554 lines
15 KiB
Plaintext
554 lines
15 KiB
Plaintext
.\" $NetBSD: t1,v 1.3 2010/08/22 02:19:07 perry Exp $
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.\"
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.\" Copyright (C) Caldera International Inc. 2001-2002. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions are
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.\" met:
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.\"
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.\" Redistributions of source code and documentation must retain the above
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.\" copyright notice, this list of conditions and the following
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.\" disclaimer.
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.\"
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.\" Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgment:
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.\"
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.\" This product includes software developed or owned by Caldera
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.\" International, Inc. Neither the name of Caldera International, Inc.
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.\" nor the names of other contributors may be used to endorse or promote
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.\" products derived from this software without specific prior written
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.\" permission.
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.\"
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.\" USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA
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.\" INTERNATIONAL, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
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.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
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.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
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.\" OR OTHERWISE) RISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
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.\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" @(#)t1 8.1 (Berkeley) 8/14/93
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.\"
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.EH 'USD:3-%''An Introduction to the UNIX Shell'
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.OH 'An Introduction to the UNIX Shell''USD:3-%'
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.\".RP
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.TL
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An Introduction to the UNIX Shell
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.AU
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S. R. Bourne
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.AI
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Murray Hill, NJ
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.AU
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(Updated for 4.3BSD by Mark Seiden)
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.AU
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(Further updated by Perry E. Metzger)\(dg
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.AB
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.FS
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\(dg This paper was updated in 2010 to reflect most features of modern
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POSIX shells, which all follow the design of S.R. Bourne's original v7
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Unix shell.
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Among these are ash, bash, ksh and others.
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Typically one of these will be installed as /bin/sh on a modern system.
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It does not describe the behavior of the c shell (csh).
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If it's the c shell (csh) you're interested in, a good place to
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begin is William Joy's paper "An Introduction to the C shell" (USD:4).
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.FE
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.LP
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The
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.ul
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shell
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is a command programming language that provides an interface
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to the
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.UX
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operating system.
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Its features include
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control-flow primitives, parameter passing, variables and
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string substitution.
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Constructs such as
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.ul
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while, if then else, case
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and
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.ul
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for
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are available.
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Two-way communication is possible between the
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.ul
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shell
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and commands.
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String-valued parameters, typically file names or flags, may be
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passed to a command.
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A return code is set by commands that may be used to determine control-flow,
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and the standard output from a command may be used
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as shell input.
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.LP
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The
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.ul
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shell
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can modify the environment
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in which commands run.
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Input and output can be redirected
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to files, and processes that communicate through `pipes'
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can be invoked.
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|
Commands are found by
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|
searching directories
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in the file system in a
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sequence that can be defined by the user.
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|
Commands can be read either from the terminal or from a file,
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which allows command procedures to be
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stored for later use.
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.AE
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.ds ST \v'.3m'\s+2*\s0\v'-.3m'
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.SH
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1.0\ Introduction
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.LP
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The shell is both a command language
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and a programming language
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that provides an interface to the UNIX
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operating system.
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This memorandum describes, with
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examples, the UNIX shell.
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The first section covers most of the
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everyday requirements
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of terminal users.
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Some familiarity with UNIX
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is an advantage when reading this section;
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see, for example,
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"UNIX for beginners".
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.[
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unix beginn kernigh 1978
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.]
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Section 2 describes those features
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of the shell primarily intended
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for use within shell procedures.
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These include the control-flow
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primitives and string-valued variables
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provided by the shell.
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A knowledge of a programming language
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would be a help when reading this section.
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The last section describes the more
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advanced features of the shell.
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References of the form "see \fIpipe\fP (2)"
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are to a section of the UNIX manual.
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.[
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seventh 1978 ritchie thompson
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.]
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.SH
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1.1\ Simple\ commands
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.LP
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Simple commands consist of one or more words
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separated by blanks.
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The first word is the name of the command
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to be executed; any remaining words
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are passed as arguments to the command.
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For example,
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.DS
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who
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.DE
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is a command that prints the names
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of users logged in.
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The command
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.DS
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ls \(mil
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.DE
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prints a list of files in the current
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directory.
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The argument \fI\(mil\fP tells \fIls\fP
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to print status information, size and
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the creation date for each file.
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.SH
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1.2\ Input\ output\ redirection
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.LP
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Most commands produce output on the standard output
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that is initially connected to the terminal.
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This output may be sent to a file
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by writing, for example,
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.DS
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ls \(mil >file
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.DE
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The notation \fI>file\fP
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is interpreted by the shell and is not passed
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as an argument to \fIls.\fP
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If \fIfile\fP does not exist then the
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shell creates it;
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otherwise the original contents of
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\fIfile\fP are replaced with the output
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from \fIls.\fP
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Output may be appended to a file
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using the notation
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.DS
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ls \(mil \*(APfile
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.DE
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In this case \fIfile\fP is also created if it does not already
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exist.
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.LP
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The standard input of a command may be taken
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from a file instead of the terminal by
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writing, for example,
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.DS
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wc <file
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.DE
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The command \fIwc\fP reads its standard input
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(in this case redirected from \fIfile\fP)
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|
and prints the number of characters, words and
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lines found.
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If only the number of lines is required
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then
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.DS
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wc \(mil <file
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.DE
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could be used.
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.\" I considered adding the following, but have thought better of it
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.\" for now.
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.\" -- Perry Metzger
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.\"
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.\" .LP
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.\" Error messages are typically printed by commands on a different
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.\" channel, called standard error, which may also be redirected using the
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.\" notation 2>\|.
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.\" For example
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.\" .DS
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.\" command some args >out 2>errors
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.\" .DE
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.\" will redirect standard output to the file `out' but standard error
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.\" (and thus all error messages) to `errors'.
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.\" The notation 2>&1 sets standard error pointing to the same
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.\" place as standard out.
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.\" Thus:
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.\" .DS
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.\" command some args 2>&1 >everything
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.\" .DE
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.\" will put both standard out and standard error into the file `everything'.
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.\" See section 3.7 below for more details.
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.SH
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1.3\ Pipelines\ and\ filters
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.LP
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The standard output of one command may be
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connected to the standard input of another
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by writing
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the `pipe' operator,
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indicated by \*(VT,
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as in,
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.DS
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ls \(mil \*(VT wc
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.DE
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Two commands connected in this way constitute
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a \fIpipeline\fP and
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the overall effect is the same as
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.DS
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ls \(mil >file; wc <file
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.DE
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except that no \fIfile\fP is used.
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Instead the two \fIprocesses\fP are connected
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by a pipe (see \fIpipe\fP(2)) and are
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run in parallel.
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Pipes are unidirectional and
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synchronization is achieved by
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halting \fIwc\fP when there is
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nothing to read and halting \fIls\fP
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when the pipe is full.
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.LP
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A \fIfilter\fP is a command
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that reads its standard input,
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transforms it in some way,
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and prints the result as output.
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One such filter, \fIgrep,\fP
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selects from its input those lines
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that contain some specified string.
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For example,
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.DS
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ls \*(VT grep old
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.DE
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prints those lines, if any, of the output
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from \fIls\fP that contain
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the string \fIold.\fP
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Another useful filter is \fIsort\fP.
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For example,
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.DS
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who \*(VT sort
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.DE
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will print an alphabetically sorted list
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of logged in users.
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.LP
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A pipeline may consist of more than two commands,
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for example,
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.DS
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ls \*(VT grep old \*(VT wc \(mil
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.DE
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prints the number of file names
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in the current directory containing
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the string \fIold.\fP
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.SH
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1.4\ Background\ commands
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.LP
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To execute a command (or pipeline) the shell normally
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creates the new \fIprocesses\fP
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and waits for them to finish.
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A command may be run without waiting
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for it to finish.
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For example,
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.DS
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cc pgm.c &
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.DE
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calls the C compiler to compile
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the file \fIpgm.c\|.\fP
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The trailing \fB&\fP is an operator that instructs the shell
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not to wait for the command to finish.
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To help keep track of such a process
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the shell reports its job number (see below) and process
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id following its creation.
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Such a command is said to be running in the \fIbackground\fP.
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By contrast, a command executed without the \fB&\fP is said to be
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running in the \fIforeground\fP.\(dg
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.FS
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\(dg Even after execution, one may move commands from the foreground
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to the background, or temporarily suspend their execution (which is
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known as \fIstopping\fP a command.
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This is described in detail in section 3.10 on \fIJob Control\fB.
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.FE
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.LP
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A list of currently active processes, including ones not associated
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with the current shell, may be obtained using the \fIps\fP(1) command.
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.SH
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1.5\ File\ name\ generation
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.LP
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|
Many commands accept arguments
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which are file names.
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For example,
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.DS
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ls \(mil main.c
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.DE
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prints information relating to the file \fImain.c\fP\|.
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.LP
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The shell provides a mechanism
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for generating a list of file names
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that match a pattern.
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For example,
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.DS
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ls \(mil \*(ST.c
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.DE
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generates, as arguments to \fIls,\fP
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all file names in the current directory that end in \fI.c\|.\fP
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The character \*(ST is a pattern that will match any string
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including the null string.
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In general \fIpatterns\fP are specified
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as follows.
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.RS
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.IP \fB\*(ST\fR 8
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Matches any string of characters
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including the null string.
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.IP \fB?\fR 8
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Matches any single character.
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.IP \fB[\*(ZZ]\fR 8
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Matches any one of the characters
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enclosed.
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A pair of characters separated by a minus will
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match any character lexically between
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the pair.
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.RE
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.LP
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|
For example,
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.DS
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[a\(miz]\*(ST
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.DE
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matches all names in the current directory
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beginning with
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one of the letters \fIa\fP through \fIz.\fP
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.DS
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/usr/fred/test/?
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.DE
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matches all names in the directory
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\fB/usr/fred/test\fP that consist of a single character.
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If no file name is found that matches
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the pattern then the pattern is passed,
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unchanged, as an argument.
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.LP
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This mechanism is useful both to save typing
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and to select names according to some pattern.
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It may also be used to find files.
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For example,
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.DS
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echo /usr/fred/\*(ST/core
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.DE
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finds and prints the names of all \fIcore\fP files in sub-directories
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|
of \fB/usr/fred\|.\fP
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(\fIecho\fP is a standard UNIX command that prints
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its arguments, separated by blanks.)
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This last feature can be expensive,
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requiring a scan of all
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sub-directories of \fB/usr/fred\|.\fP
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.LP
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There is one exception to the general
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|
rules given for patterns.
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|
The character `\fB.\fP'
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|
at the start of a file name must be explicitly
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matched.
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.DS
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echo \*(ST
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.DE
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will therefore echo all file names in the current
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directory not beginning
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with `\fB.\fP'\|.
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.DS
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echo \fB.\fP\*(ST
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.DE
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will echo all those file names that begin with `\fB.\fP'\|.
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This avoids inadvertent matching
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of the names `\fB.\fP' and `\fB..\fP'
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which mean `the current directory'
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and `the parent directory'
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respectively.
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(Notice that \fIls\fP suppresses
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information for the files `\fB.\fP' and `\fB..\fP'\|.)
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.LP
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|
Finally, the tilde character, `\fB\(ap\fP', may be used to indicate the
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home directory of a user.
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|
The `\fB\(ap\fP' at the beginning of a path name followed by a
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non-alphabetic character expands to the current user's home
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directory.
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|
If the `\fB\(ap\fP' is followed by a login name, it expands to the named
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user's home directory.
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|
For example:
|
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.DS
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ls \(ap
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cd \(apegbert/
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.DE
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will list the contents of the user's home directory and then change
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to the home directory of the user ``egbert''.
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.SH
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|
1.6\ Quoting
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.LP
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|
Characters that have a special meaning
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to the shell, such as \fB< > \*(ST ? \*(VT &\|,\fR
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are called metacharacters.
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A complete list of metacharacters is given
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in appendix B.
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Any character preceded by a \fB\\\fR is \fIquoted\fP
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and loses its special meaning, if any.
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The \fB\\\fP is elided so that
|
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.DS
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echo \\?
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.DE
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|
will echo a single \fB?\|,\fP
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and
|
|
.DS
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|
echo \\\\
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.DE
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|
will echo a single \fB\\\|.\fR
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|
To allow long strings to be continued over
|
|
more than one line
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the sequence \fB\\newline\fP
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is ignored.
|
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.LP
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|
\fB\\\fP is convenient for quoting
|
|
single characters.
|
|
When more than one character needs
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quoting the above mechanism is clumsy and
|
|
error prone.
|
|
A string of characters may be quoted
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by enclosing the string between single quotes.
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|
For example,
|
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.DS
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|
echo xx\'\*(ST\*(ST\*(ST\*(ST\'xx
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.DE
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|
will echo
|
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.DS
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|
xx\*(ST\*(ST\*(ST\*(STxx
|
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.DE
|
|
The quoted string may not contain
|
|
a single quote
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|
but may contain newlines, which are preserved.
|
|
This quoting mechanism is the most
|
|
simple and is recommended
|
|
for casual use.
|
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.LP
|
|
A third quoting mechanism using double quotes
|
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is also available
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|
that prevents interpretation of some but not all
|
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metacharacters.
|
|
Discussion of the
|
|
details is deferred
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to section 3.5\|.
|
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.SH
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1.7\ Prompting
|
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.LP
|
|
When the shell is used from a terminal it will
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|
issue a prompt before reading a command.
|
|
By default this prompt is `\fB$\ \fR'\|.
|
|
It may be changed by saying,
|
|
for example,
|
|
.DS
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|
\s-1PS1\s0="yesdear$ "
|
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.DE
|
|
that sets the prompt to be the string \fIyesdear$\|.\fP
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|
If a newline is typed and further input is needed
|
|
then the shell will issue the prompt `\fB>\ \fR'\|.
|
|
Sometimes this can be caused by mistyping
|
|
a quote mark.
|
|
If it is unexpected then entering the interrupt character
|
|
(typically \s-1CONTROL-C\s0)
|
|
will return the shell to read another command.
|
|
This prompt may be changed by saying, for example,
|
|
.DS
|
|
\s-1PS2\s0=more
|
|
.DE
|
|
Entering the interrupt character may also be used to terminate most
|
|
programs running as the current foreground job.
|
|
.LP
|
|
(\s-1PS1\s0 and \s-1PS2\s0 are \fIshell variables\fP, which will be
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|
described in section 2.4 below.)
|
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.SH
|
|
1.8\ The\ shell\ and\ login
|
|
.LP
|
|
Following \fIlogin\fP(1)
|
|
the shell is called to read and execute
|
|
commands typed at the terminal.
|
|
If the user's login directory
|
|
contains the file \fB.profile\fP
|
|
then it is assumed to contain commands
|
|
and is read by the shell before reading
|
|
any commands from the terminal.
|
|
.LP
|
|
(Most versions of the shell also specify a file that is read and
|
|
executed on start-up whether or not the shell is invoked by login.
|
|
The \s-1ENV\s0 shell variable, described in section 2.4 below, can be
|
|
used to override the name of this file.
|
|
See the shell manual page for further information.)
|
|
.SH
|
|
1.9\ Summary
|
|
.sp
|
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.RS
|
|
.IP \(bu
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|
\fBls\fP
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|
.br
|
|
Print the names of files in the current directory.
|
|
.IP \(bu
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|
\fBls >file\fP
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|
.br
|
|
Put the output from \fIls\fP into \fIfile.\fP
|
|
.IP \(bu
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|
\fBls \*(VT wc \(mil\fR
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|
.br
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|
Print the number of files in the current directory.
|
|
.IP \(bu
|
|
\fBls \*(VT grep old\fR
|
|
.br
|
|
Print those file names containing the string \fIold.\fP
|
|
.IP \(bu
|
|
\fBls \*(VT grep old \*(VT wc \(mil\fR
|
|
.br
|
|
Print the number of files whose name contains the string \fIold.\fP
|
|
.IP \(bu
|
|
\fBcc pgm.c &\fR
|
|
.br
|
|
Run \fIcc\fP in the background.
|
|
.RE
|