SH(1) SH(1)
NAME
sh, cd, wait - shell, the standard command programming
language
SYNOPSIS
sh [ -acefiknpstuvx ] [ args ]
DESCRIPTION
Sh is a command programming language that executes commands
read from a terminal or a file. See Invocation below for
the meaning of arguments to the shell.
Definitions
A blank is a tab or a space. A name is a sequence of let-
ters, digits, or underscores beginning with a letter or
underscore. A parameter is a name, a digit, or any of the
characters *, @, #, ?, -, $, and !. A word is a sequence of
characters and quoted strings, surrounded by blanks or new-
lines.
Commands
A simple-command is a sequence of words separated by blanks.
The first word specifies the name of the command to be exe-
cuted. 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 (octal) 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 exit
status of a pipeline is the exit status of the last command.
A list is a sequence of one or more pipelines separated by
;, &, &&, or ||, and optionally terminated by ; or &. Of
these four symbols, ; and & have equal precedence, which is
lower than that of && and ||. The symbols && and || also
have equal precedence. A semicolon (;) causes sequential
execution of the preceding pipeline; an ampersand (&) causes
asynchronous execution of the preceding pipeline (i.e., the
shell does not wait for that pipeline to finish). The sym-
bol && (||) causes the list following it to be executed only
if the preceding pipeline returns a zero (non-zero) exit
status. An arbitrary number of new-lines may appear in a
list, instead of semicolons, to delimit commands.
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A command is either a simple-command or one of the follow-
ing. Unless otherwise stated, the value returned by a com-
mand is that of the last simple-command executed in the com-
mand.
for name [ in word ... ; ] do list done
Each time a for command is executed, name is set to the
next word taken from the in word list. If in word ...
; is omitted, then the for command executes the do list
once for each positional parameter that is set (see
Parameter Substitution below). 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 pat-
terns is the same as that used for file-name generation
(see File Name Generation) except that a slash, a lead-
ing dot, or a dot immediately following a slash need
not be matched explicitly.
if list then list [ elif list then list ] ... [ else list ] fi
The list following if is executed and, if it returns a
zero exit status, the list following the first then is
executed. Otherwise, the list following elif is exe-
cuted and, if its value is zero, the list following the
next then is executed. Failing that, the else list is
executed. If no else list or then list is executed,
then the if command returns a zero exit status.
while list do list done
A while command repeatedly executes the while list and,
if the exit status of the last command in the list is
zero, executes the do list; otherwise the loop termi-
nates. If no commands in the do list are executed,
then the while command returns a zero exit status;
until may be used in place of while to negate the loop
termination test.
(list)
Execute list in a sub-shell.
{list}
list is simply executed.
name () command
Define a function which is referenced by name. The body
of the function is the command. The most useful form of
command is a sequence of commands enclosed by { and }.
Execution of functions is described below (see
Execution).
The following words are only recognized as the first word of
a command and when not quoted:
if then else elif fi case esac for while until do done
Comments
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A word beginning with # causes that word and all the follow-
ing characters up to a new-line to be ignored.
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 new-lines are removed.
Parameter Substitution
The character $ is used to introduce substitutable
parameters. There are two types of parameters, positional
and keyword. If parameter is a digit, it is a positional
parameter. Positional parameters may be assigned values by
set. Keyword parameters (also known as variables) may be
assigned values by writing:
name=value [ name=value ] ...
Pattern-matching is not performed on value. There cannot be
a function and a variable with the same name.
${parameter}
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 * or
@, all the positional parameters, starting with $1, are
substituted (separated by spaces). Parameter $0 is set
from argument zero when the shell is invoked.
${parameter:-word}
If parameter is set and is non-null, substitute its
value; otherwise substitute word.
${parameter:=word}
If parameter is not set or is null set it to word; the
value of the parameter is substituted. Positional
parameters may not be assigned to in this way.
${parameter:?word}
If parameter is set and is non-null, substitute its
value; otherwise, print word and exit from the shell.
If word is omitted, the message ``parameter null or not
set'' is printed.
${parameter:+word}
If parameter is set and is non-null, substitute word;
otherwise substitute nothing.
In the above, word is not evaluated unless it is to be used
as the substituted string, so that, in the following exam-
ple, pwd is executed only if d is not set or is null:
echo ${d:-`pwd`}
If the colon (:) is omitted from the above expressions, the
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shell only checks whether parameter is set or not.
The following parameters are automatically set by the shell:
# The number of positional parameters in decimal.
- Flags supplied to the shell on invocation or by
the set command.
? The decimal value returned by the last syn-
chronously executed command.
$ The process number of this shell.
! The process number of the last background command
invoked.
The following parameters are used by the shell:
HOME The default argument (home directory) for the cd
command.
PATH The search path for commands (see Execution
below).
CDPATH
The search path for the cd command.
MAIL If this parameter is set to the name of a mail
file the shell informs the user of the arrival of
mail in the specified file. The file is inspected
every three minutes.
HISTORY
If this parameter is set to the name of a writable
file, the shell appends interactive input to the
file, for use by the = command (=(1)).
PS1 Primary prompt string, by default ``$ ''.
PS2 Secondary prompt string, by default ``> ''.
IFS Internal field separators, normally space, tab,
and new-line.
The shell gives default values to PATH, PS1, PS2 and IFS.
HOME is set by login(8).
Blank Interpretation
After parameter and command substitution, the results of
substitution are scanned for internal field separator char-
acters (those found in IFS) and split into distinct argu-
ments where such characters are found. Explicit null argu-
ments ("" 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
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, the word is
left unchanged. The directories . and .. (initially or
after a /) are only matched by patterns beginning with an
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explicit period. The character / itself must be matched
explicitly.
* Matches any string, including the null string.
? Matches any single character.
[...]
Matches any one of the enclosed characters. A
pair of characters separated by - matches any
character lexically between the pair, inclusive.
If the first character following the opening ``[''
is a ``^'' any character not enclosed is matched.
Quoting
The following characters have a special meaning to the shell
and cause termination of a word unless quoted:
; & ( ) | < > new-line space tab { }
(The characters { and } need not be quoted inside a ${} con-
struction.) A character may be quoted (i.e., made to stand
for itself) by preceding it with a \. The pair \new-line is
ignored. All characters enclosed between a pair of single
quote marks (''), except a single quote, are quoted. Inside
double quote marks (""), 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 new-line is
typed and further input is needed to complete a command, the
secondary prompt (i.e., the value of PS2) is issued.
Input/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 descrip-
tor 0).
>word Use file word as standard output (file
descriptor 1). If the file does not exist it
is created; otherwise, it is truncated to zero
length.
>>word Use file word as standard output. If the file
exists output is appended to it (by first
seeking to the end-of-file); otherwise, the
file is created.
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<<word The shell input is read up to a line that is
the same as word, or to an end-of-file. The
resulting document becomes the standard input.
If any character of word is quoted, no inter-
pretation is placed upon the characters of the
document; otherwise, parameter and command
substitution occurs, (unescaped) \new-line is
ignored, and \ must be used to quote the char-
acters \, $, `, and the first character of
word.
<&digit Use the file associated with file descriptor
digit as standard input. Similarly for the
standard output using >&digit.
<&- The standard input is closed. Similarly for
the standard output using >&-.
If any of the above is preceded by a digit, the file
descriptor which will be associated with the file is that
specified by the digit (instead of the default 0 or 1). For
example:
... 2>&1
associates file descriptor 2 with the file currently associ-
ated with file descriptor 1.
The order in which redirections are specified is signifi-
cant. The shell evaluates redirections left-to-right. For
example:
... 1>xxx 2>&1
first associates file descriptor 1 with file xxx. It associ-
ates file descriptor 2 with the file associated with file
descriptor 1 (i.e. xxx). 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 xxx.
If a command is followed by & the default standard input for
the command is the empty file /dev/null. 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 (see environ(5)) 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 or function for each
name found, giving it the corresponding value. If the user
SH(1) SH(1)
modifies the value of any of these parameters or creates new
parameters, none of these affects the environment unless the
export command is used to bind the shell's parameter to the
environment (see also set -a). A parameter may be removed
from the environment with the unset command. The environ-
ment seen by any executed command is thus composed of any
unmodified name-value pairs originally inherited by the
shell, minus any pairs removed by unset, plus any modifica-
tions 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 (but
not functions). Thus:
TERM=450 cmd and
(export TERM; TERM=450; cmd)
are equivalent (as far as the execution of cmd is con-
cerned).
If the -k flag is set, all keyword arguments are placed in
the environment, even if they occur after the command name.
The following first prints a=b c then 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 &; otherwise signals
have the values inherited by the shell from its parent, with
the exception of signal 11 (but see also the trap command
below).
Execution
Each time a command is executed, the above substitutions are
carried out. If the command name matches one of the Special
Commands listed below, it is executed in the shell process.
If the command name does not match a Special Command, but
matches the name of a defined function, the function is exe-
cuted in the shell process (note how this differs from the
execution of shell procedures). The positional parameters
$1, $2, .... are set to the arguments of the function. If
the command name matches neither a Special Command nor the
name of a defined function, a new process is created and an
attempt is made to execute the command via exec(2).
The shell parameter PATH defines the search path for the
directory containing the command. Alternative directory
names are separated by a colon (:). The default path is
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:/bin:/usr/bin (specifying the current directory, /bin, and
/usr/bin, in that order). Note that the current directory
is specified by a null path name, which can appear immedi-
ately after the equal sign or between the colon delimiters
anywhere else in the path list. If the command name con-
tains a / 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
sub-shell is spawned to read it. A parenthesized command is
also executed in a sub-shell.
Special Commands
Input/output redirection is permitted for these commands.
File descriptor 1 is the default output location.
: No effect; the command does nothing. A zero exit code
is returned.
. file
Read and execute commands from file and return. The
search path specified by PATH is used to find the
directory containing file.
builtin [ command ]
Execute the built-in special command (such as break)
regardless of functions defined with the same name.
break [ n ]
Exit from the enclosing for or while loop, if any. If
n is specified break n levels.
continue [ n ]
Resume the next iteration of the enclosing for or while
loop. If n is specified resume at the n-th enclosing
loop.
cd [ arg ]
Change the current directory to arg. The shell parame-
ter HOME is the default arg. The shell parameter CDPATH
defines the search path for the directory containing
arg. Alternative directory names are separated by a
colon (:). The default path is <null> (specifying the
current directory). Note that the current directory is
specified by a null path name, which can appear immedi-
ately after the equal sign or between the colon delim-
iters anywhere else in the path list. If arg begins
with a / the search path is not used. Otherwise, each
directory in the path is searched for arg.
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
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modified.
exit [ n ]
Causes a shell to exit with the exit status specified
by n. If n is omitted the exit status is that of the
last command executed (an end-of-file will also cause
the shell to exit.)
export [ name ... ]
The given names are marked for automatic export to the
environment of subsequently-executed commands. If no
arguments are given, a list of all names that are
exported in this shell is printed.
newgrp [ arg ... ]
Equivalent to exec newgrp arg .... See newgrp(1) for
usage and description.
read [ name ... ]
One line is read from the standard input and the first
word is assigned to the first name, the second word to
the second name, etc., with leftover words assigned to
the last name. The return code is 0 unless an end-of-
file is encountered.
return [ n ]
Causes a function to exit with the return value speci-
fied by n. If n is omitted, the return status is that
of the last command executed.
set [ --aehknptuvx [ arg ... ] ]
-a Mark variables which are modified or created for
export.
-e Exit immediately if a command exits with a non-
zero exit status.
-f Disable file name generation
-k All keyword arguments are placed in the environ-
ment for a command, not just those that precede
the command name.
-n Read commands but do not execute them.
-p Remove the definitions for all functions imported
from the environment, and set IFS to blank, tab
and newline.
-t Exit after reading and executing one command.
-u Treat unset variables as an error when substitut-
ing.
-v Print shell input lines as they are read.
-x Print commands and their arguments as they are
executed.
-- Do not change any of the flags; useful in setting
$1 to -.
Using + rather than - causes these flags to be turned
off. These flags can also be used upon invocation of
the shell. The current set of flags may be found in
$-. The remaining arguments are positional parameters
and are assigned, in order, to $1, $2, .... If no
arguments are given the values of all names are
printed.
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shift [ n ]
The positional parameters from $n+1 ... are renamed $1
.... If n is not given, it is assumed to be 1.
times
Print the accumulated user and system times for pro-
cesses run from the shell.
trap [ arg ] [ n ] ...
The command arg is 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. 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 arg is absent all trap(s) n are reset to
their original values. If arg is the null string this
signal is ignored by the shell and by the commands it
invokes. If n is 0 the command arg is executed on exit
from the shell. The trap command with no arguments
prints a list of commands associated with each signal
number.
umask [ nnn ]
The user file-creation mask is set to nnn (see
umask(2)). If nnn is omitted, the current value of the
mask is printed.
unset [ name ... ]
For each name, remove the corresponding variable or
function. The variables PATH, PS1, PS2 and IFS cannot
be unset.
wait [ n ]
Wait for the specified process and report its termina-
tion status. If n is not given all currently active
child processes are waited for and the return code is
zero.
whatis [ name ... ]
For each name, print the associated value as a parame-
ter, function, builtin or executable binary as appro-
priate. In each case, the value is printed in a form
that would yield the same value if typed as input to
the shell itself: parameters are printed as assign-
ments, functions as their definitions, builtins as
calls to builtin, and binaries as their full pathnames.
Invocation
If the shell is invoked through exec(2) and the first char-
acter of argument zero is -, commands are initially read
from $HOME/.profile, if it exists. Thereafter, commands are
read as described below, which is also the case when the
shell is invoked as /bin/sh. The flags below are inter-
preted by the shell on invocation only; Note that unless the
-c or -s flag is specified, the first argument is assumed to
be the name of a file containing commands, and the remaining
SH(1) SH(1)
arguments are passed as positional parameters to that com-
mand file:
-c string If the -c flag is present commands are read from
string.
-s If the -s 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 Special
Commands) 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, this shell
is interactive. In this case TERMINATE is ignored
(so that kill 0 does not kill an interactive
shell) and INTERRUPT is caught and ignored (so
that wait is interruptible). In all cases, QUIT
is ignored by the shell.
The remaining flags and arguments are described under the
set command above.
EXIT STATUS
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 exit command above).
FILES
$HOME/.profile
/tmp/sh*
/dev/null
SEE ALSO
=(1), cd(1), echo(1), newgrp(1), test(1)
dup(2), exec(2), fork(2), pipe(2), signal(2), umask(2),
wait(2), a.out(5), environ(5)
BUGS
A function invocation overwrites the arguments of the invok-
ing shell.