ash.1

操作系统设计与实现源码

.\" Copyright (c) 1991 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
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
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.\"    notice, this list of conditions and the following disclaimer in the
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.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
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.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
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.\"	@(#)sh.1	5.1 (Berkeley) 3/7/91
.\"
.TH SH 1 "March 7, 1991"
.UC 7
.de h \" subheading
.sp
.ti -0.3i
.B "\\$1"
.PP
..
.de d \" begin display
.sp
.in +4
.nf
..
.de e \" end display
.in -4
.fi
.sp
..
.de c \" command, etc.
.br
.HP 3
\fB\\$1\fR
.br
..
.de b \" begin builtin command
.HP 3
.B \\$1
..
.SH NAME
ash, command, getopts, hash, jobs, local, return, setvar, unset \- a shell
.SH SYNOPSIS
.B ash
[
.B -efIijnsxz
] [
.B +efIijnsxz
] [
.B -c
.I command
] [
.I arg
] ...
.SH COPYRIGHT
Copyright 1989 by Kenneth Almquist.
.SH DESCRIPTION
.I Ash
is a version of
.I sh
with features similar to those of the System V shell.
This manual page lists all the features of
.I ash
but concentrates on the ones not in other shells.
.h "Invocation"
If the
.B -c
options is given, then the shell executes the specified shell command.
The
.B -s
flag cause the shell to read commands from the standard input (after
executing any command specified with the
.B -c
option.
If neither the
.B -s
or
.B -c
options are set, then the first
.I arg
is taken as the name of a file to read commands from.
If this is impossible because there are no arguments following
the options, then
.I ash
will set the
.B -s
flag and will read commands from the standard input.
.PP
The shell sets the initial value of the positional parameters from the
.IR arg s
remaining after any
.I arg
used as the name of a file of commands is deleted.
.PP
The flags (other than
.BR -c )
are set by preceding them with ``-'' and cleared by preceding them
with ``+''; see the
.I set
builtin command for a list of flags.
If no value is specified for the
.B -i
flag, the
.B -s
flag is set, and the standard input and output of the shell
are connected to terminals, then the
.B -i
flag will be set.
If no value is specified for the
.B -j
flag, then the
.B -j
flag will be set if the
.B -i
flag is set.
.PP
When the shell is invoked with the
.B -c
option, it is good practice to include the
.I -i
flag if the command was entered interactively by a user.
For compatibility with the System V shell, the
.I -i
option should come after the
.B -c
option.
.PP
If the first character of argument zero to the shell is ``-'',
the shell is assumed to be a login shell, and the files
.B /etc/profile
and
.B .profile
are read if they exist.
If the environment variable SHINIT is set on entry to the shell,
the commands in SHINIT are normally parsed and executed.  SHINIT is
not examined if the shell is a login shell, or if it the shell is running a
shell procedure.   (A shell is considered to be running a shell
procedure if neither the
.B -s
nor the
.B -c
options are set.)
.h "Control Structures"
A
.I list
is a sequence of zero or more commands separated by newlines,
semicolons, or ampersands, and optionally terminated by one of these
three characters.  (This differs from the System V shell, which
requires a list to contain at least one command in most cases.)  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 proceed onto the next command; otherwise it waits
for the command to terminate before proceeding to the next one.
.PP
``&&'' and ``||'' are binary operators.
``&&'' executes the first command, and then executes the second command
iff the exit status of the first command is zero.  ``||'' is similar,
but executes the second command iff the exit status of the first command
is nonzero.  ``&&'' and ``||'' both have the same priority.
.PP
The ``|'' operator is a binary operator which feeds the standard output
of the first command into the standard input of the second command.
The exit status of the ``|'' operator is the exit status of the second
command.  ``|'' has a higher priority than ``||'' or ``&&''.
.PP
An
.I if
command looks like
.d
\fBif\fR list
\fBthen\fR	list
.ti -\w'[ 'u
[ \fBelif\fR list
  \fBthen\fR	list ] ...
.ti -\w'[ 'u
[ \fBelse\fR	list ]
\fBfi\fR
.e
.PP
A
.I while
command looks like
.d
\fBwhile\fR list
\fBdo\fR	list
\fBdone\fR
.e
The two lists are executed repeatedly while the exit status of the first
list is zero.  The
.I until
command is similar, but has the word
.B until
in place of
.B while
 repeats until the exit status of the first list
is zero.
.PP
The
.I for
command looks like
.d
\fBfor\fR variable \fBin\fR word...
\fBdo\fR	list
\fBdone\fR
.e
The words are expanded, and then the list is executed repeatedly with
the variable set to each word in turn.
.B do
and
.B done
may be replaced with
``{'' and ``}''.
.PP
The
.I break
and
.I continue
commands look like
.d
\fBbreak\fR [ num ]
\fBcontinue\fR [ num ]
.e
.I Break
terminates the
.I num
innermost
.I for
or
.I while
loops.
.I Continue
continues with the next iteration of the
.IRnum'th
innermost loop.
These are implemented as builtin commands.
.PP
The
.I case
command looks like
.d
\fBcase\fR word \fBin\fR
pattern\fB)\fR list \fB;;\fR
\&...
\fBesac\fR
.e
The pattern can actually be one or more patterns (see
.I Patterns
below), separated by ``|'' characters.
.PP
Commands may be grouped by writing either
.d
\fB(\fRlist\fB)\fR
.e
or
.d
\fB{\fR list; \fB}\fR
.e
The first of these executes the commands in a subshell.
.PP
A function definition looks like
.d
name \fB( )\fR command
.e
A function definition is an executable statement; when executed it installs
a function named
.B name
and returns an exit status of zero.
The command is normally a list enclosed between ``{'' and ``}''.
.PP
Variables may be declared to be local to a function by using a
.I local
command.  This should appear as the first staement of a function,
and looks like
.d
\fBlocal\fR [ variable | \fB-\fR ] ...
.e
.I Local
is implemented as a builtin command.
.PP
When a variable is made local, it inherits the initial value and
exported and readonly flags from the variable with the same name in the
surrounding scope, if there is one.  Otherwise, the variable is
initially unset.
.I Ash
uses dynamic scoping, so that if you make the variable
.B x
local to function
.IR f ,
which then calls function
.IR g ,
references to the variable
.B x
made inside
.I g
will refer to the variable
.B x
declared inside
.IR f ,
not to the global variable named
.BR x .
.PP
The only special parameter than can be made local is ``\fB-\fR''.
Making ``\fB-\fR'' local any shell options that are changed via the
.I set
command inside the function to be restored to their original values
when the function returns.
.PP
The
.I return
command looks like
.d
\fBreturn\fR [ exitstatus ]
.e
It terminates the currently executing function.
.I Return
is implemented as a builtin command.
.h "Simple Commands"
A simple command is a sequence of words.  The execution of a simple
command proceeds as follows.  First, the leading words of the form
``name=value'' are stripped off and assigned to the environment of
the command.  Second, the words are expanded.  Third, the first
remaining word is taken as the command name that command is located.
Fourth, any redirections are performed.  Fifth, the command is
executed.  We look at these operations in reverse order.
.PP
The execution of the command varies with the type of command.
There are three types of commands:  shell functions, builtin commands,
and normal programs.
.PP
When a shell function is executed, all of the shell positional parameters
(except $0, which remains unchanged) are set to the parameters to 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 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.
.PP
Shell builtins are executed internally to the shell, without spawning
a new process.
.PP
When a normal program is executed, the shell runs the program, passing
the parameters and the environment to the program.  If the program is
a shell procedure, the shell will interpret the program in a subshell.
The shell will reinitialize itself in this case, so that the effect
will be as if a new shell had been invoked to handle the shell procedure,
except that the location of commands located in the parent shell will
be remembered by the child.  If the program is a file beginning with
``#!'', the remainder of the first line specifies an interpreter for
the program.  The shell (or the operating system, under Berkeley UNIX)
will run the interpreter in this case.  The arguments to the interpreter