SH(1) User Commands SH(1)NAME
sh, jsh - the standard command interpreter
SYNOPSIS
sh [-acefhikmnprstuvx] [arg] ...
jsh [-acefhikmnprstuvx] [arg] ...
DESCRIPTION
Sh is a command programming language that executes commands read from a
terminal or a file. See the invocation section for the meaning of
arguments to the shell.
Commands
A simple-command is a sequence of non blank words separated by blanks
(a blank is a tab or a space). 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 (see exec(2)). The value of a simple-command
is its exit status if it terminates normally or 200+status if it termi‐
nates abnormally (see signal(2) for a list of status values).
A pipeline is a sequence of one or more commands separated by |. The
standard output of each command but the last is connected by a 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.
The value of a pipeline is the exit status of its last command.
A list is a sequence of one or more pipelines separated by ;, &, && or
|| and optionally terminated by ; or &. ; and & have equal precedence
which is lower than that of && and ||, && and || also have equal prece‐
dence. A semicolon causes sequential execution; an ampersand causes
the preceding pipeline to be executed without waiting for it to finish.
The symbol && (||) causes the list following to be executed only if the
preceding pipeline returns a zero (non zero) value. Newlines may
appear in a list, instead of semicolons, to delimit commands.
A # at the beginning of a word starts a comment and causes the rest of
the line to be ignored.
A 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.
for name [in word ...] do list done
Each time a for command is executed name is set to the next word
in the for word list If in word ... is omitted then in "$@" is
assumed. Execution ends when there are no more words in the
list.
case word in [pattern [| pattern ] ... ) list ;;] ... esac
A case command executes the list associated with the first pat‐
tern that matches word. The form of the patterns is the same as
that used for file name generation.
if list then list [elif list then list] ... [else list] fi
The list following if is executed and if it returns zero the
list following then is executed. Otherwise, the list following
elif is executed and if its value is zero the list following
then is executed. Failing that the else list is executed.
while list [do list] done
A while command repeatedly executes the while list and if its
value is zero executes the do list; otherwise the loop termi‐
nates. The value returned by a while command is that of the
last executed command in the do list. until may be used in
place of while to negate the loop termination test.
( list )
Execute list in a subshell.
{ list; }
list is simply executed.
name() { list; }
Defines the shell function name. Each time when name is recog‐
nized as a command, list is executed, with the positional param‐
eters $1, $2... set to the arguments of the command. After the
function returns, the previous positional parameters are
restored.
The following words are only recognized as the first word of a command
and when not quoted.
if then else elif fi case in esac for while until do done { }
Command substitution
The standard output from a command enclosed in a pair of grave accents
(``) may be used as part or all of a word; trailing newlines are
removed.
Parameter substitution
The character $ is used to introduce substitutable parameters. Posi‐
tional parameters may be assigned values by set. Variables may be set
by writing
name=value [ name=value ] ...
${parameter}
A parameter is a sequence of letters, digits or underscores (a
name), a digit, or any of the characters * @ # ? - $ !. The
value, if any, of the parameter is substituted. The braces are
required only when parameter is followed by a letter, digit, or
underscore that is not to be interpreted as part of its name.
If parameter is a digit then it is a positional parameter. If
parameter is * or @ then all the positional parameters, starting
with $1, are substituted separated by spaces. $0 is set from
argument zero when the shell is invoked.
${parameter:-word}
If parameter is set and not empty then substitute its value;
otherwise substitute word.
${parameter:=word}
If parameter is not set and not empty then set it to word; the
value of the parameter is then substituted. Positional parame‐
ters may not be assigned to in this way.
${parameter:?word}
If parameter is set and not empty then substitute its value;
otherwise, print word and exit from the shell. If word is omit‐
ted then a standard message is printed.
${parameter:+word}
If parameter is set and not empty then substitute word; other‐
wise substitute nothing.
If the : is omitted, the substitutions are only executed if the parame‐
ter is set, even if it is empty.
In the above word is not evaluated unless it is to be used as the sub‐
stituted string. (So that, for example, echo ${d-`pwd`} will only exe‐
cute pwd if d is unset.)
The following parameters are automatically set by the shell.
# The number of positional parameters in decimal.
- Options supplied to the shell on invocation or by set.
? The value returned by the last executed command in deci‐
mal.
$ The process number of this shell.
! The process number of the last background command
invoked.
The following parameters are used by the shell:
CDPATH The search path for the cd command (see above).
HOME The default argument (home directory) for the cd command.
OPTARG The value of the last option argument processed by the
getopts special command.
OPTIND The index of the last option processed by the getopts
special command.
PATH The search path for commands (see execution).
MAIL If this variable is set to the name of a mail file then
the shell informs the user of the arrival of mail in the
specified file.
MAILCHECK
If this variable is set, it is interpreted as a value in
seconds to wait between checks for new mail. The default
is 600 (10 minutes). If the value is zero, mail is
checked before each prompt.
MAILPATH
A colon-separated list of files that are checked for new
mail. MAIL is ignored if this variable is set.
PS1 Primary prompt string, by default `$ '.
PS2 Secondary prompt string, by default `> '.
IFS Internal field separators, normally space, tab, and new‐
line.
LANG, LC_ALL
See locale(7).
LC_CTYPE
Affects the mapping of bytes to characters for file name
generation, for the interpretation of `\', and for han‐
dling $IFS.
SHACCT If this variable is set in the initial environment passed
to the shell and points to a file writable by the user,
accounting statistics are written to it.
TIMEOUT
The shell exists when prompting for input if no command
is entered for more than the given value in seconds. A
value of zero means no timeout and is the default.
Blank interpretation
After parameter and command substitution, any results of substitution
are scanned for internal field separator characters (those found in
$IFS) and split into distinct arguments where such characters are
found. Explicit null arguments ("" or ´´) are retained. Implicit null
arguments (those resulting from parameters that have no values) are
removed.
File name generation
Following substitution, each command word is scanned for the characters
*, ? and [. If one of these characters appears then the word is
regarded as a pattern. The word is replaced with alphabetically sorted
file names that match the pattern. If no file name is found that
matches the pattern then the word is left unchanged. The character .
at the start of a file name or immediately following a /, and the char‐
acter /, must be matched explicitly.
* Matches any string, including the null string.
? Matches any single character.
[...] Matches any one of the characters enclosed. A pair of charac‐
ters separated by - matches any character lexically between the
pair.
[!...] Matches any character except the enclosed ones.
Quoting
The following characters have a special meaning to the shell and cause
termination of a word unless quoted.
; & ( ) | ^ < > newline space tab
A character may be quoted by preceding it with a \. \newline is
ignored. All characters enclosed between a pair of quote marks (´´),
except a single quote, are quoted. Inside double quotes ("") parameter
and command substitution occurs and \ quotes the characters \ ` " and
$.
"$*" is equivalent to "$1 $2 ..." whereas
"$@" is equivalent to "$1" "$2" ... .
Prompting
When used interactively, the shell prompts with the value of $PS1
before reading a command. If at any time a newline is typed and fur‐
ther input is needed to complete a command then the secondary prompt
($PS2) is issued.
Input and 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 command
and are not passed on to the invoked command. Substitution occurs
before word or digit is used.
<word Use file word as standard input (file descriptor 0).
>word Use file word as standard output (file descriptor 1). If the
file does not exist then it is created; otherwise it is trun‐
cated to zero length.
>>word Use file word as standard output. If the file exists then out‐
put is appended (by seeking to the end); otherwise the file is
created.
<<[-]word
The shell input is read up to a line the same as word, or end of
file. The resulting document becomes the standard input. If
any character of word is quoted then no interpretation is placed
upon the characters of the document; otherwise, parameter and
command substitution occurs, \newline is ignored, and \ is used
to quote the characters \ $ ` and the first character of word.
The optional - causes leading tabulator character to be stripped
from the resulting document; word may then also be prefixed by a
tabulator.
<&digit
The standard input is duplicated from file descriptor digit; see
dup(2). Similarly for the standard output using >.
<&- The standard input is closed. Similarly for the standard output
using >.
If one of the above is preceded by a digit then the file descriptor
created is that specified by the digit (instead of the default 0 or 1).
For example,
... 2>&1
creates file descriptor 2 to be a duplicate of file descriptor 1.
If a command is followed by & then the default standard input for the
command is the empty file (/dev/null), unless job control is enabled.
Otherwise, the environment for the execution of a command contains the
file descriptors of the invoking shell as modified by input output
specifications.
Environment
The environment is a list of name-value pairs that is passed to an exe‐
cuted program in the same way as a normal argument list; see exec(2)
and environ(5). 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. Exe‐
cuted commands inherit the same environment. If the user modifies the
values of these parameters or creates new ones, none of these affects
the environment unless the export command is used to bind the shell's
parameter to the environment. The environment seen by any executed
command is thus composed of any unmodified name-value pairs originally
inherited by the shell, plus any modifications or additions, all of
which must be noted in export commands.
The environment for any simple-command may be augmented by prefixing it
with one or more assignments to parameters. Thus these two lines are
equivalent
TERM=450 cmd args
(export TERM; TERM=450; cmd args)
If the -k flag is set, all keyword arguments are placed in the environ‐
ment, even if the occur after the command name. The following prints
`a=b c' and `c':
echo a=b c
set -k
echo a=b c
Signals
The INTERRUPT and QUIT signals for an invoked command are ignored if
the command is followed by & (unless job control is enabled); otherwise
signals have the values inherited by the shell from its parent. (But
see also trap.)
Execution
Each time a command is executed the above substitutions are carried
out. The shell then first looks if a function with the command name
was defined; if so, it is chosen for execution. Otherwise, except for
the `special commands' listed below a new process is created and an
attempt is made to execute the command via an exec(2).
The shell parameter $PATH defines the search path for the directory
containing the command. Each alternative directory name is separated
by a colon (:). The default path is `/usr/5bin:/bin:/usr/bin:'. If
the command name contains a / then the search path is not used. Other‐
wise, each directory in the path is searched for an executable file.
If the file has execute permission but is not an a.out file, it is
assumed to be a file containing shell commands. A subshell (i.e., a
separate process) is spawned to read it. A parenthesized command is
also executed in a subshell.
Special commands
The following commands are executed in the shell process itself:
: No effect; the command does nothing.
. file Read and execute commands from file and return. The search path
$PATH is used to find the directory containing file.
break [n]
Exit from the enclosing for or while loop, if any. If n is
specified then break n levels.
continue [n]
Resume the next iteration of the enclosing for or while loop.
If n is specified then resume at the n-th enclosing loop.
cd [arg]
Change the current directory to arg. The shell parameter $HOME
is the default arg. If no directory arg is found and the
$CDPATH parameter contains a list of directories separated by
colons, each of these directories is used as a prefix to arg in
the given order, and the current directory is set to the first
one that is found.
echo [arg ...]
Each arg is printed to standard output; afterwards, a newline is
printed. The following escapes sequences are recognized in arg:
\b Prints a backspace character.
\c Causes the command to return immediately. Any following
characters are ignored, and the terminating newline is
not printed.
\f Prints a formfeed character.
\n Prints a newline character.
\r Prints a carriage-return character.
\t Prints a tabulator character.
\v Prints a vertical tabulator character.
\\ Prints a backslash character.
\0nnn Prints the character (byte) with octal value nnn.
If /usr/ucb precedes /usr/5bin or /usr/bin in the current set‐
ting of the $PATH variable and the first argument is -n, the
terminating newline is not printed, and no escape sequences are
recognized. If the $SYSV3 variable is set in the initial envi‐
ronment passed to the shell, the -n argument is also inter‐
preted, but escape sequences are processed as usual.
eval [arg ...]
The arguments are read as input to the shell and the resulting
command(s) executed.
exec [arg ...]
The command specified by the arguments is executed in place of
this shell without creating a new process. Input output argu‐
ments may appear and if no other arguments are given cause the
shell input output to be modified.
exit [n]
Causes the shell to exit with the exit status specified by n.
If n is omitted then the exit status is that of the last command
executed. (An end of file will also exit from the shell.)
export [name ...]
The given names are marked for automatic export to the environ‐
ment of subsequently-executed commands. If no arguments are
given then a list of exportable names is printed.
getopts optstring variable [arg ...]
Retrieves options and option-arguments from arg (or the posi‐
tional parameters) similar to getopt(3). optstring is a list of
characters (bytes); each character represents an option letter.
A character followed by : indicates that the option has an argu‐
ment. Calling getopts repeatedly causes one option to be
retrieved per call. The index of the current option is stored
in the variable OPTIND; it is initialized to 1 when the shell
starts. The option-argument, if any, is stored in the OPTARG
variable. The option character is stored in the variable named.
When the end of the options is reached, getopts returns with a
non-zero value. A missing argument or an illegal option also
causes a non-zero return value, and an error message is printed
to standard error.
hash [name ...]
The shell maintains a hash table of the locations of external
commands. If name arguments are given, each one is looked up
and is inserted into the table if it is found. Otherwise, a
list of the commands currently in the table is printed.
newgrp [arg ...]
Equivalent to `exec newgrp arg ...'.
pwd Prints the name of the current directory.
read [-r] name ...
One line is read from the standard input; successive words of
the input are assigned to the variables name in order, with
leftover words to the last variable. The return code is 0
unless the end-of-file is encountered. Normally, backslashes in
the line read escape the following character; this is inhibited
if the -r option is given.
readonly [name ...]
The given names are marked readonly and the values of the these
names may not be changed by subsequent assignment. If no argu‐
ments are given then a list of all readonly names is printed.
return [n]
Return from a shell function to the execution level above. With
the argument n, the special variable $? is set to the given
value.
set [--aefhknptuvx [arg ...]]
-- No effect; useful if the first arg begins with -.
-a Export any variables that are modified or created from now
on.
-e If non interactive then exit immediately if a command
fails.
-f File name generation is disabled.
-h When a function is defined, look up all external commands
it contains as described for the hash special command.
Normally, these commands are looked up when they are exe‐
cuted.
-k All keyword arguments are placed in the environment for a
command, not just those that precede the command name.
-m Enables job control (see below).
-n Read commands but do not execute them.
-p Makes the shell privileged. A privileged shell does not
execute the system and user profiles; if an non-privileged
shell (the default) has an effective user or group id dif‐
ferent to its real user or group id or if it has an effec‐
tive user or group id below 100, it resets its effective
user or group id, respectively, to the corresponding real
id at startup.
-t Exit after reading and executing one command.
-u Treat unset variables as an error when substituting.
-v Print shell input lines as they are read.
-x Print commands and their arguments as they are executed.
- Turn off the -x and -v options.
These flags can also be used upon invocation of the shell. The
current set of flags may be found in $-.
If + is used instead of -, the given flags are disabled.
Remaining arguments are positional parameters and are assigned,
in order, to $1, $2, etc. If no arguments are given then the
values of all names are printed.
shift [n]
The positional parameters from $2... are renamed $1... The n
argument causes a shift by the given number, i.e. $n+1 is
renamed to $1 and so forth.
times Print the accumulated user and system times for processes run
from the shell.
test [expr]
test evaluates the expression expr, and if its value is true
then returns zero exit status; otherwise, a non zero exit status
is returned. test returns a non zero exit if there are no argu‐
ments.
The following primitives are used to construct expr:
-r file true if the file exists and is readable.
-w file true if the file exists and is writable.
-u file true if the file exists and has the setuid bit set.
-g file true if the file exists and has the setgid bit set.
-k file true if the file exists and has the sticky bit set.
-f file true if the file exists and is a regular file (or
any file other than a directory if /usr/ucb occurs
early in the current $PATH parameter).
-d file true if the file exists and is a directory.
-h file true if the file exists and is a symbolic link.
-L file true if the file exists and is a symbolic link.
-p file true if the file exists and is a named pipe.
-b file true if the file exists and is a block device.
-c file true if the file exists and is a character device.
-s file true if the file exists and has a size greater than
zero.
-t [fildes] true if the open file whose file descriptor number
is fildes (1 by default) is associated with a termi‐
nal device.
-z s1 true if the length of string s1 is zero.
-n s1 true if the length of the string s1 is nonzero.
s1 = s2 true if the strings s1 and s2 are equal.
s1 != s2 true if the strings s1 and s2 are not equal.
s1 true if s1 is not the null string.
n1 -eq n2 true if the integers n1 and n2 are algebraically
equal. Any of the comparisons -ne, -gt, -ge, -lt,
or -le may be used in place of -eq.
These primaries may be combined with the following operators:
! unary negation operator
-a binary and operator
-o binary or operator
( expr ) parentheses for grouping.
-a has higher precedence than -o. Notice that all
the operators and flags are separate arguments to
test. Notice also that parentheses are meaningful
as command separators and must be escaped.
trap [arg] [n|name] ...
Arg is a command to be read and executed when the
shell receives signal(s) n. (Note that arg is
scanned once when the trap is set and once when
the trap is taken.) Trap commands are executed in
order of signal number. If arg is absent then all
trap(s) n are reset to their original values. If
arg is the null string then this signal is ignored
by the shell and by invoked commands. If n is 0
then the command arg is executed on exit from the
shell, otherwise upon receipt of signal n as num‐
bered in signal(2). Trap with no arguments prints
a list of commands associated with each signal
number. A symbolic name can be used instead of
the n argument; it is formed by the signal name in
the `C' language minus the SIG prefix, e.g. TERM
for SIGTERM. EXIT is the same as a `0' argument.
type name ...
For each name, prints if it would be executed as a
shell function, as a special command, or as an
external command. In the last case, the full path
name to the command is also printed.
ulimit [-[HS][a|cdfmnstuv]]
ulimit [-[HS][c|d|f|m|n|s|t|u|v]] [limit]
Handles resource limits for the shell and pro‐
cesses created by it, as described in getr‐
limit(2). Without a limit argument, the current
settings are printed; otherwise, a new limit is
set. The following options are accepted:
-H Sets a hard limit. Only the super-user may
raise a hard limit.
-S Sets a soft limit. A soft limit must not
exceed the hard limit.
If neither -H or -S is given, the soft
limit is printed, or both limits are set,
respectively.
-a Chooses all limits described.
-c The maximum size of a core dump in 512-byte
blocks.
-d The maximum size of the data segment in
kbytes.
-f The maximum size of a file in 512-byte
blocks. This is the default if no limit is
explicitly selected.
-l The maximum size of locked memory in
kbytes.
-m The maximum resident set size in kbytes.
-n The maximum number of open file descrip‐
tors.
-s The maximum size of the stack segment in
kbytes.
-t The maximum processor time in seconds.
-u The maximum number of child processes.
-v The maximum address space size in kbytes.
umask [-S] [nnn]
The user file creation mask is set to the octal
value nnn (see umask(2)). Symbolic modes as
described in chmod(1) are also accepted. If nnn
is omitted, the current value of the mask is
printed. With the -S option, the current mask is
printed as a symbolic string.
unset variable ...
Unsets each variable named.
wait [n]
Wait for the specified process and report its ter‐
mination status. If n is not given then all cur‐
rently active child processes are waited for. The
return code from this command is that of the
process waited for. If n does not refer to a
child process of the shell, wait returns immedi‐
ately with code 0.
Invocation
If the first character of argument zero is -, commands
are read from /etc/profile and $HOME/.profile, if the
respective file exists. Commands are then read as
described below. The following flags are interpreted by
the shell when it is invoked.
-c string If the -c flag is present then commands are
read from string.
-s If the -s flag is present or if no arguments
remain then commands are read from the stan‐
dard input. Shell output is written to file
descriptor 2.
-i If the -i flag is present or if the shell
input and output are attached to a terminal
(as told by isatty(3)) then this shell is
interactive. In this case the terminate sig‐
nal SIGTERM (see signal(2)) is ignored (so
that `kill 0' does not kill an interactive
shell) and the interrupt signal SIGINT is
caught and ignored (so that wait is interrupt‐
able). In all cases SIGQUIT is ignored by the
shell.
The remaining flags and arguments are described under the
set command.
Job Control
When an interactive shell is invoked as jsh, job control
is enabled. Job control allows to stop and resume pro‐
cesses, and to switch between foreground and background
jobs. A job consists of the commands of a single pipe‐
line. Each job is placed in a separate process group; a
login shell and all jobs created by it form a session.
Interrupt, quit, and other terminal control characters
only affect the current foreground process group. The
foreground job can be stopped pressing the suspend key,
typically ^Z; any job can be stopped by sending the STOP
signal to it. Jobs are identified by jod ids of the fol‐
lowing form:
% or %% or %+
The current job.
%- The job that was previously the current job.
?string
The only job whose name contains string.
%number
The job with the given number.
number The job with process group id number.
string The only job for which string is a prefix of its
name.
The following built-in commands are additionally avail‐
able with job control:
bg [jobid ...]
Places each jobid in the background. The default
job id is the current job.
fg [jobid ...]
Sequentially selects each jobid as the foreground
job. The default job id is the current job.
jobs [-p|-l] [jobid ...] | [-x command [arguments ...]]
Prints information about each jobid, or executes
command.
-l Includes the process group id and the
starting directory.
-p Includes the process group id.
-x command [arguments ...]
Executes command with arguments; each argu‐
ment that forms a job id is replaced by the
process group id of the respective job. It
is an error if a given job does not exist.
kill [[-s signal | -signal] jobid ... | -l [status]
A special version of the kill(1) command that rec‐
ognizes job ids in its arguments.
stop jobid ...
Stops the given jobs (i.e. sends a STOP signal to
them).
suspend
Stops the shell itself. This is not allowed if
the shell is a session leader.
wait [jobid]
The wait command (see above) recognizes job ids in
its arguments.
FILES
/etc/profile
$HOME/.profile
/tmp/sh*
/dev/null
SEE ALSOenv(1), exec(2), signal(2)DIAGNOSTICS
Errors detected by the shell, such as syntax errors or
fatal errors in special commands, cause the shell to
return a non zero exit status. If the shell is being
used non interactively then execution of the shell file
is abandoned. Otherwise, the shell returns the exit sta‐
tus of the last command executed (see also exit).
NOTES
For historical reasons, ^ is a synonym for | as pipeline
separator. Its use in new applications is discouraged.
If a command other than a simple-command (i.e. `for ...',
`case ...' etc.) is redirected, it is executed in a sub‐
shell. If variable assignments must be visible in the
parent shell after the input has been redirected, the
exec special command can be used:
exec 5<&0 <input
while read line
do
...
variable=value
...
done
exec <&5 5<&-
If parameters that have been inherited from the initial
environment are modified, they must be explicitly
exported to make the change visible to external commands,
as described under `Environment' above.
The $IFS parameter is applied to any unquoted word. Thus
IFS=X
echoXfoo
executes the `echo' command with the argument `foo'.
The command `set --' without further arguments is a no-
op. The shift special command can be used to delete all
positional parameters.
There is only one namespace for both functions and param‐
eters. A function definition will delete a parameter
with the same name and vice-versa.
Parameter assignments that precede a special command
affect the shell itself; parameter assignments that pre‐
cede the call of a function are ignored.
Heirloom Bourne Shell 7/3/05 SH(1)
