term% cat index.txt SH(1) General Commands Manual 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 letters, 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 newlines.
Commands
A simple-command is a sequence of words separated by blanks. The first
word specifies the name of the command to be executed. Except as spec‐
ified 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 symbol && (⎪⎪) 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.
A command is either a simple-command or one of the following. Unless
otherwise stated, 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 taken from the in word list. If in word ... ; is omitted,
then the for command executes the do list once for each posi‐
tional 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 pat‐
tern that matches word. The form of the patterns is the same as
that used for file-name generation (see File Name Generation)
except that a slash, a leading dot, or a dot immediately follow‐
ing 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. Other‐
wise, the list following elif is executed 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 terminates. 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 func‐
tions 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
A word beginning with # causes that word and all the following charac‐
ters 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 re‐
moved.
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 as‐
signed 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; oth‐
erwise 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; oth‐
erwise, 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 sub‐
stituted string, so that, in the following example, 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 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 synchronously exe‐
cuted command.
$ The process number of this shell.
! The process number of the last background command in‐
voked.
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 min‐
utes.
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 characters (those found in
IFS) and split into distinct arguments where such characters are found.
Explicit null arguments ("" or '') are retained. Implicit null argu‐
ments (those resulting from parameters that have no values) are re‐
moved.
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
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 fol‐
lowing the opening ``['' is a ``^'' any character not en‐
closed 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 ${} construction.)
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 substi‐
tution 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 in‐
put 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 be‐
fore 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 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.
<<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 be‐
comes the standard input. If any character of word is
quoted, no interpretation is placed upon the characters
of the document; otherwise, parameter and command substi‐
tution occurs, (unescaped) \new-line is ignored, and \
must be used to quote the characters \, $, `, 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 (in‐
stead of the default 0 or 1). For example:
... 2>&1
associates file descriptor 2 with the file currently associated with
file descriptor 1.
The order in which redirections are specified is significant. The
shell evaluates redirections left-to-right. For example:
... 1>xxx 2>&1
first associates file descriptor 1 with file xxx. It associates 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 com‐
mand 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 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 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 unset, plus any modifications or ad‐
ditions, 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 concerned).
If the -k flag is set, all keyword arguments are placed in the environ‐
ment, 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 inher‐
ited 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 func‐
tion, the function is executed in the shell process (note how this dif‐
fers from the execution of shell procedures). The positional parame‐
ters $1, $2, .... are set to the arguments of the function. If the
command name matches neither a Special Command nor the name of a de‐
fined function, a new process is created and an attempt is made to exe‐
cute the command via exec(2).
The shell parameter PATH defines the search path for the directory con‐
taining the command. Alternative directory names are separated by a
colon (:). The default path is :/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 contains 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 exe‐
cuted in a sub-shell.
Special Commands
Input/output redirection is permitted for these commands. File de‐
scriptor 1 is the default output location.
: No effect; the command does nothing. A zero exit code is re‐
turned.
. 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 parameter HOME
is the default arg. The shell parameter CDPATH defines the
search path for the directory containing arg. Alternative di‐
rectory names are separated by a colon (:). The default path is
<null> (specifying the current directory). Note that the cur‐
rent directory is specified by a null path name, which can ap‐
pear immediately after the equal sign or between the colon de‐
limiters 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 argu‐
ments may appear and, if no other arguments are given, cause the
shell input/output to be 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 exe‐
cuted (an end-of-file will also cause the shell to exit.)
export [ name ... ]
The given names are marked for automatic export to the environ‐
ment 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 specified 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 environment 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 substituting.
-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 argu‐
ments are positional parameters and are assigned, in order, to
$1, $2, .... If no arguments are given the values of all names
are printed.
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 processes run
from the shell.
trap [ arg ] [ n ] ...
The command arg is to be read and executed when the shell re‐
ceives 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 termination sta‐
tus. 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 parameter, func‐
tion, builtin or executable binary as appropriate. 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 assignments, 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 character of ar‐
gument 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 interpreted 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 arguments are
passed as positional parameters to that command 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-interac‐
tively 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 invoking shell.
SH(1)