sh.1

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 1 代码 · 共 906 行 · 第 1/2 页

.\" SCCSID: @(#)sh.1	8.3	11/26/90
.TH sh 1 
.SH Name
sh, for, case, if, while, :, ., break, continue, cd, eval, exec, exit, export, login, read, readonly, set, shift, times, trap, umask, wait \- command language
.SH Syntax
.B sh
[
.B \-ceiknrstuvx
] [
.I arg...
] 
.SH Description
.NX R "sh command interpreter"
The
.PN sh
command is a command programming language that executes commands read
from a terminal or a file.  See
.B Invocation
for the meaning of arguments to the shell.
.PP
.B Commands
.br
A
.I simple command
is a sequence of nonblank
.I words
separated by blanks (a blank is a
.B tab
or a
.BR space ).
.NX R "simple command" "defined"
The first word specifies the name of the command to be executed.
Except as specified below, the remaining words are passed as arguments
to the invoked command.  The command name is passed as argument 0.  For
further information, see
.MS execve 2 .
The
.I value
of a simple command is its exit status if it terminates normally or
.RI 200+ status
if it terminates abnormally.  For a list of status values, see 
.MS sigvec 2 .
.PP
A
.I pipeline
is a sequence of one or more
.I commands
separated by
.BR \(or .
.NX R "pipeline" "defined"
The standard output of each command but the last is connected by a
.MS pipe 2
to the standard input of the next command.  Each command is run as a
separate process; the shell waits for the last command to terminate.
.PP
A
.I list
is a sequence of one or more
.I pipelines
separated by
.BR ; ,
.BR & ,
.B &&
or
.B \(or\|\(or
and optionally terminated by
.B ;
or
.BR & . 
.NX R "pipeline" "lists"
.B ;
and
.B &
have equal precedence which is lower than that of
.B &&
and
.BR \(or\|\(or ,
.B &&
and
.B \(or\|\(or
also have equal precedence.  A semicolon causes sequential execution.
An ampersand causes the preceding
.I pipeline
to be executed without waiting for it to finish.  The symbol
.B &&
.RB ( \(or\|\(or )
causes the
.I list
following to be executed only if the preceding
.I pipeline
returns a zero (nonzero) value.  Newlines may appear in a
.I list,
instead of semicolons, to delimit commands.
.PP
A
.I command
is either a simple command or one of the following.  The value returned
by a command is that of the last simple command executed in the
command.
.IP "\fBfor \fIname \fR[ \fBin \fIword \&...\fR ] \fBdo \fIlist \fBdone"
.NX R "for command (sh)"
Each time a
.B for
command is executed, 
.I name
is set to the next word in the
.B for
word list. 
If
.B in
.I word
\&...
is omitted,
.B in
"$@"
is assumed.  Execution ends when there are no more words in the list.
.IP "\fBcase \fIword \fBin \fR[ \fIpattern \fR[ | \fIpattern \fR] \&...) \fIlist \fR;; ] ... \fBesac"
.NX R "case command (sh)"
A
.B case
command executes the
.I list
associated with the first pattern that matches
.I word.
The form of the patterns is the same as that used for file name
generation.
.IP "\fBif \fIlist \fBthen \fIlist \fR[ \fBelif \fIlist \fBthen \fIlist \fR] ... [ \fBelse \fIlist \fR] \fBfi"
.NX R "if command (sh)"
The
.I list
following
.B if
is executed and if it returns zero, the
.I list
following
.B then
is executed.  Otherwise, the
.I list
following
.B elif
is executed and if its value is zero, the
.I list
following
.B then
is executed.  Failing that, the
.B else
.I list
is executed.
.IP "\fBwhile \fIlist \fR[ \fBdo \fIlist \fR] \fBdone"
.NX R "while command (sh)"
A
.B while
command repeatedly executes the
.B while
.I list
and, if its value is zero, executes the
.B do
.I list;
otherwise the loop terminates.  The value returned by a
.B while
command is that of the last executed command in the
.B do
.I list.\ 
Use
.B until
in place of
.B while
to negate the loop termination test.
.TP
.BI ( " list " )
Execute
.I list
in a subshell.
.TP
.BI { " list " }
.I list
is simply executed.
.PP
The following words are only recognized as the first word of a command
and when not quoted.
.IP
.B
if then else elif fi case in esac for while until do done { }
.PP
.B Command substitution
.br
.NX R "sh command interpreter" "command substitution"
The standard output from a command enclosed in a pair of back quotes 
.RB ( \`\|\` )
may be used as part or all of a word; trailing new lines are removed.
.PP
.B Parameter substitution
.br
.NX R "sh command interpreter" "parameter substitution"
The character
.B $
is used to introduce substitutable parameters.  Positional parameters
may be assigned values by
.BR set .
Variables may be set by writing
.IP
.IB name = value
[\ 
.IB name = value
\ ] ...
.TP
$\fB\|{\fIparameter\fB\|}\fR
A
.I parameter
is a sequence of letters, digits or underscores (a
.IR name ),
a digit, or any of the characters
.B
* @ # ? \- $ !\|. 
.NX R "parameter" "defined"
The value, if any, of the parameter is substituted.  The braces are
required only when
.I parameter
is followed by a letter, digit, or underscore that is not to be
interpreted as part of its name.  If
.I parameter
is a digit, it is a positional parameter.  If
.I parameter
is
.BR * " or" " @"
then all the positional parameters, starting with
.BR $1 ,
are substituted separated by spaces.
.B $0
is set from argument zero when the shell is invoked.
.TP
$\fB\|{\fIparameter\|\-word\|\fB}\fR
If
.I parameter
is set, substitute its value; otherwise substitute
.I word.
.TP
$\fB\|{\fIparameter\|\(eq\|word\|\fB}\fR
If
.I parameter
is not set, set it to
.I word;
the value of the parameter is then substituted.  Positional parameters
may not be assigned to in this way.
.TP
$\fB\|{\fIparameter\|?\|word\|\fB}\fR
If
.I parameter
is set, substitute its value; otherwise, print
.I word
and exit from the shell.  If
.I word
is omitted, a standard message is printed.
.TP
$\fB\|{\fIparameter\|\(plword\|\fB}\fR
If
.I parameter
is set, substitute
.I word;
otherwise substitute nothing.
.PP
In the above
.I word
is not evaluated unless it is to be used as the substituted string.
(So that, for example, echo ${d\-\`pwd\`} will only execute
.I pwd
if
.I d
is unset.)
.PP
The following
.I parameters
are automatically set by the shell.
.RS
.TP
.B #
The number of positional parameters in decimal.
.PD 0
.TP
.B \-
Options supplied to the shell on invocation or by
.BR set .
.TP
.B ?
The value returned by the last executed command in decimal.
.TP
.B $
The process number of this shell.
.TP
.B !
The process number of the last background command invoked.
.PD
.RE
.PP
The following
.I parameters
are used but not set by the shell.
.RS
.TP
.B
HOME
The default argument (home directory) for the
.B cd
command.
.PD 0
.TP
.B
PATH
The search path for commands (see
.BR execution ).
.TP
.B
MAIL
If this variable is set to the name of
a mail file, the shell informs the user of
the arrival of mail in the specified file.
.TP
.B PS1
Primary prompt string, by default `$ '.
.TP
.B PS2
Secondary prompt string, by default `> '.
.TP
.B IFS
Internal field separators, normally
.BR space ,
.BR tab ,
and
.BR new line .
.PD
.RE
.PP
.B Blank interpretation
.br
After parameter and command substitution,
any results of substitution are scanned for internal field separator
characters (those found in
.BR $IFS \*S)
and split into distinct arguments where such characters are found.
Explicit null arguments ("" or \'\') are retained.
Implicit null arguments (those resulting from
.I parameters
that have no values) are removed.
.PP
.B File name generation
.br
Following substitution, each command word is scanned for the characters
.BR * ,
.B ?
and
.B [\ .
If one of these characters appears, the word is regarded as a pattern.
.NX R "pattern" "matching"
The word is replaced with alphabetically sorted file names that match the
pattern.  If no file name is found that matches the pattern,
the word is left unchanged.  The character
.B .
at the start of a file name or immediately following a
.BR / ,
and the character
.BR / ,
must be matched explicitly.
.TP 
.B *
Matches any string, including the null string.
.PD 0
.TP 
.B ?
Matches any single character.
.TP 
.B [\ ...\ ]
Matches any one of the characters enclosed.
A pair of characters separated by
.B \-
matches any character lexically between the pair.
.PD
.PP
.B Quoting.
.br
.NX R "sh command interpreter" "quoting characters"
The following characters have a special meaning to the shell
and cause termination of a word unless quoted.
.IP
.B
;   &   (   )   \(or   <   >   new line   space   tab
.PP
A character may be
.I quoted
by preceding it with a
.B
\e\|.  \enew-line
is ignored.  All characters enclosed between a pair of quote marks
.RB ( \'\|\' ),
except a single quote, are quoted.  Inside double quotes
.RB ( "\|" )
parameter and command substitution occurs and
.B
\e
quotes the characters
.B
\e \' "
and
.BR $ \|.
.PP
.B
"$*"
is equivalent to
.B
"$1 $2 ..."
whereas
.br
.B
"$@"
is equivalent to
.B
"$1" "$2" ... .
.PP
.B Prompting
.br
.NX R "sh command interpreter" "prompts"
When used interactively, the shell prompts with the value of
PS1
before reading a command.
If at any time a new line is typed and further input is needed
to complete a command, the secondary prompt
.RB ( \s-2$PS2\s0 )
is issued.
.PP
.B Input output
.br
.NX R "sh command interpreter" "directing input"
.NX R "sh command interpreter" "directing output"
Before a command is executed, its input and output may be redirected
using a special notation interpreted by the shell.  The following may
appear anywhere in a simple command or may precede or follow a
.I command
and are not passed on to the invoked command.  Substitution occurs
before
.I word
or
.I digit
is used.
.IP <\fI\|word
Use file
.I word
as standard input (file descriptor 0).
.PD
.IP >\fI\|word
Use file
.I word
as standard output (file descriptor 1).  If the file does not exist, it
is created; otherwise it is truncated to zero length.
.IP >>\fI\|word