell is reported.
.Pp
Examples:
.Pp
.Dl trap
.Pp
List trapped signals and their corresponding actions.
.Pp
.Dl trap -l
.Pp
Print a list of valid signals.
.Pp
.Dl trap '' INT QUIT tstp 30
.Pp
Ignore signals INT QUIT TSTP USR1.
.Pp
.Dl trap date INT
.Pp
Run the
.Dq date
command (print the date) upon receiving signal INT.
.Pp
.Dl trap HUP INT
.Pp
Run the
.Dq HUP
command, or function, upon receiving signal INT.
.Pp
.Dl Ic eval Qo \&$( trap -P QUIT \&) Qc
.Pp
Parse and execute the action that would be invoked were a
.Dv SIGQUIT
received.
.Pp
.Dl trap 1 2
.Pp
Reset the actions for signals 1 (HUP) and 2 (INT) to their defaults.
.Bd -literal -offset indent
traps=$(trap -p)
   # more commands ...
trap 'action' SIG
   # more commands ...
eval "$traps"
.Ed
.Pp
Save the trap status, execute commands, changing some traps,
and then reset all traps to their values at the start of the sequence.
The
.Fl p
option is required in the first command here,
or any signals that were previously
untrapped (in their default states)
and which were altered during the intermediate code,
would not be reset by the final
.Ic eval .
.\"
.It Ic type Op Ar name ...
Interpret each
.Ar name
as a command and print the resolution of the command search.
Possible resolutions are:
shell keyword, alias, shell built-in,
command, tracked alias and not found.
For aliases the alias expansion is
printed; for commands and tracked aliases the complete pathname of the
command is printed.
.\"
.It Ic ulimit Oo Fl H Ns \*(Ba Ns Fl S Oc Op Fl a \*(Ba Fl btfdscmlrpnv Op Ar value
Inquire about or set the hard or soft limits on processes or set new
limits.
The choice between hard limit (which no process is allowed to
violate, and which may not be raised once it has been lowered) and soft
limit (which causes processes to be signaled but not necessarily killed,
and which may be raised) is made with these flags:
.Bl -tag -width Fl
.It Fl H
set or inquire about hard limits
.It Fl S
set or inquire about soft limits.
.El
.Pp
If neither
.Fl H
nor
.Fl S
is specified, the soft limit is displayed or both limits are set.
If both are specified, the last one wins.
.Pp
The limit to be interrogated or set, then, is chosen by specifying
any one of these flags:
.Bl -tag -width Fl
.It Fl a
show all the current limits
.It Fl b
the socket buffer size of a process (bytes)
.It Fl c
the largest core dump size that can be produced
(512-byte blocks)
.It Fl d
the data segment size of a process (kilobytes)
.It Fl f
the largest file that can be created
(512-byte blocks)
.It Fl l
how much memory a process can lock with
.Xr mlock 2
(kilobytes)
.It Fl m
the total physical memory that can be
in use by a process (kilobytes)
.It Fl n
the number of files a process can have open at once
.It Fl p
the number of processes this user can
have at one time
.It Fl r
the number of threads this user can
have at one time
.It Fl s
the stack size of a process (kilobytes)
.It Fl t
CPU time (seconds)
.It Fl v
how large a process address space can be
.El
.Pp
If none of these is specified, it is the limit on file size that is shown
or set.
If value is specified, the limit is set to that number; otherwise
the current limit is displayed.
.Pp
Limits of an arbitrary process can be displayed or set using the
.Xr sysctl 8
utility.
.It Ic umask Oo Fl S Oc Op Ar mask
Set the value of umask (see
.Xr umask 2 )
to the specified octal value.
If the argument is omitted, the umask value is printed.
With
.Fl S
a symbolic form is used instead of an octal number.
.It Ic unalias Oo Fl a Oc Op Ar name
If
.Ar name
is specified, the shell removes that alias.
If
.Fl a
is specified, all aliases are removed.
.It Ic unset Oo Fl efvx Oc Ar name ...
If
.Fl v
is specified, the specified variables are unset and unexported.
Readonly variables cannot be unset.
If
.Fl f
is specified, the specified functions are undefined.
If
.Fl e
is given, the specified variables are unexported, but otherwise unchanged,
alternatively, if
.Fl x
is given, the exported status of the variable will be retained,
even after it is unset.
.Pp
If no flags are provided
.Fl v
is assumed.
If
.Fl f
is given with one of the other flags,
then the named variables will be unset, or unexported, and functions
of the same names will be undefined.
The
.Fl e
and
.Fl x
flags both imply
.Fl v .
If
.Fl e
is given, the
.Fl x
flag is ignored.
.Pp
The exit status is 0, unless an attempt was made to unset
a readonly variable, in which case the exit status is 1.
It is not an error to unset (or undefine) a variable (or function)
that is not currently set (or defined.)
.\"
.It Ic wait Oo Fl n Oc Oo Fl p Ar var Oc Op Ar job ...
Wait for the specified jobs to complete
and return the exit status of the last job in the parameter list,
or 127 if that job is not a current child of the shell.
.Pp
If no
.Ar job
arguments are given, wait for all jobs to
complete and then return an exit status of zero
(including when there were no jobs, and so nothing exited.)
.Pp
With the
.Fl n
option, wait instead for any one of the given
.Ar job Ns s,
or if none are given, any job, to complete, and
return the exit status of that job.
If none of the given
.Ar job
arguments is a current child of the shell,
or if no
.Ar job
arguments are given and the shell has no unwaited for children,
then the exit status will be 127.
.Pp
The
.Fl p Ar var
option allows the process (or job) identifier of the
job for which the exit status is returned to be obtained.
The variable named (which must not be readonly) will be
unset initially, then if a job has exited and its status is
being returned, set to the identifier from the
arg list (if given) of that job,
or the lead process identifier of the job to exit when used with
.Fl n
and no job arguments.
Note that
.Fl p
with neither
.Fl n
nor
.Ar job
arguments is useless, as in that case no job status is
returned, the variable named is simply unset.
.Pp
If the wait is interrupted by a signal,
its exit status will be greater than 128,
and
.Ar var ,
if given, will remain unset.
.Pp
Once waited upon, by specific process number or job-id,
or by a
.Ic wait
with no arguments,
knowledge of the child is removed from the system,
and it cannot be waited upon again.
.Pp
Note than when a list of jobs are given, more that
one argument might refer to the same job.
In that case, if the final argument represents a job
that is also given earlier in the list, it is not
defined whether the status returned will be the
exit status of the job, or 127 indicating that
the child no longer existed when the wait command
reached the later argument in the list.
In this
.Nm
the exit status will be that from the job.
.Nm
waits for each job exactly once, regardless of
how many times (or how many different ways) it
is listed in the arguments to
.Ic wait .
That is
.Bd -literal -offset indent -compact
wait 100 100 100
.Ed
is identical to
.Bd -literal -offset indent -compact
wait 100
.Ed
.El
.\"
.\"
.Ss Job Control
.\"
Each process (or set of processes) started by
.Nm
is created as a
.Dq job
and added to the jobs table.
When enabled by the
.Fl m
option
.Pq aka Fl o Cm monitor
when the job is created,
.Nm
places each job (if run from the top level shell)
into a process group of its own, which allows control
of the process(es), and its/their descendants, as a unit.
When the
.Fl m
option is off, or when started from a sub-shell environment,
jobs share the same process group as the parent shell.
The
.Fl m
option is enabled by default in interactive shells with
a terminal as standard input and standard error.
.Pp
Jobs with separate process groups may be stopped, and then later
resumed in the foreground (with access to the terminal)
or in the background (where attempting to read from the
terminal will result in the job stopping.)
A list of current jobs can be obtained using the
.Ic jobs
built-in command.
Jobs are identified using either the process identifier
of the lead process of the job (the value available in
the special parameter
.Dq Dv \&!
if the job is started in the background), or using percent
notation.
Each job is given a
.Dq job number
which is a small integer, starting from 1, and can be
referenced as
.Dq Li \&% Ns Ar n
where
.Ar n
is that number.
Note that this applies to jobs both with and without their own process groups.
Job numbers are shown in the output from the
.Ic jobs
command enclosed in brackets
.Po
.Sq Li \&[
and
.Sq Li \&]
.Pc .
Whenever the job table becomes empty, the numbers begin at one again.
In addition, there is the concept of a current, and a previous job,
identified by
.Dq Li \&%+
.Po
or
.Dq Li \&%%
or even just
.Dq Li \&%
.Pc ,
and a previous job, identified by
.Dq Li \&%\- .
Whenever a background job is started,
or a job is resumed in the background,
it becomes the current job.
The job that was the current job
(prepare for a big surprise here, drum roll..., wait for it...\&)
becomes the previous job.
When the current job terminates, the previous job is
promoted to be the current job.
In addition the form
.Dq Li \&% Ns Ar string\^
finds the job for which the command starts with
.Ar string
and the form
.Dq Li \&%? Ns Ar string\^
finds the job which contains the
.Ar string
in its command somewhere.
Both forms require the result to be unambiguous.
For this purpose the
.Dq command
is that shown in the output from the
.Ic jobs
command, not the original command line.
.Pp
The
.Ic bg ,
.Ic fg ,
.Ic jobid ,
.Ic jobs ,
.Ic kill ,
and
.Ic wait
commands all accept job identifiers as arguments, in addition to
process identifiers (larger integers).
See the
.Sx Built-ins
section above, and
.Xr kill 1 ,
for more details of those commands.
In addition, a job identifier
(using one of the
.Dq \&% forms )
issued as a command, without arguments, is interpreted as
if it had been given as the argument to the
.Ic fg
command.
.Pp
To cause a foreground process to stop, enter the terminal's
.Ic stop
character (usually control-Z).
To cause a background process to stop, send it a
.Dv STOP
signal, using the kill command.
A useful function to define is
.Bd -literal -offset indent
stop() { kill -s STOP "${@:-%%}"; }
.Ed
.Pp
The
.Ic fg
command resumes a stopped job, placing it in the foreground,
and
.Ic bg
resumes a stopped job in the background.
The
.Ic jobid
command provides information about process identifiers, job identifiers,
and the process group identifier, for a job.
.Pp
Whenever a sub-shell is created, the jobs table becomes invalid
(the sub-shell has no children.)
However, to enable uses like
.Bd -literal -offset indent
PID=$(jobid -p %1)
.Ed
.Pp
the table is only actually cleared in a sub-shell when needed to
create the first job there (built-in commands run in the foreground
do not create jobs.)
Note that in this environment, there is no useful current job
.Dq ( Li \&%%
actually refers to the sub-shell itself, but is not accessible)
but the job which is the current job in the parent can be accessed as
.Dq Li \&%\- .
.\"
.\"
.Ss Command Line Editing
.\"
When
.Nm
is being used interactively from a terminal, the current command
and the command history (see
.Ic fc
in the
.Sx Built-ins
section)
can be edited using emacs-mode or vi-mode command-line editing.
The command
.Ql set -o emacs
(or
.Fl E
option)
enables emacs-mode editing.
The command
.Ql set -o vi
(or
.Fl V
option)
enables vi-mode editing and places the current shell process into
vi insert mode.
(See the
.Sx Argument List Processing
section above.)
.Pp
The vi-mode uses commands similar to a subset of those described in the
.Xr vi 1
man page.
With vi-mode
enabled,
.Nm sh
can be switched between insert mode and command mode.
It's similar to
.Ic vi :
pressing the
.Aq ESC
key will throw you into vi command mode.
Pressing the
.Aq return
key while in command mode will pass the line to the shell.
.Pp
The emacs-mode uses commands similar to a subset available in the
.Ic emacs
editor.
With emacs-mode enabled, special keys can be used to modify the text
in the buffer using the control key.
.Pp
.Nm
uses the
.Xr editline 3
library.
See
.Xr editline 7
for a list of the possible command bindings,
and the default settings in emacs and vi modes.
Also see
.Xr editrc 5
for the commands that can be given to configure
.Xr editline 7
in the file named by the
.Ev EDITRC
parameter,
or a file used with the
.Ic inputrc
built-in command,
or using
.Xr editline 7 Ap s
configuration command line.
.Pp
When command line editing is enabled, the
.Xr editline 7
functions control printing of the
.Ev PS1
and
.Ev PS2
prompts when required.
As, in this mode, the command line editor needs to
keep track of what characters are in what position on
the command line, care needs to be taken when setting
the prompts.
Normal printing characters are handled automatically,
however mode setting sequences, which do not actually display
on the terminal, need to be identified to
.Xr editline 7 .
This is done, when needed, by choosing a character that
is not needed anywhere in the prompt, including in the mode
setting sequences, any single character is acceptable,
and assigning it to the shell parameter
.Dv PSlit .
Then that character should be used, in pairs, in the
prompt string.
Between each pair of
.Dv PSlit
characters are mode setting sequences, which affect the printing
attributes of the following (normal) characters of the prompt,
but do not themselves appear visibly, nor change the terminal's
cursor position.
.Pp
Each such sequence, that is
.Dv PSlit
character, mode setting character sequence, and another
.Dv PSlit
character, must currently be followed by at least one following
normal prompt character, or it will be ignored.
That is, a
.Dv PSlit
character cannot be the final character of
.Ev PS1
or
.Ev PS2 ,
nor may two
.Dv PSlit
delimited sequences appear adjacent to each other.
Each sequence can contain as many mode altering sequences as are
required however.
Only the first character from
.Dv PSlit
will be used.
When set
.Dv PSlit
should usually be set to a string containing just one
character, then it can simply be embedded in
.Ev PS1
(or
.Ev PS2 )
as in
.Pp
.D1 Li PS1=\*q${PSlit} Ns Ar mset\^ Ns Li ${PSlit}XYZ${PSlit} Ns Ar mclr\^ Ns Li ${PSlit}ABC\*q
.Pp
The prompt visible will be
.Dq XYZABC
with the
.Dq XYZ
part shown according as defined by the mode setting characters
.Ar mset ,
and then cleared again by
.Ar mclr .
See
.Xr tput 1
for one method to generate appropriate mode sequences.
Note that both parts, XYZ and ABC, must each contain at least one
character.
.Pp
If
.Dv PSlit
is unset, which is its initial state, or set to a null string,
no literal character will be defined,
and all characters of the prompt strings will be assumed
to be visible characters (which includes spaces etc.)
To allow smooth use of prompts, without needing redefinition, when
.Xr editline 7
is disabled, the character chosen should be one which will be
ignored by the terminal if received, as when
.Xr editline 7
is not in use, the prompt strings are simply written to the terminal.
For example, setting:
.\" XXX: PS1 line is too long for -offset indent
.Bd -literal -offset left
  PSlit="$(printf\ '\e1')"
  PS1="${PSlit}$(tput\ bold\ blink)${PSlit}\e$${PSlit}$(tput\ sgr0)${PSlit}\ "
.Ed
.Pp
will arrange for the primary prompt to be a bold blinking dollar sign,
if supported by the current terminal, followed by an (ordinary) space,
and, as the SOH (control-A) character
.Pq Sq \e1
will not normally affect
a terminal, this same prompt will usually work with
.Xr editline 7
enabled or disabled.
.Sh ENVIRONMENT
.Bl -tag -width MAILCHECK
.It Ev CDPATH
The search path used with the
.Ic cd
built-in.
.It Ev EDITRC
Gives the name of the file containing commands for
.Xr editline 7 .
See
.Xr editrc 5
for possible content and format.
The file is processed, when in interactive mode with
command line editing enabled, whenever
.Ev EDITRC
is set (even with no actual value change,)
and if command line editing changes from disabled to enabled,
or the editor style used is changed.
(See the
.Fl E
and
.Fl V
options of the
.Ic set
built-in command, described in
.Sx Built-ins
above, which are documented further above in
.Sx Argument List Processing . )
If unset
.Dq $HOME/.editrc
is used.
.It Ev ENV
Names the file sourced at startup by the shell.
Unused by this shell after initialization,
but is usually passed through the environment to
descendant shells.
See the
.Sx Invocation
section above for details of how
.Ev ENV
is processed and used.
.It Ev EUSER
Set to the login name of the effective user id running the shell,
as returned by
.Bd -literal -offset indent -compact
getpwuid(geteuid())->pw_name
.Ed
.Po
See
.Xr getpwuid 3
and
.Xr geteuid 2
for more details.
.Pc
This is obtained each time
.Ev EUSER
is expanded, so changes to the shell's execution identity
cause updates without further action.
If unset, it returns nothing.
If set it loses its special properties, and is simply a variable.
See the
.Ic specialvar
built-in command for remedial action.
.It Ev HISTSIZE
The number of lines in the history buffer for the shell.
.It Ev HOME
Set automatically by
.Xr login 1
from the user's login directory in the password file
.Pq Xr passwd 5 .
This environment variable also functions as the default argument for the
.Ic cd
built-in.
.It Ev HOSTNAME
Set to the current hostname of the system, as returned by
.Xr gethostname 3 .
This is obtained each time
.Ev HOSTNAME
is expanded, so changes to the system's name are reflected
without further action.
If unset, it returns nothing.
If set it loses its special properties, and is simply a variable.
See the
.Ic specialvar
built-in command for remedial action.
.It Ev IFS
Input Field Separators.
This is normally set to
.Aq space ,
.Aq tab ,
and
.Aq newline .
.Sx White Space Splitting
section for more details.
.It Ev LANG
The string used to specify localization information that allows users
to work with different culture-specific and language conventions.
See
.Xr nls 7 .
.It Dv LINENO
The current line number in the script or function.
See the section
.Sx LINENO
below for more details.
.It Ev MAIL
The name of a mail file, that will be checked for the arrival of new mail.
Overridden by
.Ev MAILPATH .
The check occurs just before
.Ev PS1
is written, immediately after reporting jobs which have changed status,
in interactive shells only.
New mail is considered to have arrived if the monitored file
has increased in size since the last check.
.\" .It Ev MAILCHECK
.\" The frequency in seconds that the shell checks for the arrival of mail
.\" in the files specified by the
.\" .Ev MAILPATH
.\" or the
.\" .Ev MAIL
.\" file.
.\" If set to 0, the check will occur at each prompt.
.It Ev MAILPATH
A colon
.Dq \&:
separated list of file names, for the shell to check for incoming mail.
This environment setting overrides the
.Ev MAIL
setting.
There is a maximum of 10 mailboxes that can be monitored at once.
.It Ev PATH
The default search path for executables.
See the
.Sx Path Search
section above.
.It Ev POSIXLY_CORRECT
If set in the environment upon initialization of the shell,
then the shell option
.Ic posix
will be set.
.Po
See the description of the
.Ic set
command in the
.Sx Built-ins
section.
.Pc
After initialization it is unused by the shell,
but is usually passed through the environment to
descendant processes, including other instances of the shell,
which may interpret it in a similar way.
.It Ev PPID
The process identified of the parent process of the
current shell.
This value is set at shell startup, ignoring
any value in the environment, and then made readonly.
.It Ev PS1
The primary prompt string, which defaults to
.Dq Li "$ " ,
unless you are the superuser, in which case it defaults to
.Dq Li "# " .
This string is subject to parameter, arithmetic, and if
enabled by setting the
.Ic promptcmds
option, command substitution before being output.
During execution of commands used by command substitution,
execution tracing, the
.Ic xtrace
.Ic ( set Fl x )
option is temporarily disabled.
If
.Ic promptcmds
is not set and the prompt string uses command substitution,
the prompt used will be an appropriate error string.
For other expansion errors, the prompt will become an
empty string, without an error message.
To verify parsing of
.Ev PS1 ,
the method suggested for
.Ev ENV
can be used.
.It Ev PS2
The secondary prompt string, which defaults to
.Dq Li "> " .
After expansion (as for
.Ev PS1 )
it is written whenever more input is required to complete the
current command.
.It Ev PS4
is Output, after expansion like
.Ev PS1 ,
as a prefix for each line when execution trace
.Ic ( set Fl x )
is enabled.
.Ev PS4
defaults to
.Dq Li "+ " .
.It Ev PSc
Initialized by the shell, ignoring any value from the environment,
to a single character string, either
.Sq \&#
or
.Sq \&$ ,
depending upon whether the current user is the superuser or not.
This is intended for use when building a custom
.Ev PS1 .
.It Ev PSlit
Defines the character which may be embedded in pairs, in
.Ev PS1
or
.Ev PS2
to indicate to
.Xr editline 7
that the characters between each pair of occurrences of the
.Dv PSlit
character will not appear in the visible prompt, and will not
cause the terminal's cursor to change position, but rather set terminal
attributes for the following prompt character(s) at least one of
which must be present.
See
.Sx Command Line Editing
above for more information.
.It Ev RANDOM
Returns a different pseudo-random integer,
in the range [0,32767] each time it is accessed.
.Ev RANDOM
can be assigned an integer value to seed the PRNG.
If the value assigned is a constant, then the
sequence of values produces on subsequent references of
.Ev RANDOM
will repeat after the next time the same constant is assigned.
Note, this is not guaranteed to remain constant from one version
of the shell to another \(en the PRNG algorithm, or seeding
method is subject to change.
If
.Ev RANDOM
is assigned an empty value (null string) then the next time
.Ev RANDOM
is accessed, it will be seeded from a more genuinely random source.
The sequence of pseudo-random numbers generated will not be able to
be generated again (except by luck, whether good or bad, depends!)
This is also how the initial seed is generated, if none has been
assigned before
.Ev RANDOM
is first accessed after shell initialization.
Should the error message
.Dq "RANDOM initialisation failed"
appear on standard error, it indicates that the source
of good random numbers was not available, and
.Ev RANDOM
has instead been seeded with a more predictable value.
The following sequence of random numbers will
not be as unpredictable as they otherwise would be.
.It Ev SECONDS
Returns the number of seconds since the current shell was started.
If unset, it remains unset, and returns nothing, unless set again.
If set, it loses its special properties, and becomes a normal variable.
See the
.Ic specialvar
built-in command for remedial action.
.It Ev START_TIME
Initialized by the shell to the number of seconds since the Epoch
(see
.Xr localtime 3 )
when the shell was started.
The value of
.Dl $(( Ns Ev START_TIME + Ev SECONDS Ns ))
represents the current time, if
.Ev START_TIME
has not been modified, and
.Ev SECONDS
has not been set or unset.
.It Ev TERM
The default terminal setting for the shell.
This is inherited by
children of the shell, and is used in the history editing modes.
.\" This is explicitly last, not in sort order - please leave!
.It Ev ToD
When referenced, uses the value of
.Ev ToD_FORMAT
(or
.Dq \&%T
if
.Ev ToD_FORMAT
is unset) as the format argument to
.Xr strftime 3
to encode the current time of day, in the time zone
defined by
.Ev TZ
if set, or current local time if not, and returns the result.
If unset
.Ev ToD
returns nothing.
If set, it loses its special properties, and becomes a normal variable.
See the
.Ic specialvar
built-in command for remedial action.
.It Ev ToD_FORMAT
Can be set to the
.Xr strftime 3
format string to be used when expanding
.Ev ToD .
Initially unset.
.It Ev TZ
If set, gives the time zone
(see
.Xr localtime 3 ,
.Xr environ 7 )
to use when formatting
.Ev ToD
and if exported, other utilities that deal with times.
If unset, the system's local wall clock time zone is used.
.It Ev NETBSD_SHELL
Unlike the variables previously mentioned,
this variable is somewhat strange,
in that it cannot be set,
inherited from the environment,
modified, or exported from the shell.
If set, by the shell, it indicates that the shell is the
.Ic sh
defined by this manual page, and gives its version information.
It can also give information in additional space separated words,
after the version string.
If the shell was built as part of a reproducible build,
the relevant date that was used for that build will be included.
Finally, any non-standard compilation options,
which may affect features available,
that were used when building the shell will be listed.
.Ev NETBSD_SHELL
behaves like any other variable that has the read-only
and un-exportable attributes set.
.El
.Ss Dv LINENO
.Dv LINENO
is in many respects a normal shell variable, containing an
integer value, and can be expanded using any of the forms
mentioned above which can be used for any other variable.
.Pp
.Dv LINENO
can be exported, made readonly, or unset, as with any other
variable, with similar effects.
Note that while being readonly prevents later attempts to
set, or unset,
.Dv LINENO ,
it does not prevent its value changing.
References to
.Dv LINENO
.Pq "when not unset"
always obtain the current line number.
However,
.Dv LINENO
should normally not ever be set or unset.
In this shell setting
.Dv LINENO
reverses the effect of an earlier
.Ic unset ,
but does not otherwise affect the value obtained.
If unset,
.Dv LINENO
should not normally be set again, doing so is not portable.
If
.Dv LINENO
is set or unset, different shells act differently.
The value of
.Dv LINENO
is never imported from the environment when the shell is
started, though if present there, as with any other variable,
.Dv LINENO
will be exported by this shell.
.Pp
.Dv LINENO
is set automatically by the shell to be the number of the source
line on which it occurs.
When exported,
.Dv LINENO
is exported with its value set to the line number it would have
had had it been referenced on the command line of the command to
which it is exported.
Line numbers are counted from 1, which is the first line the shell
reads from any particular file.
For this shell, standard input, including in an interactive shell,
the user's terminal, is just another file and lines are counted
there as well.
However note that not all shells count interactive
lines this way, it is not wise to rely upon
.Dv LINENO
having a useful value, except in a script, or a function.
.Pp
The role of
.Dv LINENO
in functions is less clear.
In some shells,
.Dv LINENO
continues to refer to the line number in the script which defines
the function,
in others lines count from one within the function, always (and
resume counting normally once the function definition is complete)
and others count in functions from one if the function is defined
interactively, but otherwise just reference the line number in the
script in which the function is defined.
This shell gives the user the option to choose.
If the
.Fl L
flag (the
.Ic local_lineno
option, see
.Sx Argument List Processing )
is set, when the function is defined, then the function
defaults to counting lines with one being the first line of the
function.
When the
.Fl L
flag is not set, the shell counts lines in a function definition
in the same continuous sequence as the lines that surround the
function definition.
Further, if
.Dv LINENO
is made local
(see
.Sx Built-ins
above)
inside the function, the function can decide which
behavior it prefers.
If
.Dv LINENO
is made local and inherited, and not given a value, as in
.Dl local Fl I Dv LINENO
then from that point in the function,
.Dv LINENO
will give the line number as if lines are counted in sequence
with the lines that surround the function definition (and
any other function definitions in which this is nested.)
If
.Dv LINENO
is made local, and in that same command, given a value, as
.Dl local Oo Fl I Ns | Ns Fl N Oc Dv LINENO Ns = Ns Ar value
then
.Dv LINENO
will give the line number as if lines are counted from one
from the beginning of the function.
The value nominally assigned in this case is irrelevant, and ignored.
For completeness, if lineno is made local and unset, as in
.Dl local Fl N Dv LINENO
then
.Dv LINENO
is simply unset inside the function, and gives no value at all.
.Pp
Now for some technical details.
The line on which
.Dv LINENO
occurs in a parameter expansion, is the line that contains the
.Sq \&$
that begins the expansion of
.Dv LINENO .
In the case of nested expansions, that
.Sq \&$
is the one that actually has
.Dv LINENO
as its parameter.
In an arithmetic expansion, where no
.Sq \&$
is used to evaluate
.Dv LINENO
but
.Dv LINENO
is simply referenced as a variable, then the value is the
line number of the line that contains the
.Sq L
of
.Dv LINENO .
For functions line one of the function definition (when relevant)
is the line that contains the first character of the
function name in the definition.
When exported, the line number of the command is the line number
where the first character of the word which becomes the command name occurs.
.Pp
When the shell opens a new file, for any reason,
it counts lines from one in that file,
and then resumes its original counting once it resumes reading the
previous input stream.
When handling a string passed to
.Ic eval
the line number starts at the line on which the string starts,
and then if the string contains internal newline characters,
those characters increase the line number.
This means that references to
.Dv LINENO
in such a case can produce values larger than would be
produced by a reference on the line after the
.Ic eval .
.Sh FILES
.Bl -item
.It
.Pa $HOME/.profile
.It
.Pa /etc/profile
.El
.Sh EXIT STATUS
Errors that are detected by the shell, such as a syntax error, will cause the
shell to exit with a non-zero exit status.
If the shell is not an
interactive shell, the execution of the shell file will be aborted.
Otherwise
the shell will return the exit status of the last command executed, or
if the exit built-in is used with a numeric argument, it will return the
argument.
.Sh SEE ALSO
.Xr csh 1 ,
.Xr echo 1 ,
.Xr getopt 1 ,
.Xr ksh 1 ,
.Xr login 1 ,
.Xr printf 1 ,
.Xr test 1 ,
.Xr editline 3 ,
.Xr getopt 3 ,
.\" .Xr profile 4 ,
.Xr editrc 5 ,
.Xr passwd 5 ,
.Xr editline 7 ,
.Xr environ 7 ,
.Xr nls 7 ,
.Xr sysctl 8
.Sh HISTORY
A
.Nm
command appeared in
.At v1 .
It was replaced in
.At v7
with a version that introduced the basis of the current syntax.
That was, however, unmaintainable so we wrote this one.
.Sh BUGS
Setuid shell scripts should be avoided at all costs, as they are a
significant security risk.
.Pp
The characters generated by filename completion should probably be quoted
to ensure that the filename is still valid after the input line has been
processed.
.Pp
Job control of compound statements (loops, etc) is a complete mess.
.Pp
Many, many, more.
(But less than there were...)