path: root/usr/src/man/man1has/sh.1has

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.TH SH 1HAS "April 9, 2016"
.SH NAME
sh, jsh \- standard and job control shell and command interpreter
.SH SYNOPSIS
.LP
.nf
\fB/usr/bin/sh\fR  [\fB-acefhiknprstuvx\fR] [\fIargument\fR]...
.fi

.LP
.nf
\fB/usr/xpg4/bin/sh\fR  [\(+- abCefhikmnoprstuvx]
     [\(+- o \fIoption\fR]... [\fB-c\fR \fIstring\fR] [\fIarg\fR]...
.fi

.LP
.nf
\fB/usr/bin/jsh\fR  [\fB-acefhiknprstuvx\fR] [\fIargument\fR]...
.fi

.SH DESCRIPTION
.LP
The \fB/usr/bin/sh\fR utility is a command programming language that executes
commands read from a terminal or a file.
.sp
.LP
The \fB/usr/xpg4/bin/sh\fR utility is a standards compliant shell. This utility
provides all the functionality of \fBksh\fR(1), except in cases discussed in
\fBksh\fR(1) where differences in behavior exist.
.sp
.LP
The \fBjsh\fR utility is an interface to the shell that provides all of the
functionality of \fBsh\fR and enables job control (see \fBJob Control\fR
section below).
.sp
.LP
Arguments to the shell are listed in the \fBInvocation\fR section below.
.SS "Definitions"
.LP
A \fIblank\fR is a tab or a space. A \fIname\fR is a sequence of \fBASCII\fR
letters, digits, or underscores, beginning with a letter or an underscore. A
\fIparameter\fR is a name, a digit, or any of the characters \fB*\fR, \fB@\fR,
\fB#\fR, \fB?\fR, \fB\(mi\fR, \fB$\fR, and \fB!\fR.
.SH USAGE
.SS "Commands"
.LP
A \fIsimple-command\fR is a sequence of non-blank \fIword\fRs separated by
\fIblank\fRs. The first \fIword\fR specifies the name of the command to be
executed. Except as specified below, the remaining \fIword\fRs are passed as
arguments to the invoked command. The command name is passed as argument 0 (see
\fBexec\fR(2)). The \fIvalue\fR of a \fIsimple-command\fR is its exit status if
it terminates normally, or (octal) \fB200\fR+\fIstatus\fR if it terminates
abnormally. See \fBsignal.h\fR(3HEAD) for a list of status values.
.sp
.LP
A \fIpipeline\fR is a sequence of one or more \fIcommand\fRs separated by
\fB|\fR. The standard output of each \fIcommand\fR but the last is connected by
a \fBpipe\fR(2) to the standard input of the next \fIcommand\fR. Each
\fIcommand\fR is run as a separate process. The shell waits for the last
\fIcommand\fR to terminate. The exit status of a \fIpipeline\fR is the exit
status of the last command in the \fIpipeline\fR.
.sp
.LP
A \fIlist\fR is a sequence of one or more \fIpipeline\fRs separated by \fB;\fR,
\fB&\fR, \fB&&\fR, or \fB|\||\fR, and optionally terminated by \fB;\fR or
\fB&\fR\&. Of these four symbols, \fB;\fR and \fB&\fR have equal precedence,
which is lower than that of \fB&&\fR and \fB|\||\fR. The symbols \fB&&\fR and
\fB|\||\fR also have equal precedence. A semicolon (\fB;\fR) causes sequential
execution of the preceding \fIpipeline\fR, that is, the shell waits for the
\fIpipeline\fR to finish before executing any commands following the semicolon.
An ampersand (\fB&\fR) causes asynchronous execution of the preceding pipeline,
that is, the shell does \fBnot\fR wait for that pipeline to finish. The symbol
\fB&&\fR (\|\fB|\||\fR) causes the \fIlist\fR following it to be executed only
if the preceding pipeline returns a zero (non-zero) exit status. An arbitrary
number of newlines can appear in a \fIlist\fR, instead of semicolons, to
delimit commands.
.sp
.LP
A \fIcommand\fR is either a \fIsimple-command\fR or one of the following.
Unless otherwise stated, the value returned by a command is that of the last
\fIsimple-command\fR executed in the command.
.sp
.ne 2
.na
\fB\fBfor\fR \fIname\fR [ \fBin\fR \fIword\fR .\|.\|. ] \fBdo\fR \fIlist\fR
\fBdone\fR\fR
.ad
.sp .6
.RS 4n
Each time a \fBfor\fR command is executed, \fIname\fR is set to the next
\fIword\fR taken from the \fBin\fR \fIword\fR list. If \fBin\fR
\fIword\fR .\|.\|. is omitted, then the \fBfor\fR command executes the \fBdo\fR \fIlist\fR
once for each positional parameter that is set (see \fBParameter
Substitution\fR section below). Execution ends when there are no more words in
the list.
.RE

.sp
.ne 2
.na
\fB\fBcase\fR \fIword\fR \fBin\fR [ \fIpattern\fR [ | \fIpattern\fR ] \fB)\fR
\fIlist\fR \fB;\|;\fR ] .\|.\|.  \fBesac\fR\fR
.ad
.sp .6
.RS 4n
A \fBcase\fR command executes the \fIlist\fR associated with the first
\fIpattern\fR that matches \fIword\fR. The form of the patterns is the same as
that used for file-name generation (see \fBFile Name Generation\fR section),
except that a slash, a leading dot, or a dot immediately following a slash need
not be matched explicitly.
.RE

.sp
.LP
\fBif\fR \fIlist\fR \fB; then\fR \fIlist\fR \fBelif\fR \fIlist\fR \fB; then\fR
\fIlist\fR \fB;\fR ] .\|.\|. [ \fBelse\fR \fIlist\fR \fB;\fR ] \fBfi\fR
.sp
.LP
The \fIlist\fR following \fBif\fR is executed and, if it returns a zero exit
status, the \fIlist\fR following the first \fBthen\fR is executed. Otherwise,
the \fIlist\fR following \fBelif\fR is executed and, if its value is zero, the
\fIlist\fR following the next \fBthen\fR is executed. Failing that, the
\fBelse\fR \fIlist\fR is executed. If no \fBelse\fR \fIlist\fR or \fBthen\fR
\fIlist\fR is executed, then the \fBif\fR command returns a zero exit status.
.sp
.ne 2
.na
\fB\fBwhile\fR \fIlist\fR \fBdo\fR \fIlist\fR \fBdone\fR\fR
.ad
.RS 27n
A \fBwhile\fR command repeatedly executes the \fBwhile\fR \fIlist\fR and, if
the exit status of the last command in the list is zero, executes the \fBdo\fR
\fIlist\fR; otherwise the loop terminates. If no commands in the \fBdo\fR
\fIlist\fR are executed, then the \fBwhile\fR command returns a zero exit
status; \fBuntil\fR can be used in place of \fBwhile\fR to negate the loop
termination test.
.RE

.sp
.ne 2
.na
\fB\fB(\fR\fIlist\fR\fB)\fR\fR
.ad
.RS 27n
Execute \fIlist\fR in a sub-shell.
.RE

.sp
.ne 2
.na
\fB\fB{\fR \fIlist\fR\fB;}\fR\fR
.ad
.RS 27n
\fIlist\fR is executed in the current (that is, parent) shell. The \fB{\fR must
be followed by a space.
.RE

.sp
.ne 2
.na
\fB\fIname\fR \fB(\|) {\fR \fIlist\fR\fB;}\fR\fR
.ad
.RS 27n
Define a function which is referenced by \fIname\fR. The body of the function
is the \fIlist\fR of commands between \fB{\fR and \fB}\fR. The \fB{\fR must be
followed by a space. Execution of functions is described below (see
\fBExecution\fR section). The \fB{\fR and \fB}\fR are unnecessary if the body
of the function is a \fIcommand\fR as defined above, under \fBCommands\fR.
.RE

.sp
.LP
The following words are only recognized as the first word of a command and when
not quoted:
.sp
.LP
\fBif  then  else  elif  fi  case  esac  for  while  until  do  done  {  }\fR
.SS "Comments Lines"
.LP
A word beginning with \fB#\fR causes that word and all the following characters
up to a newline to be ignored.
.SS "Command Substitution"
.LP
The shell reads commands from the string between two grave accents (\fB``\fR)
and the standard output from these commands can be used as all or part of a
word. Trailing newlines from the standard output are removed.
.sp
.LP
No interpretation is done on the string before the string is read, except to
remove backslashes (\fB\e\fR) used to escape other characters. Backslashes can
be used to escape a grave accent (\fB`\fR) or another backslash (\fB\e\fR) and
are removed before the command string is read. Escaping grave accents allows
nested command substitution. If the command substitution lies within a pair of
double quotes (\fB" .\|.\|.\|` .\|.\|.\|` .\|.\|.\| "\fR), a backslash used to
escape a double quote (\fB\e"\fR) is removed. Otherwise, it is left intact.
.sp
.LP
If a backslash is used to escape a newline character (\fB\enewline\fR), both
the backslash and the newline are removed (see the later section on
\fBQuoting\fR). In addition, backslashes used to escape dollar signs
(\fB\e$\fR) are removed. Since no parameter substitution is done on the command
string before it is read, inserting a backslash to escape a dollar sign has no
effect. Backslashes that precede characters other than \fB\e\fR, \fB`\fR,
\fB"\fR, \fBnewline\fR, and \fB$\fR are left intact when the command string is
read.
.SS "Parameter Substitution"
.LP
The character \fB$\fR is used to introduce substitutable \fIparameter\fRs.
There are two types of parameters, positional and keyword. If \fIparameter\fR
is a digit, it is a positional parameter. Positional parameters can be assigned
values by \fBset\fR. Keyword parameters (also known as variables) can be
assigned values by writing:
.sp
.LP
\fIname\fR\fB=\fR\fIvalue\fR [ \fIname\fR\fB=\fR\fIvalue\fR ] .\|.\|.
.sp
.LP
Pattern-matching is not performed on \fIvalue\fR. There cannot be a function
and a variable with the same \fIname\fR.
.sp
.ne 2
.na
\fB\fB${\fR\fIparameter\fR\fB}\fR\fR
.ad
.RS 25n
The value, if any, of the parameter is substituted. The braces are required
only when \fIparameter\fR is followed by a letter, digit, or underscore that is
not to be interpreted as part of its name. If \fIparameter\fR is \fB*\fR or
\fB@\fR, all the positional parameters, starting with \fB$1\fR, are substituted
(separated by spaces). Parameter \fB$0\fR is set from argument zero when the
shell is invoked.
.RE

.sp
.ne 2
.na
\fB\fB${\fR\fIparameter\fR\fB:\(mi\fR\fIword\fR\fB}\fR\fR
.ad
.RS 25n
Use Default Values. If \fIparameter\fR is unset or null, the expansion of
\fIword\fR is substituted; otherwise, the value of \fIparameter\fR is
substituted.
.RE

.sp
.ne 2
.na
\fB\fB${\fR\fIparameter\fR\fB:=\fR\fIword\fR\fB}\fR\fR
.ad
.RS 25n
Assign Default Values. If \fIparameter\fR is unset or null, the expansion of
\fIword\fR is assigned to \fIparameter\fR. In all cases, the final value of
\fIparameter\fR is substituted. Only variables, not positional parameters or
special parameters, can be assigned in this way.
.RE

.sp
.ne 2
.na
\fB\fB${\fR\fIparameter\fR\fB:?\fR\fIword\fR\fB}\fR\fR
.ad
.RS 25n
If \fIparameter\fR is set and is non-null, substitute its value; otherwise,
print \fIword\fR and exit from the shell. If \fIword\fR is omitted, the message
"parameter null or not set" is printed.
.RE

.sp
.ne 2
.na
\fB\fB${\fR\fIparameter\fR\fB:+\fR\fIword\fR\fB}\fR\fR
.ad
.RS 25n
If \fIparameter\fR is set and is non-null, substitute \fIword\fR; otherwise
substitute nothing.
.RE

.sp
.LP
In the above, \fIword\fR is not evaluated unless it is to be used as the
substituted string, so that, in the following example, \fBpwd\fR is executed
only if \fBd\fR is not set or is null:
.sp
.in +2
.nf
\fBecho  ${d:\(mi`pwd`}\fR
.fi
.in -2
.sp

.sp
.LP
If the colon (\fB:\fR) is omitted from the above expressions, the shell only
checks whether \fIparameter\fR is set or not.
.sp
.LP
The following parameters are automatically set by the shell.
.sp
.ne 2
.na
\fB\fB#\fR\fR
.ad
.RS 8n
The number of positional parameters in decimal.
.RE

.sp
.ne 2
.na
\fB\fB\(mi\fR\fR
.ad
.RS 8n
Flags supplied to the shell on invocation or by the \fBset\fR command.
.RE

.sp
.ne 2
.na
\fB\fB?\fR\fR
.ad
.RS 8n
The decimal value returned by the last synchronously executed command.
.RE

.sp
.ne 2
.na
\fB\fB$\fR\fR
.ad
.RS 8n
The process number of this shell.
.RE

.sp
.ne 2
.na
\fB\fB!\fR\fR
.ad
.RS 8n
The process number of the last background command invoked.
.RE

.sp
.LP
The following parameters are used by the shell. The parameters in this section
are also referred to as environment variables.
.sp
.ne 2
.na
\fB\fBHOME\fR\fR
.ad
.RS 13n
The default argument (home directory) for the \fBcd\fR command, set to the
user's login directory by \fBlogin\fR(1) from the password file (see
\fBpasswd\fR(5)).
.RE

.sp
.ne 2
.na
\fB\fBPATH\fR\fR
.ad
.RS 13n
The search path for commands (see \fBExecution\fR section below).
.RE

.sp
.ne 2
.na
\fB\fBCDPATH\fR\fR
.ad
.RS 13n
The search path for the \fBcd\fR command.
.RE

.sp
.ne 2
.na
\fB\fBMAIL\fR\fR
.ad
.RS 13n
If this parameter is set to the name of a mail file \fIand\fR the
\fBMAILPATH\fR  parameter is not set, the shell informs the user of the arrival
of mail in the specified file.
.RE

.sp
.ne 2
.na
\fB\fBMAILCHECK\fR\fR
.ad
.RS 13n
This parameter specifies how often (in seconds) the shell checks for the
arrival of mail in the files specified by the \fBMAILPATH\fR or \fBMAIL\fR
parameters. The default value is \fB600\fR seconds (10 minutes). If set to 0,
the shell checks before each prompt.
.RE

.sp
.ne 2
.na
\fB\fBMAILPATH\fR\fR
.ad
.RS 13n
A colon-separated list of file names. If this parameter is set, the shell
informs the user of the arrival of mail in any of the specified files. Each
file name can be followed by % and a message that is e printed when the
modification time changes. The default message is, \fByou have mail\fR.
.RE

.sp
.ne 2
.na
\fB\fBPS1\fR\fR
.ad
.RS 13n
Primary prompt string, by default " $ \|".
.RE

.sp
.ne 2
.na
\fB\fBPS2\fR\fR
.ad
.RS 13n
Secondary prompt string, by default " > \|".
.RE

.sp
.ne 2
.na
\fB\fBIFS\fR\fR
.ad
.RS 13n
Internal field separators, normally \fBspace\fR, \fBtab\fR, and \fBnewline\fR
(see \fBBlank Interpretation\fR section).
.RE

.sp
.ne 2
.na
\fB\fBSHACCT\fR\fR
.ad
.RS 13n
If this parameter is set to the name of a file writable by the user, the shell
writes an accounting record in the file for each shell procedure executed.
.RE

.sp
.ne 2
.na
\fB\fBSHELL\fR\fR
.ad
.RS 13n
When the shell is invoked, it scans the environment (see \fBEnvironment\fR
section below) for this name.
.RE

.sp
.LP
See \fBenviron\fR(7) for descriptions of the following environment variables
that affect the execution of \fBsh\fR: \fBLC_CTYPE\fR and \fBLC_MESSAGES\fR.
.sp
.LP
The shell gives default values to \fBPATH\fR, \fBPS1\fR, \fBPS2\fR,
\fBMAILCHECK\fR, and \fBIFS\fR. Default values for \fBHOME\fR and \fBMAIL\fR
are set by \fBlogin\fR(1).
.SS "Blank Interpretation"
.LP
After parameter and command substitution, the results of substitution are
scanned for internal field separator characters (those found in \fBIFS\fR) and
split into distinct arguments where such characters are found. Explicit null
arguments (\fB""\fR or \fB\&''\fR) are retained. Implicit null arguments (those
resulting from \fIparameter\fRs that have no values) are removed.
.SS "Input/Output Redirection"
.LP
A command's input and output can be redirected using a special notation
interpreted by the shell. The following can appear anywhere in a
\fIsimple-command\fR or can precede or follow a \fIcommand\fR and are \fBnot\fR
passed on as arguments to the invoked command. \fBNote:\fR Parameter and
command substitution occurs before \fIword\fR or \fIdigit\fR is used.
.sp
.ne 2
.na
\fB\fB<\fR\fIword\fR\fR
.ad
.RS 16n
Use file \fIword\fR as standard input (file descriptor 0).
.RE

.sp
.ne 2
.na
\fB\fB>\fR\fIword\fR\fR
.ad
.RS 16n
Use file \fIword\fR as standard output (file descriptor 1). If the file does
not exist, it is created; otherwise, it is truncated to zero length.
.RE

.sp
.ne 2
.na
\fB\fB>>\fR\fIword\fR\fR
.ad
.RS 16n
Use file \fIword\fR as standard output. If the file exists, output is appended
to it by first seeking to the \fBEOF\fR. Otherwise, the file is created.
.RE

.sp
.ne 2
.na
\fB\fB<\|>\fR\fIword\fR\fR
.ad
.RS 16n
Open file \fIword\fR for reading and writing as standard input.
.RE

.sp
.ne 2
.na
\fB\fB<<\fR[\fB\(mi\fR]\fIword\fR\fR
.ad
.RS 16n
After parameter and command substitution is done on \fIword\fR, the shell input
is read up to the first line that literally matches the resulting \fIword\fR,
or to an \fBEOF\fR. If, however, the hyphen (\fB\(mi\fR) is appended to
\fB<<\fR:
.RS +4
.TP
1.
leading tabs are stripped from \fIword\fR before the shell input is read
(but after parameter and command substitution is done on \fIword\fR);
.RE
.RS +4
.TP
2.
leading tabs are stripped from the shell input as it is read and before each
line is compared with \fIword\fR; and
.RE
.RS +4
.TP
3.
shell input is read up to the first line that literally matches the
resulting \fIword\fR, or to an \fBEOF\fR.
.RE
If any character of \fIword\fR is quoted (see \fBQuoting\fR section later), no
additional processing is done to the shell input. If no characters of
\fIword\fR are quoted:
.RS +4
.TP
1.
parameter and command substitution occurs;
.RE
.RS +4
.TP
2.
(escaped) \fB\enewline\fRs are removed; and
.RE
.RS +4
.TP
3.
\fB\e\fR must be used to quote the characters \fB\e\fR, \fB$\fR, and
\fB`\fR.
.RE
The resulting document becomes the standard input.
.RE

.sp
.ne 2
.na
\fB\fB<&\fR\fIdigit\fR\fR
.ad
.RS 16n
Use the file associated with file descriptor \fIdigit\fR as standard input.
Similarly for the standard output using \fB>&\fR\fIdigit\fR.
.RE

.sp
.ne 2
.na
\fB\fB<&\(mi\fR\fR
.ad
.RS 16n
The standard input is closed. Similarly for the standard output using
\fB>&\(mi\fR.
.RE

.sp
.LP
If any of the above is preceded by a digit, the file descriptor which is
associated with the file is that specified by the digit (instead of the default
\fB0\fR or \fB1\fR). For example:
.sp
.in +2
.nf
\fB\&... 2>&1\fR
.fi
.in -2
.sp

.sp
.LP
associates file descriptor 2 with the file currently associated with file
descriptor 1.
.sp
.LP
The order in which redirections are specified is significant. The shell
evaluates redirections left-to-right. For example:
.sp
.in +2
.nf
\fB\&... 1>\fIxxx\fR 2>&1\fR
.fi
.in -2
.sp

.sp
.LP
first associates file descriptor 1 with file \fIxxx\fR. It associates file
descriptor 2 with the file associated with file descriptor 1 (that is,
\fIxxx\fR). If the order of redirections were reversed, file descriptor 2 would
be associated with the terminal (assuming file descriptor 1 had been) and file
descriptor 1 would be associated with file \fIxxx\fR.
.sp
.LP
Using the terminology introduced on the first page, under \fBCommands\fR, if a
\fIcommand\fR is composed of several \fIsimple commands\fR, redirection is
evaluated for the entire \fIcommand\fR before it is evaluated for each
\fIsimple command\fR. That is, the shell evaluates redirection for the entire
\fIlist\fR, then each \fIpipeline\fR within the \fIlist\fR, then each
\fIcommand\fR within each \fIpipeline\fR, then each \fIlist\fR within each
\fIcommand\fR.
.sp
.LP
If a command is followed by \fB&\fR, the default standard input for the command
is the empty file, \fB/dev/null\fR. Otherwise, the environment for the
execution of a command contains the file descriptors of the invoking shell as
modified by input/output specifications.
.SS "File Name Generation"
.LP
Before a command is executed, each command \fIword\fR is scanned for the
characters \fB*\fR, \fB?\fR, and \fB[\fR. If one of these characters appears
the word is regarded as a \fIpattern\fR. 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
\fB\&.\fR at the start of a file name or immediately following a \fB/\fR, as
well as the character \fB/\fR itself, must be matched explicitly.
.sp
.ne 2
.na
\fB\fB*\fR\fR
.ad
.RS 13n
Matches any string, including the null string.
.RE

.sp
.ne 2
.na
\fB\fB?\fR\fR
.ad
.RS 13n
Matches any single character.
.RE

.sp
.ne 2
.na
\fB\fB[\fR.\|.\|.\fB]\fR\fR
.ad
.RS 13n
Matches any one of the enclosed characters. A pair of characters separated by
\fB\(mi\fR matches any character lexically between the pair, inclusive. If the
first character following the opening \fB[\fR is a \fB!\fR, any character not
enclosed is matched.
.RE

.sp
.LP
Notice that all quoted characters (see below) must be matched explicitly in a
filename.
.SS "Quoting"
.LP
The following characters have a special meaning to the shell and cause
termination of a word unless quoted:
.sp
.LP
\fB;  &  (  )  |  ^  <  >  newline  space  tab\fR
.sp
.LP
A character can be \fIquoted\fR (that is, made to stand for itself) by
preceding it with a backslash (\fB\e\fR) or inserting it between a pair of
quote marks (\fB\|'\|'\fR or \fB""\fR). During processing, the shell can quote
certain characters to prevent them from taking on a special meaning.
Backslashes used to quote a single character are removed from the word before
the command is executed. The pair \fB\enewline\fR is removed from a word before
command and parameter substitution.
.sp
.LP
All characters enclosed between a pair of single quote marks (\fB\|'\|'\fR),
except a single quote, are quoted by the shell. Backslash has no special
meaning inside a pair of single quotes. A single quote can be quoted inside a
pair of double quote marks (for example, \fB"\|'"\fR), but a single quote can
not be quoted inside a pair of single quotes.
.sp
.LP
Inside a pair of double quote marks (\fB""\fR), parameter and command
substitution occurs and the shell quotes the results to avoid blank
interpretation and file name generation. If \fB$*\fR is within a pair of double
quotes, the positional parameters are substituted and quoted, separated by
quoted spaces (\fB"$1 \|$2\fR \|.\|.\|.\fB"\fR). However, if \fB$@\fR is within
a pair of double quotes, the positional parameters are substituted and quoted,
separated by unquoted spaces (\fB"$1"\|"$2"\fR \| .\|.\|. ). \fB\e\fR quotes
the characters \fB\e\fR, \fB`\fR, \fB,\fR (comma), and \fB$\fR. The pair
\fB\enewline\fR is removed before parameter and command substitution. If a
backslash precedes characters other than \fB\e\fR, \fB`\fR, \fB,\fR (comma),
\fB$\fR, and newline, then the backslash itself is quoted by the shell.
.SS "Prompting"
.LP
When used interactively, the shell prompts with the value of \fBPS1\fR before
reading a command. If at any time a newline is typed and further input is
needed to complete a command, the secondary prompt (that is, the value of
\fBPS2\fR) is issued.
.SS "Environment"
.LP
The \fIenvironment\fR (see \fBenviron\fR(7)) is a list of name-value pairs that
is passed to an executed program in the same way as a normal argument list. The
shell interacts with the environment in several ways. On invocation, the shell
scans the environment and creates a parameter for each name found, giving it
the corresponding value. If the user modifies the value of any of these
parameters or creates new parameters, none of these affects the environment
unless the \fBexport\fR command is used to bind the shell's parameter to the
environment (see also \fBset\fR \fB-a\fR). A parameter can be removed from the
environment with the \fBunset\fR command. The environment seen by any executed
command is thus composed of any unmodified name-value pairs originally
inherited by the shell, minus any pairs removed by \fBunset\fR, plus any
modifications or additions, all of which must be noted in \fBexport\fR
commands.
.sp
.LP
The environment for any \fIsimple-command\fR can be augmented by prefixing it
with one or more assignments to parameters. Thus:
.sp
.in +2
.nf
\fBTERM=450  \fIcommand\fR\fR
.fi
.in -2
.sp

.sp
.LP
and
.sp
.in +2
.nf
\fB(export TERM; TERM=450;   \fIcommand\fR\fR
.fi
.in -2
.sp

.sp
.LP
are equivalent as far as the execution of \fIcommand\fR is concerned if
\fIcommand\fR is not a Special Command. If \fIcommand\fR is a Special Command,
then
.sp
.in +2
.nf
\fBTERM=450   \fIcommand\fR\fR
.fi
.in -2
.sp

.sp
.LP
modifies the \fBTERM\fR variable in the current shell.
.sp
.LP
If the \fB-k\fR flag is set, \fIall\fR keyword arguments are placed in the
environment, even if they occur after the command name. The following example
first prints \fBa=b c\fR and \fBc\fR:
.sp
.in +2
.nf
\fBecho a=b  c

a=b  c

set  \(mik

echo a=b  c

c\fR
.fi
.in -2
.sp

.SS "Signals"
.LP
The \fBINTERRUPT\fR and \fBQUIT\fR signals for an invoked command are ignored
if the command is followed by \fB&\fR\&. Otherwise, signals have the values
inherited by the shell from its parent, with the exception of signal 11 (but
see also the \fBtrap\fR command below).
.SS "Execution"
.LP
Each time a command is executed, the command substitution, parameter
substitution, blank interpretation, input/output redirection, and filename
generation listed above are carried out. If the command name matches the name
of a defined function, the function is executed in the shell process (note how
this differs from the execution of shell script files, which require a
sub-shell for invocation). If the command name does not match the name of a
defined function, but matches one of the \fBSpecial Commands\fR listed below,
it is executed in the shell process.
.sp
.LP
The positional parameters \fB$1\fR, \fB$2\fR, .\|.\|. are set to the arguments
of the function. If the command name matches neither a \fBSpecial Command\fR
nor the name of a defined function, a new process is created and an attempt is
made to execute the command via \fBexec\fR(2).
.sp
.LP
The shell parameter \fBPATH\fR defines the search path for the directory
containing the command. Alternative directory names are separated by a colon
(\fB:\fR). The default path is \fB/usr/bin\fR. The current directory is
specified by a null path name, which can appear immediately after the equal
sign, between two colon delimiters anywhere in the path list, or at the end of
the path list. If the command name contains a \fB/\fR the search path is not
used. Otherwise, each directory in the path is searched for an executable file.
If the file has execute permission but is not an \fBa.out\fR file, it is
assumed to be a file containing shell commands. A sub-shell is spawned to read
it. A parenthesized command is also executed in a sub-shell.
.sp
.LP
The location in the search path where a command was found is remembered by the
shell (to help avoid unnecessary \fIexec\fRs later). If the command was found
in a relative directory, its location must be re-determined whenever the
current directory changes. The shell forgets all remembered locations whenever
the \fBPATH\fR variable is changed or the \fBhash\fR \fB-r\fR command is
executed (see below).
.SS "Special Commands"
.LP
Input/output redirection is now permitted for these commands. File descriptor 1
is the default output location. When Job Control is enabled, additional
\fBSpecial Commands\fR are added to the shell's environment (see \fBJob
Control\fR section below).
.sp
.ne 2
.na
\fB\fB:\fR\fR
.ad
.sp .6
.RS 4n
No effect; the command does nothing. A zero exit code is returned.
.RE

.sp
.ne 2
.na
\fB\fB\&.\|\fR \fIfilename\fR\fR
.ad
.sp .6
.RS 4n
Read and execute commands from \fIfilename\fR and return. The search path
specified by \fBPATH\fR is used to find the directory containing
\fIfilename\fR.
.RE

.sp
.ne 2
.na
\fB\fBbg\fR [\fB%\fR\fIjobid .\|.\|.\fR]\fR
.ad
.sp .6
.RS 4n
When Job Control is enabled, the \fBbg\fR command is added to the user's
environment to manipulate jobs. Resumes the execution of a stopped job in the
background. If \fB%\fR\fIjobid\fR is omitted the current job is assumed. (See
\fBJob Control\fR section below for more detail.)
.RE

.sp
.ne 2
.na
\fB\fBbreak\fR [ \fIn\fR ]\fR
.ad
.sp .6
.RS 4n
Exit from the enclosing \fBfor\fR or \fBwhile\fR loop, if any. If \fIn\fR is
specified, break \fIn\fR levels.
.RE

.sp
.ne 2
.na
\fB\fBcd\fR [ \fIargument\fR ]\fR
.ad
.sp .6
.RS 4n
Change the current directory to \fIargument\fR. The shell parameter \fBHOME\fR
is the default \fIargument\fR. The shell parameter \fBCDPATH\fR defines the
search path for the directory containing \fIargument\fR. Alternative directory
names are separated by a colon (\fB:\fR). The default path is \fB<null>\fR
(specifying the current directory). \fBNote:\fR The current directory is
specified by a null path name, which can appear immediately after the equal
sign or between the colon delimiters anywhere else in the path list. If
\fIargument\fR begins with a \fB/\fR the search path is not used. Otherwise,
each directory in the path is searched for \fIargument\fR.
.RE

.sp
.ne 2
.na
\fB\fBchdir\fR [ \fIdir\fR ]\fR
.ad
.sp .6
.RS 4n
\fBchdir\fR changes the shell's working directory to directory \fIdir\fR. If no
argument is given, change to the home directory of the user. If \fIdir\fR is a
relative pathname not found in the current directory, check for it in those
directories listed in the \fBCDPATH\fR variable. If \fIdir\fR is the name of a
shell variable whose value starts with a \fB/\fR, change to the directory named
by that value.
.RE

.sp
.ne 2
.na
\fB\fBcontinue\fR [ \fIn\fR ]\fR
.ad
.sp .6
.RS 4n
Resume the next iteration of the enclosing \fBfor\fR or \fBwhile\fR loop. If
\fIn\fR is specified, resume at the \fIn\fR-th enclosing loop.
.RE

.sp
.ne 2
.na
\fB\fBecho\fR [ \fIarguments\fR .\|.\|. ]\fR
.ad
.sp .6
.RS 4n
The words in \fIarguments\fR are written to the shell's standard output,
separated by space characters. See \fBecho\fR(1) for fuller usage and
description.
.RE

.sp
.ne 2
.na
\fB\fBeval\fR [ \fIargument\fR .\|.\|. ]\fR
.ad
.sp .6
.RS 4n
The arguments are read as input to the shell and the resulting command(s)
executed.
.RE

.sp
.ne 2
.na
\fB\fBexec\fR [ \fIargument\fR .\|.\|. ]\fR
.ad
.sp .6
.RS 4n
The command specified by the arguments is executed in place of this shell
without creating a new process. Input/output arguments can appear and, if no
other arguments are given, cause the shell input/output to be modified.
.RE

.sp
.ne 2
.na
\fB\fBexit\fR [ \fIn\fR ]\fR
.ad
.sp .6
.RS 4n
Causes the calling shell or shell script to exit with the exit status specified
by \fIn\fR. If \fIn\fR is omitted the exit status is that of the last command
executed (an \fBEOF\fR also causes the shell to exit.)
.RE

.sp
.ne 2
.na
\fB\fBexport\fR [ \fIname\fR .\|.\|. ]\fR
.ad
.sp .6
.RS 4n
The given \fIname\fRs are marked for automatic export to the \fIenvironment\fR
of subsequently executed commands. If no arguments are given, variable names
that have been marked for export during the current shell's execution are
listed. (Variable names exported from a parent shell are listed only if they
have been exported again during the current shell's execution.) Function names
are \fBnot\fR exported.
.RE

.sp
.ne 2
.na
\fB\fBfg\fR [\fB%\fR\fIjobid .\|.\|.\fR]\fR
.ad
.sp .6
.RS 4n
When Job Control is enabled, the \fBfg\fR command is added to the user's
environment to manipulate jobs. This command resumes the execution of a stopped
job in the foreground and also moves an executing background job into the
foreground. If \fB%\fR\fIjobid\fR is omitted, the current job is assumed. (See
\fBJob Control\fR section below for more detail.)
.RE

.sp
.ne 2
.na
\fB\fBgetopts\fR\fR
.ad
.sp .6
.RS 4n
Use in shell scripts to support command syntax standards (see \fBIntro\fR(1)).
This command parses positional parameters and checks for legal options. See
\fBgetoptcvt\fR(1) for usage and description.
.RE

.sp
.ne 2
.na
\fB\fBhash\fR [ \fB-r\fR ] [ \fIname\fR .\|.\|. ]\fR
.ad
.sp .6
.RS 4n
For each \fIname\fR, the location in the search path of the command specified
by \fIname\fR is determined and remembered by the shell. The \fB-r\fR option
causes the shell to forget all remembered locations. If no arguments are given,
information about remembered commands is presented. \fIHits\fR is the number of
times a command has been invoked by the shell process. \fICost\fR is a measure
of the work required to locate a command in the search path. If a command is
found in a "relative" directory in the search path, after changing to that
directory, the stored location of that command is recalculated. Commands for
which this are done are indicated by an asterisk (\fB*\fR) adjacent to the
\fIhits\fR information. \fICost\fR is incremented when the recalculation is
done.
.RE

.sp
.ne 2
.na
\fB\fBjobs\fR [\fB\fR\fB-p\fR\fB|\fR\fB-l\fR] [\fB%\fR\fIjobid ...\fR]\fR
.ad
.br
.na
\fB\fBjobs\fR \fB-x\fR \fIcommand\fR [\fIarguments\fR]\fR
.ad
.sp .6
.RS 4n
Reports all jobs that are stopped or executing in the background. If
\fB%\fR\fIjobid\fR is omitted, all jobs that are stopped or running in the
background are reported. (See \fBJob Control\fR section below for more detail.)
.RE

.sp
.ne 2
.na
\fB\fBkill\fR [ \fB-\fR\fIsig\fR ] \fB%\fR\fIjob\fR .\|.\|.\fR
.ad
.br
.na
\fB\fBkill\fR \fB-l\fR\fR
.ad
.sp .6
.RS 4n
Sends either the \fBTERM\fR (terminate) signal or the specified signal to the
specified jobs or processes. Signals are either given by number or by names (as
given in \fBsignal.h\fR(3HEAD) stripped of the prefix "SIG" with the exception
that \fBSIGCHD\fR is named \fBCHLD\fR). If the signal being sent is \fBTERM\fR
(terminate) or \fBHUP\fR (hangup), then the job or process is sent a \fBCONT\fR
(continue) signal if it is stopped. The argument \fIjob\fR can be the process
id of a process that is not a member of one of the active jobs. See \fBJob
Control\fR section below for a description of the format of \fIjob\fR. In the
second form, \fBkill\fR \fB-l\fR, the signal numbers and names are listed. (See
\fBkill\fR(1)).
.RE

.sp
.ne 2
.na
\fB\fBlogin\fR [ \fIargument\fR .\|.\|. ]\fR
.ad
.sp .6
.RS 4n
Equivalent to `\fBexec\fR \fBlogin\fR \fIargument\fR.\|.\|.\|.' See
\fBlogin\fR(1) for usage and description.
.RE

.sp
.ne 2
.na
\fB\fBnewgrp\fR [ \fIargument\fR ]\fR
.ad
.sp .6
.RS 4n
Equivalent to \fBexec\fR \fBnewgrp\fR \fIargument\fR. See \fBnewgrp\fR(1) for
usage and description.
.RE

.sp
.ne 2
.na
\fB\fBpwd\fR\fR
.ad
.sp .6
.RS 4n
Print the current working directory. See \fBpwd\fR(1) for usage and
description.
.RE

.sp
.ne 2
.na
\fB\fBread\fR \fIname\fR .\|.\|.\fR
.ad
.sp .6
.RS 4n
One line is read from the standard input and, using the internal field
separator, \fBIFS\fR (normally space or tab), to delimit word boundaries, the
first word is assigned to the first \fIname\fR, the second word to the second
\fIname\fR, and so forth, with leftover words assigned to the last \fIname\fR.
Lines can be continued using \fB\enewline\fR\&. Characters other than
\fBnewline\fR can be quoted by preceding them with a backslash. These
backslashes are removed before words are assigned to \fInames\fR, and no
interpretation is done on the character that follows the backslash. The return
code is \fB0\fR, unless an \fBEOF\fR is encountered.
.RE

.sp
.ne 2
.na
\fB\fBreadonly\fR [ \fIname\fR .\|.\|. ]\fR
.ad
.sp .6
.RS 4n
The given \fIname\fRs are marked \fBreadonly\fR and the values of these
\fIname\fRs can not be changed by subsequent assignment. If no arguments are
given, a list of all \fBreadonly\fR names is printed.
.RE

.sp
.ne 2
.na
\fB\fBreturn\fR [ \fIn\fR ]\fR
.ad
.sp .6
.RS 4n
Causes a function to exit with the return value specified by \fIn\fR. If
\fIn\fR is omitted, the return status is that of the last command executed.
.RE

.sp
.ne 2
.na
\fB\fBset\fR [ \fB-aefhkntuvx\fR [ \fIargument\fR .\|.\|. ] ]\fR
.ad
.sp .6
.RS 4n
.sp
.ne 2
.na
\fB\fB-a\fR\fR
.ad
.RS 6n
Mark variables which are modified or created for export.
.RE

.sp
.ne 2
.na
\fB\fB-e\fR\fR
.ad
.RS 6n
Exit immediately if a command exits with a non-zero exit status.
.RE

.sp
.ne 2
.na
\fB\fB-f\fR\fR
.ad
.RS 6n
Disable file name generation.
.RE

.sp
.ne 2
.na
\fB\fB-h\fR\fR
.ad
.RS 6n
Locate and remember function commands as functions are defined (function
commands are normally located when the function is executed).
.RE

.sp
.ne 2
.na
\fB\fB-k\fR\fR
.ad
.RS 6n
All keyword arguments are placed in the environment for a command, not just
those that precede the command name.
.RE

.sp
.ne 2
.na
\fB\fB-n\fR\fR
.ad
.RS 6n
Read commands but do not execute them.
.RE

.sp
.ne 2
.na
\fB\fB-t\fR\fR
.ad
.RS 6n
Exit after reading and executing one command.
.RE

.sp
.ne 2
.na
\fB\fB-u\fR\fR
.ad
.RS 6n
Treat unset variables as an error when substituting.
.RE

.sp
.ne 2
.na
\fB\fB-v\fR\fR
.ad
.RS 6n
Print shell input lines as they are read.
.RE

.sp
.ne 2
.na
\fB\fB-x\fR\fR
.ad
.RS 6n
Print commands and their arguments as they are executed.
.RE

.sp
.ne 2
.na
\fB\fB-\fR\fR
.ad
.RS 6n
Do not change any of the flags; useful in setting \fB$1\fR to \fB\(mi\fR\&.
.RE

Using \fB+\fR rather than \fB\(mi\fR causes these flags to be turned off. These
flags can also be used upon invocation of the shell. The current set of flags
can be found in \fB$\(mi\fR. The remaining arguments are positional parameters
and are assigned, in order, to \fB$1\fR, \fB$2\fR, .\|.\|. If no arguments are
given, the values of all names are printed.
.RE

.sp
.ne 2
.na
\fB\fBshift\fR [ \fIn\fR ]\fR
.ad
.sp .6
.RS 4n
The positional parameters from \fB$\fR\fIn\fR\fB+1\fR .\|.\|. are renamed
\fB$1\fR .\|.\|. . If \fIn\fR is not given, it is assumed to be 1.
.RE

.sp
.ne 2
.na
\fB\fBstop\fR \fIpid .\|.\|.\fR\fR
.ad
.sp .6
.RS 4n
Halt execution of the process number \fIpid\fR. (see \fBps\fR(1)).
.RE

.sp
.ne 2
.na
\fB\fBsuspend\fR\fR
.ad
.sp .6
.RS 4n
Stops the execution of the current shell (but not if it is the login shell).
.RE

.sp
.ne 2
.na
\fB\fBtest\fR\fR
.ad
.sp .6
.RS 4n
Evaluate conditional expressions. See \fBtest\fR(1) for usage and description.
.RE

.sp
.ne 2
.na
\fB\fBtimes\fR\fR
.ad
.sp .6
.RS 4n
Print the accumulated user and system times for processes run from the shell.
.RE

.sp
.ne 2
.na
\fB\fBtrap\fR [ \fIargument\fR \fIn\fR [ \fIn2\fR .\|.\|. ]]\fR
.ad
.sp .6
.RS 4n
The command \fIargument\fR is to be read and executed when the shell receives
numeric or symbolic signal(s) (\fIn\fR). (\fBNote:\fR \fIargument\fR is scanned
once when the trap is set and once when the trap is taken.) Trap commands are
executed in order of signal number or corresponding symbolic names. Any attempt
to set a trap on a signal that was ignored on entry to the current shell is
ineffective. An attempt to trap on signal 11 (memory fault) produces an error.
If \fIargument\fR is absent, all trap(s) \fIn\fR are reset to their original
values. If \fIargument\fR is the null string, this signal is ignored by the
shell and by the commands it invokes. If \fIn\fR is 0, the command
\fIargument\fR is executed on exit from the shell. The \fBtrap\fR command with
no arguments prints a list of commands associated with each signal number.
.RE

.sp
.ne 2
.na
\fB\fBtype\fR [ \fIname\fR .\|.\|. ]\fR
.ad
.sp .6
.RS 4n
For each \fIname\fR, indicate how it would be interpreted if used as a command
name.
.RE

.sp
.ne 2
.na
\fB\fBulimit\fR [ [\fB-HS\fR] [\fB-a\fR | \fB-cdfnstv\fR] ]\fR
.ad
.br
.na
\fB\fBulimit\fR [ [\fB-HS\fR] [\fB-c\fR | \fB-d\fR | \fB-f\fR | \fB-n\fR |
\fB-s\fR | \fB-t\fR | \fB-v\fR] ] \fBlimit\fR\fR
.ad
.sp .6
.RS 4n
\fBulimit\fR prints or sets hard or soft resource limits. These limits are
described in \fBgetrlimit\fR(2).
.sp
If \fIlimit\fR is not present, \fBulimit\fR prints the specified limits. Any
number of limits can be printed at one time. The \fB-a\fR option prints all
limits.
.sp
If \fIlimit\fR is present, \fBulimit\fR sets the specified limit to
\fIlimit\fR. The string \fBunlimited\fR requests that the current limit, if
any, be removed. Any user can set a soft limit  to any value less than or equal
to  the hard limit. Any user can lower a hard limit.  Only a  user with
appropriate privileges can raise or remove a hard limit.  See
\fBgetrlimit\fR(2).
.sp
The \fB-H\fR option specifies a hard limit. The \fB-S\fR option specifies a
soft limit. If neither option is specified, \fBulimit\fR sets both limits and
print the soft limit.
.sp
The following options specify the resource whose limits are to be printed or
set. If no option is specified, the file size limit is printed or set.
.sp
.ne 2
.na
\fB\fB-c\fR\fR
.ad
.RS 6n
maximum core file size (in 512-byte blocks)
.RE

.sp
.ne 2
.na
\fB\fB-d\fR\fR
.ad
.RS 6n
maximum size of data segment or heap (in kbytes)
.RE

.sp
.ne 2
.na
\fB\fB-f\fR\fR
.ad
.RS 6n
maximum file size (in 512-byte blocks)
.RE

.sp
.ne 2
.na
\fB\fB-n\fR\fR
.ad
.RS 6n
maximum file descriptor plus 1
.RE

.sp
.ne 2
.na
\fB\fB-s\fR\fR
.ad
.RS 6n
maximum size of stack segment (in kbytes)
.RE

.sp
.ne 2
.na
\fB\fB-t\fR\fR
.ad
.RS 6n
maximum CPU time (in seconds)
.RE

.sp
.ne 2
.na
\fB\fB-v\fR\fR
.ad
.RS 6n
maximum size of virtual memory (in kbytes)
.RE

Run the \fBsysdef\fR(8) command to obtain the maximum possible limits for your
system. The values reported are in hexadecimal, but can be translated into
decimal numbers using the \fBbc\fR(1) utility. See \fBswap\fR(8).)
.sp
As an example of \fBulimit\fR, to limit the size of a core file dump to 0
Megabytes, type the following:
.sp
.in +2
.nf
\fBulimit -c 0\fR
.fi
.in -2
.sp

.RE

.sp
.ne 2
.na
\fB\fBumask\fR [ \fInnn\fR ]\fR
.ad
.sp .6
.RS 4n
The user file-creation mask is set to \fInnn\fR (see \fBumask\fR(1)). If
\fInnn\fR is omitted, the current value of the mask is printed.
.RE

.sp
.ne 2
.na
\fB\fBunset\fR [ \fIname\fR .\|.\|. ]\fR
.ad
.sp .6
.RS 4n
For each \fIname\fR, remove the corresponding variable or function value. The
variables \fBPATH\fR, \fBPS1\fR, \fBPS2\fR, \fBMAILCHECK\fR, and \fBIFS\fR
cannot be unset.
.RE

.sp
.ne 2
.na
\fB\fBwait\fR [ \fIn\fR ]\fR
.ad
.sp .6
.RS 4n
Wait for your background process whose process id is \fIn\fR and report its
termination status. If \fIn\fR is omitted, all your shell's currently active
background processes are waited for and the return code is zero.
.RE

.SS "Invocation"
.LP
If the shell is invoked through \fBexec\fR(2) and the first character of
argument zero is \fB\(mi\fR, commands are initially read from
\fB/etc/profile\fR and from \fB$HOME/.profile\fR, if such files exist.
Thereafter, commands are read as described below, which is also the case when
the shell is invoked as \fB/usr/bin/sh\fR. The flags below are interpreted by
the shell on invocation only. \fBNote:\fR Unless the \fB-c\fR or \fB-s\fR flag
is specified, the first argument is assumed to be the name of a file containing
commands, and the remaining arguments are passed as positional parameters to
that command file:
.sp
.ne 2
.na
\fB\fB-c\fR\fI\| string\fR\fR
.ad
.RS 15n
If the \fB-c\fR flag is present commands are read from \fIstring\fR.
.RE

.sp
.ne 2
.na
\fB\fB-i\fR\fR
.ad
.RS 15n
If the \fB-i\fR flag is present or if the shell input and output are attached
to a terminal, this shell is \fBinteractive\fR. In this case, TERMINATE is
ignored (so that \fBkill 0\fR does not kill an interactive shell) and INTERRUPT
is caught and ignored (so that \fBwait\fR is interruptible). In all cases, QUIT
is ignored by the shell.
.RE

.sp
.ne 2
.na
\fB\fB-p\fR\fR
.ad
.RS 15n
If the \fB-p\fR flag is present, the shell does not set the effective user and
group IDs to the real user and group IDs.
.RE

.sp
.ne 2
.na
\fB\fB-r\fR\fR
.ad
.RS 15n
If the \fB-r\fR flag is present the shell is a restricted shell (see
\fBrsh\fR(8)).
.RE

.sp
.ne 2
.na
\fB\fB-s\fR\fR
.ad
.RS 15n
If the \fB-s\fR flag is present or if no arguments remain, commands are read
from the standard input. Any remaining arguments specify the positional
parameters. Shell output (except for \fBSpecial Commands\fR) is written to file
descriptor 2.
.RE

.sp
.LP
The remaining flags and arguments are described under the \fBset\fR command
above.
.SS "Job Control (jsh)"
.LP
When the shell is invoked as \fBjsh\fR, Job Control is enabled in addition to
all of the functionality described previously for \fBsh\fR. Typically, Job
Control is enabled for the interactive shell only. Non-interactive shells
typically do not benefit from the added functionality of Job Control.
.sp
.LP
With Job Control enabled, every command or pipeline the user enters at the
terminal is called a \fIjob\fR. All jobs exist in one of the following states:
foreground, background, or stopped. These terms are defined as follows:
.RS +4
.TP
1.
A job in the foreground has read and write access to the controlling
terminal.
.RE
.RS +4
.TP
2.
A job in the background is denied read access and has conditional write
access to the controlling terminal (see \fBstty\fR(1)).
.RE
.RS +4
.TP
3.
A stopped job is a job that has been placed in a suspended state, usually as
a result of a \fBSIGTSTP\fR signal (see \fBsignal.h\fR(3HEAD)).
.RE
.sp
.LP
Every job that the shell starts is assigned a positive integer, called a \fIjob
number\fR which is tracked by the shell and is used as an identifier to
indicate a specific job. Additionally, the shell keeps track of the
\fIcurrent\fR and \fIprevious\fR jobs. The \fIcurrent job\fR is the most recent
job to be started or restarted. The \fIprevious job\fR is the first non-current
job.
.sp
.LP
The acceptable syntax for a Job Identifier is of the form:
.sp
.LP
\fB%\fR\fIjobid\fR
.sp
.LP
where \fIjobid\fR can be specified in any of the following formats:
.sp
.ne 2
.na
\fB\fB%\fR or \fB+\fR\fR
.ad
.RS 13n
For the current job.
.RE

.sp
.ne 2
.na
\fB\fB\(mi\fR\fR
.ad
.RS 13n
For the previous job.
.RE

.sp
.ne 2
.na
\fB\fB?\fR\fI<string>\fR\fR
.ad
.RS 13n
Specify the job for which the command line uniquely contains \fIstring\fR.
.RE

.sp
.ne 2
.na
\fB\fIn\fR\fR
.ad
.RS 13n
For job number \fIn\fR.
.RE

.sp
.ne 2
.na
\fB\fIpref\fR\fR
.ad
.RS 13n
Where \fIpref\fR is a unique prefix of the command name. For example, if the
command \fBls\fR \fB-l\fR \fIname\fR were running in the background, it could
be referred to as \fB%ls\fR. \fIpref\fR cannot contain blanks unless it is
quoted.
.RE

.sp
.LP
When Job Control is enabled, the following commands are added to the user's
environment to manipulate jobs:
.sp
.ne 2
.na
\fB\fBbg\fR [\fB%\fR\fIjobid .\|.\|.\fR]\fR
.ad
.sp .6
.RS 4n
Resumes the execution of a stopped job in the background. If \fB%\fR\fIjobid\fR
is omitted the current job is assumed.
.RE

.sp
.ne 2
.na
\fB\fBfg\fR [\fB%\fR\fIjobid .\|.\|.\fR]\fR
.ad
.sp .6
.RS 4n
Resumes the execution of a stopped job in the foreground, also moves an
executing background job into the foreground. If \fB%\fR\fIjobid\fR is omitted
the current job is assumed.
.RE

.sp
.ne 2
.na
\fB\fBjobs\fR [\fB-p\fR|\fB-l\fR] [\fB%\fR\fIjobid .\|.\|.\fR]\fR
.ad
.br
.na
\fB\fBjobs\fR \fB-x\fR \fBcommand\fR [\fIarguments\fR]\fR
.ad
.sp .6
.RS 4n
Reports all jobs that are stopped or executing in the background. If
\fB%\fR\fIjobid\fR is omitted, all jobs that are stopped or running in the
background is reported. The following options modify/enhance the output of
\fBjobs\fR:
.sp
.ne 2
.na
\fB\fB-l\fR\fR
.ad
.RS 6n
Report the process group ID and working directory of the jobs.
.RE

.sp
.ne 2
.na
\fB\fB-p\fR\fR
.ad
.RS 6n
Report only the process group ID of the jobs.
.RE

.sp
.ne 2
.na
\fB\fB-x\fR\fR
.ad
.RS 6n
Replace any \fIjobid\fR found in \fIcommand\fR or \fIarguments\fR with the
corresponding process group ID, and then execute \fIcommand\fR passing it
\fIarguments\fR.
.RE

.RE

.sp
.ne 2
.na
\fB\fBkill\fR [ \fB-signal\fR ] \fB%\fR\fIjobid\fR\fR
.ad
.sp .6
.RS 4n
Builtin version of \fBkill\fR to provide the functionality of the \fBkill\fR
command for processes identified with a \fIjobid\fR.
.RE

.sp
.ne 2
.na
\fB\fBstop\fR \fB%\fR\fIjobid .\|.\|.\fR\fR
.ad
.sp .6
.RS 4n
Stops the execution of a background job(s).
.RE

.sp
.ne 2
.na
\fB\fBsuspend\fR\fR
.ad
.sp .6
.RS 4n
Stops the execution of the current shell (but not if it is the login shell).
.RE

.sp
.ne 2
.na
\fB\fBwait\fR [\fB%\fR\fIjobid .\|.\|.\fR]\fR
.ad
.sp .6
.RS 4n
\fBwait\fR builtin accepts a job identifier. If \fB%\fR\fIjobid\fR is omitted
\fBwait\fR behaves as described above under \fBSpecial Commands\fR.
.RE

.SS "Large File Behavior"
.LP
See \fBlargefile\fR(7) for the description of the behavior of \fBsh\fR and
\fBjsh\fR when encountering files greater than or equal to 2 Gbyte ( 2^31
bytes).
.SH EXIT STATUS
.LP
Errors detected by the shell, such as syntax errors, cause the shell to return
a non-zero exit status. If the shell is being used non-interactively execution
of the shell file is abandoned. Otherwise, the shell returns the exit status of
the last command executed (see also the \fBexit\fR command above).
.SS "jsh Only"
.LP
If the shell is invoked as \fBjsh\fR and an attempt is made to exit the shell
while there are stopped jobs, the shell issues one warning:
.sp
.LP
\fBThere are stopped jobs.\fR
.sp
.LP
This is the only message. If another exit attempt is made, and there are still
stopped jobs they are sent a \fBSIGHUP\fR signal from the kernel and the shell
is exited.
.SH FILES
.LP
\fB$HOME/.profile\fR
.sp
.LP
\fB/dev/null\fR
.sp
.LP
\fB/etc/profile\fR
.sp
.LP
\fB/tmp/sh*\fR
.SH ATTRIBUTES
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.SS "/usr/bin/sh, /usr/bin/jsh"
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
CSI	Enabled
.TE

.SS "/usr/xpg4/bin/sh"
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
CSI	Enabled
.TE

.SH SEE ALSO
.LP
.BR Intro (1),
.BR bc (1),
.BR echo (1),
.BR getoptcvt (1),
.BR kill (1),
.BR ksh (1),
.BR login (1),
.BR newgrp (1),
.BR pfexec (1),
.BR pfsh (1),
.BR ps (1),
.BR pwd (1),
.BR set (1),
.BR shell_builtins (1),
.BR stty (1),
.BR test (1),
.BR umask (1),
.BR wait (1),
.BR dup (2),
.BR exec (2),
.BR fork (2),
.BR getrlimit (2),
.BR pipe (2),
.BR ulimit (2),
.BR setlocale (3C),
.BR signal.h (3HEAD),
.BR passwd (5),
.BR profile (5),
.BR XPG4 (7),
.BR attributes (7),
.BR environ (7),
.BR largefile (7),
.BR rsh (8),
.BR su (8),
.BR swap (8),
.BR sysdef (8)
.SH WARNINGS
.LP
The use of \fBsetuid\fR shell scripts is \fBstrongly\fR discouraged.
.SH NOTES
.LP
Words used for filenames in input/output redirection are not interpreted for
filename generation (see \fBFile Name Generation\fR section above). For
example, \fBcat file1 >a*\fR createsa file named \fBa*\fR.
.sp
.LP
Because commands in pipelines are run as separate processes, variables set in a
pipeline have no effect on the parent shell.
.sp
.LP
If the input or the output of a \fBwhile\fR or \fBuntil\fR loop is redirected,
the commands in the loop are run in a sub-shell, and variables set or changed
there have no effect on the parent process:
.sp
.in +2
.nf
   lastline=
   while read line
   do

           lastline=$line
   done < /etc/passwd
   echo "lastline=$lastline"       # lastline is empty!
.fi
.in -2
.sp

.sp
.LP
In these cases, the input or output can be redirected by using \fBexec\fR, as
in the following example:
.sp
.in +2
.nf
   # Save standard input (file descriptor 0) as file
   # descriptor 3, and redirect standard input from the file
   /etc/passwd:

   exec 3<&0               # save standard input as fd 3
   exec </etc/passwd       # redirect input from file

   lastline=
   while read line
   do
           lastline=$line
   done

   exec 0<&3               # restore standard input
   exec 3<&-               # close file descriptor 3
   echo "$lastline"        # lastline
.fi
.in -2
.sp

.sp
.LP
If you get the error message, "\fBcannot fork, too many processes\fR", try
using the \fBwait\fR(1) command to clean up your background processes. If this
doesn't help, the system process table is probably full or you have too many
active foreground processes. There is a limit to the number of process ids
associated with your login, and to the number the system can keep track of.
.sp
.LP
Only the last process in a pipeline can be waited for.
.sp
.LP
If a command is executed, and a command with the same name is installed in a
directory in the search path before the directory where the original command
was found, the shell continues to \fBexec\fR the original command. Use the
\fBhash\fR command to correct this situation.
.sp
.LP
The Bourne shell has a limitation on the effective \fBUID\fR for a process. If
this \fBUID\fR is less than 100 (and not equal to the real UID of the process),
then the \fBUID\fR is reset to the real UID of the process.
.sp
.LP
Because the shell implements both foreground and background jobs in the same
process group, they all receive the same signals, which can lead to unexpected
behavior. It is, therefore, recommended that other job control shells be used,
especially in an interactive environment.
.sp
.LP
When the shell executes a shell script that attempts to execute a non-existent
command interpreter, the shell returns an erroneous diagnostic message that the
shell script file does not exist.