.\" $NetBSD: sh.1,v 1.123 2016/05/12 13:15:43 kre 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. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)sh.1 8.6 (Berkeley) 5/4/95 .\" .Dd May 9, 2016 .Dt SH 1 .ds flags abCEeFfhnuvxIimpqV .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 Oo Ar argument ... Oc .Ek .Nm .Fl c .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 .Ar command_string .Op Ar command_name Oo Ar argument ... Oc .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 with the .Tn 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 ) . Only features designated by .Tn POSIX , plus a few Berkeley extensions, are being incorporated into this shell. .\" We expect .\" .Tn POSIX .\" conformance by the time 4.4 BSD is released. 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. It is the program that is running when a user logs into the system (although a user can select a different shell 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 of the shell is connected to a terminal (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 from the files .Pa /etc/profile and .Pa .profile 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, the shell next reads commands from the file named in .Ev ENV . 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 is 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 case and .Dq esac below (these commands are described later): .Pp .Bl -item -compact -offset indent .It .Li case $- in *i*) .Bl -item -compact -offset indent .It .Li # commands for interactive use only .It .Li ... .El .It .Li esac .El .Pp If command line arguments besides the options have been specified, then the shell treats the first argument as the name of a file from which to read commands (a shell script), and the remaining arguments are set as the positional parameters of the shell ($1, $2, etc). Otherwise, the shell reads commands from its standard input. .Ss Argument List Processing All of the single letter options have a corresponding name that can be used as an argument to the .Fl o option. The set .Fl o name is provided next to the single letter option in the description below. Specifying a dash .Dq - turns the option on, while using a plus .Dq + disables the option. The following options can be set from the command line or with the .Ic set built-in (described later). .Bl -tag -width aaaallexportfoo -offset indent .It Fl a Em allexport Export all variables assigned to. .It Fl c Read commands from the .Ar command_string operand instead of from the standard input. Special parameter 0 will be set from the .Ar command_name operand and the positional parameters ($1, $2, etc.) set from the remaining argument operands. .Fl c is only available at invocation, it cannot be .Ic set , and there is no form using .Dq \&+ . .It Fl C Em noclobber Don't overwrite existing files with .Dq \*[Gt] . .It Fl e Em errexit If not interactive, exit immediately if any untested command fails. 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 \*[Am]\*[Am] 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 noglob Disable pathname expansion. .It Fl n Em noexec If not interactive, read commands but do not execute them. This is useful for checking the syntax of shell scripts. .It Fl u Em nounset Write a message to standard error when attempting to expand a variable that is not set, and if the shell is not interactive, exit immediately. .It Fl v Em verbose The shell writes its input to standard error as it is read. Useful for debugging. .It Fl x Em xtrace Write each command to standard error (preceded by a .Sq +\ ) before it is executed. Useful for debugging. .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 I Em ignoreeof Ignore EOFs from input when interactive. .It Fl i Em interactive Force the shell to behave interactively. .It Fl m Em monitor Turn on job control (set automatically when interactive). .It Fl s Em stdin Read commands from standard input (set automatically if no file arguments are present). This option has no effect when set or reset after the shell has already started running (i.e. with .Ic set ) . .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 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 b Em notify Enable asynchronous notification of background job completion. (Not implemented.) .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 h Em trackall Bind commands in functions to file system paths when the function is defined. When off, the file system is searched for commands each time the function is invoked. (Not implemented.) .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 "\ \ " Em cdprint Make an interactive shell always print the new directory name when changed by the .Ic cd command. .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 posix Enables closer adherence to the shell standard. This option will default set at shell startup if the environment variable .Ev POSIXLY_CORRECT is present. That can be overridden 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. Consequently, while it can be manipulated by the .Ic set command, doing so has no current purpose. .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). Following is a list of operators: .Bl -ohang -offset indent .It "Control operators:" .Dl \*[Am] \*[Am]\*[Am] \&( \&) \&; ;; | || \*[Lt]newline\*[Gt] .It "Redirection operators:" .Dl \*[Lt] \*[Gt] \*[Gt]| \*[Lt]\*[Lt] \*[Gt]\*[Gt] \*[Lt]\*[Am] \*[Gt]\*[Am] \*[Lt]\*[Lt]- \*[Lt]\*[Gt] .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 three types of quoting: matched single quotes, matched double quotes, and backslash. .Ss Backslash A backslash preserves the literal meaning of the following character, with the exception of .Aq newline . A backslash preceding a .Aq newline is treated as a line continuation. .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 $ , backquote .Pq ` , and backslash .Pq \e . The backslash inside double quotes is historically weird, and serves to quote only the following characters: .Dl $ ` \*q \e \*[Lt]newline\*[Gt] . Otherwise it remains literal. .Ss Reserved Words Reserved words are words that have special meaning to the shell and are recognized at the beginning of a line and after a control operator. The following are reserved words: .Bl -column while while while while while -offset indent .It ! Ta elif Ta fi Ta while Ta case .It else Ta for Ta then Ta { Ta } .It do Ta done Ta until Ta if Ta esac .El .Pp Their meaning is discussed later. .Ss Aliases An alias is a name and corresponding value set using the .Ic alias built-in command. Whenever a reserved word may occur (see above), 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 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 .Tn 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 name=value are stripped off and assigned to the environment of the simple command. Redirection operators and their arguments (as described below) are stripped off and saved for processing. .It The remaining words are expanded as described in the .Sx Word Expansions section below, and the first remaining word is considered the command name and the command is located. The remaining words are considered the arguments of the command. If no command name resulted, then the .Dq name=value variable assignments recognized in item 1 affect the current shell. .It Redirections are performed 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 [n] Va redir-op Ar file .Pp where .Va redir-op is one of the redirection operators mentioned previously. Following is a list of the possible redirections. The .Bq n is an optional number, as in .Sq 3 (not .Sq Bq 3 ) , that refers to a file descriptor. .Bl -tag -width aaabsfiles -offset indent .It [n] Ns \*[Gt] file Redirect standard output (or n) to file. .It [n] Ns \*[Gt]| file Same, but override the .Fl C option. .It [n] Ns \*[Gt]\*[Gt] file Append standard output (or n) to file. .It [n] Ns \*[Lt] file Redirect standard input (or n) from file. .It [n1] Ns \*[Lt]\*[Am] Ns n2 Duplicate standard input (or n1) from file descriptor n2. .It [n] Ns \*[Lt]\*[Am]- Close standard input (or n). .It [n1] Ns \*[Gt]\*[Am] Ns n2 Duplicate standard output (or n1) to n2. .It [n] Ns \*[Gt]\*[Am]- Close standard output (or n). .It [n] Ns \*[Lt]\*[Gt] file Open file for reading and writing on standard input (or n). .El .Pp The following redirection is often called a .Dq here-document . .Bl -item -offset indent .It .Li [n]\*[Lt]\*[Lt] delimiter .Dl here-doc-text ... .Li delimiter .El .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 subjected to parameter expansion, command substitution, and arithmetic expansion as described in the .Sx Word Expansions section below. If the operator is .Dq \*[Lt]\*[Lt]\(mi instead of .Dq \*[Lt]\*[Lt] , 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 \e are joined to the following line, the \e and following newline are simply removed while reading the here-doc, which thus guarantees that neither of those lines can be the end delimiter. .Ss Search and Execution There are three types of commands: shell functions, built-in commands, and normal programs -- and the command is searched for (by name) in that order. They each are executed in a different way. .Pp When a shell function is executed, all of the shell positional parameters (except $0, which 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. Then the command given in the function definition is executed. The positional parameters are restored to their original values when the command completes. This all occurs within the current shell. .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 (i.e., if it does not begin with the "magic number" whose .Tn ASCII representation is "#!", so .Xr execve 2 returns .Er ENOEXEC then) the shell will interpret the program in a subshell. 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 "shell procedure". .Ss Path Search When locating a command, the shell first looks to see if it has a shell function by that name. Then it looks for a built-in command by that name. If a built-in command is not found, one of two things happen: .Bl -enum .It Command names containing a slash are simply executed without performing any searches. .It 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. .El .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 for normal or 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. .Ss Complex Commands Complex commands are combinations of simple commands with control operators or reserved words, together creating a larger complex command. More generally, a command is one of the following: .Bl -bullet .It simple command .It pipeline .It list or compound-list .It compound command .It function definition .El .Pp Unless otherwise stated, the exit status of a command is that of the last simple command executed by the command. .Ss Pipelines A pipeline is a sequence of one or more commands separated by the control operator |. The standard output of all but the last command is connected to the standard input of the next command. The standard output of the last command is inherited from the shell, as usual. .Pp The format for a pipeline is: .Pp .Dl [!] command1 [ | command2 ...] .Pp The standard output of command1 is connected to the standard input of command2. The standard input, standard output, or both of a command is considered to be assigned by the pipeline before any redirection specified by redirection operators that are part of the command. .Pp If the pipeline is not in the background (discussed later), the shell waits for all commands to complete. .Pp If the reserved word ! does not precede the pipeline, the exit status is the exit status of the last command specified in the pipeline. Otherwise, the exit status is the logical NOT of the exit status of the last command. That is, if the last command returns zero, the exit status is 1; if the last command returns greater than zero, the exit status is zero. .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\*[Gt]\*[Am]1 | command2 .Pp sends both the standard output and standard error of command1 to the standard input of command2. .Pp A ; or .Aq newline terminator causes the preceding AND-OR-list (described next) to be executed sequentially; a \*[Am] causes asynchronous execution of the preceding AND-OR-list. .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 -- but any effect it has on the environment is wiped). .Ss Background Commands -- \*[Am] If a command is terminated by the control operator ampersand (\*[Am]), the shell executes the command asynchronously -- 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 \*[Am] [command2 \*[Am] ...] .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 \&! (see .Sx Special Parameters ) . .Ss Lists -- Generally Speaking A list is a sequence of zero or more commands separated by newlines, semicolons, or ampersands, and optionally terminated by one of these three characters. 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. .Ss Short-Circuit List Operators .Dq \*[Am]\*[Am] and .Dq || are AND-OR list operators. .Dq \*[Am]\*[Am] executes the first command, and then executes the second command if and only if the exit status of the first command is zero. .Dq || is similar, but executes the second command if and only if the exit status of the first command is nonzero. .Dq \*[Am]\*[Am] and .Dq || both have the same priority. Note that these operators are left-associative, so .Dq true || echo bar \*[Am]\*[Am] echo baz writes .Dq baz and nothing else. This is not the way it works in C. .Ss Flow-Control Constructs -- if, while, for, case The syntax of the if command is .Bd -literal -offset indent if list then list [ elif list then list ] ... [ else list ] fi .Ed .Pp The syntax of the while command is .Bd -literal -offset indent while list do list done .Ed .Pp The two lists are executed repeatedly while the exit status of the first list is zero. The until command is similar, but has the word until in place of while, which causes it to repeat until the exit status of the first list is zero. .Pp The syntax of the for command is .Bd -literal -offset indent for variable [ in word ... ] do list done .Ed .Pp The words are expanded, or "$@" if no words are given, and then the list is executed repeatedly with the variable set to each word in turn. do and done may be replaced with .Dq { and .Dq } , but doing so is non-standard and not recommended. .Pp The syntax of the break and continue command is .Bd -literal -offset indent break [ num ] continue [ num ] .Ed .Pp Break terminates the num innermost for or while loops. Continue continues with the next iteration of the innermost loop. These are implemented as built-in commands. .Pp The syntax of the case command is .Bd -literal -offset indent case word in pattern) list ;; \&... esac .Ed .Pp The pattern can actually be one or more patterns (see .Sx Shell Patterns described later), separated by .Dq \*(Ba characters. .Ss Grouping Commands Together Commands may be grouped by writing either .Pp .Dl (list) .Pp or .Pp .Dl { list; } .Pp The first of these executes the commands in a subshell. Built-in commands grouped into a (list) will not affect the current shell. The second form does not fork another shell so is slightly more efficient. Grouping commands together this way allows you to redirect their output as though they were one program: .Bd -literal -offset indent { echo -n \*q hello \*q ; echo \*q world" ; } \*[Gt] greeting .Ed .Pp Note that .Dq } must follow a control operator (here, .Dq \&; ) so that it is recognized as a reserved word and not as another command argument. .Ss Functions The syntax of a function definition is .Pp .Dl name ( ) command [ redirect... ] .Pp A function definition is an executable statement; when executed it installs a function named name and returns an exit status of zero. The command is normally a list enclosed between .Dq { and .Dq } . The standard syntax also allows the command to be any of the other compound commands, or a sub-shell, all of which are supported. As an extension, this shell also allows a simple command to be used, though users should be aware this is non-standard syntax. This means that .Dl l() ls "$@" 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: .Dl hello() cat <