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SH(1) |
User Commands |
SH(1) |
sh, jsh - the standard command interpreter
sh [-acefhikmnprstuvx] [arg] ...
jsh [-acefhikmnprstuvx] [arg] ...
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.
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 terminates 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 precedence. 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
pattern 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 terminates. 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
recognized as a command, list is executed, with the positional
parameters $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 { }
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.
The character $ is used to introduce substitutable parameters. Positional
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 parameters 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
omitted then a standard message is printed.
- ${parameter:+word}
- If parameter is set and not empty then substitute word;
otherwise substitute nothing.
If the : is omitted, the substitutions are only executed if
the parameter is set, even if it is empty.
In the above word is not evaluated unless it is to be used
as the substituted string. (So that, for example, echo ${d-`pwd`} will only
execute 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 decimal.
- $
- 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
newline.
- 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 handling $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.
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.
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 character
/, 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 characters separated
by - matches any character lexically between the pair.
- [!...]
- Matches any character except the enclosed ones.
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 \.
wline 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"
... .
When used interactively, the shell prompts with the value of $PS1
before reading a command. If at any time a newline is typed and further input
is needed to complete a command then the secondary prompt ($PS2) is issued.
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 truncated to zero
length.
- >>word
- Use file word as standard output. If the file exists then output 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.
The environment is a list of name-value pairs that is passed to an executed
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. Executed 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 environment, 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
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.)
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. Otherwise, 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.
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 setting 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 environment passed to the shell, the -n argument is also
interpreted, 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 arguments 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 environment
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 positional
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 argument.
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 arguments 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 executed.
- -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 different to its real user or group id or if it
has an effective 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
arguments.
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 terminal 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 numbered 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 processes created by it, as
described in getrlimit(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 descriptors.
- -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 termination status. If
n is not given then all currently 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 immediately with code 0.
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 standard 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 signal 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 interruptable). In all cases SIGQUIT is ignored by
the shell.
The remaining flags and arguments are described under the
set command.
When an interactive shell is invoked as jsh, job control is enabled. Job
control allows to stop and resume processes, and to switch between foreground
and background jobs. A job consists of the commands of a single pipeline. 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 following 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 available 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 argument 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 recognizes 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.
/etc/profile
$HOME/.profile
/tmp/sh*
/dev/null
env(1), exec(2), signal(2)
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 status of the last command executed (see
also exit).
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 subshell. 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
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 parameters. 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 precede the call of a function are
ignored.
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