Computer Science
SH(1) UNIX Reference Manual SH(1)
NAME
sh - command interpreter (shell)
SYNOPSIS
sh [-/+abCEefIimnpsTuVvx] [-/+o longname] [-c string] [arg ...]
DESCRIPTION
Sh is the standard command interpreter for the system. The current ver-
sion of sh is in the process of being changed to conform with the IEEE
Std1003.2 (``POSIX'') specification for the shell. This version has many
features which make it appear similar in some respects to the Korn shell,
but it is not a Korn shell clone (run GNU's bash if you want that). Only
features designated by POSIX, plus a few Berkeley extensions, are being
incorporated into this shell. This man page is not intended to be a tu-
torial or a complete specification of the shell.
Overview
The shell is a command that reads lines from either a file or the termi-
nal, interprets them, and generally executes other commands. It is the
program that is running when a user logs into the system (although a user
can select a different shell with the chsh(1) command). The shell imple-
ments a language that has flow control constructs, a macro facility that
provides a variety of features in addition to data storage, along with
built in history and line editing capabilities. It incorporates many
features to aid interactive use and has the advantage that the interpre-
tative language is common to both interactive and non-interactive use
(shell scripts). That is, commands can be typed directly to the running
shell or can be put into a file and the file can be executed directly by
the shell.
Invocation
If no args are present and if the standard input of the shell is connect-
ed to a terminal (or if the -i flag is set), the shell is considered an
interactive shell. An interactive shell generally prompts before each
command and handles programming and command errors differently (as de-
scribed below). When first starting, the shell inspects argument 0, and
if it begins with a dash '-', the shell is also considered a login shell.
This is normally done automatically by the system when the user first
logs in. A login shell first reads commands from the files /etc/profile
and .profile if they exist. If the environment variable ENV is set on
entry to a shell, or is set in the .profile of a login shell, the shell
next reads commands from the file named in ENV. Therefore, a user should
place commands that are to be executed only at login time in the .profile
file, and commands that are executed for every shell inside the ENV file.
To set the ENV variable to some file, place the following line in your
.profile of your home directory
ENV=$HOME/.shinit; export ENV
substituting for .shinit any filename you wish. If commandline arguments
besides the options have been specified, then the shell treats the first
argument as the name of a file from which to read commands (a shell
script), and the remaining arguments are set as the positional parameters
of the shell ($1, $2, etc). Otherwise, the shell reads commands from its
standard input.
Unlike older versions of sh the ENV script is only sourced on invocation
of interactive shells. This closes a well-known, and sometimes easily
exploitable security hole related to poorly thought out ENV scripts.
Argument List Processing
All of the single letter options to sh have a corresponding name that can
be used as an argument to the set(1) builtin (described later). These
names are provided next to the single letter option in the descriptions
below. Specifying a dash ``-'' enables the option, while using a plus
``+'' disables the option. A ``--'' or plain ``-'' will stop option pro-
cessing and will force the remaining words on the command line to be
treated as arguments.
-a allexport
Export all variables assigned to.
-b notify
Enable asynchronous notification of background job completion.
(UNIMPLEMENTED)
-C noclobber
Don't overwrite existing files with ``>.'' (UNIMPLEMENTED)
-E emacs
Enable the built-in emacs(1) commandline editor (disables -V if
it has been set).
-e errexit
If not interactive, exit immediately if any untested command
fails. The exit status of a command is considered to be explic-
itly tested if the command is used to control an if, elif, while,
or until; or if the command is the left hand operand of an ``&&''
or ``||'' operator.
-f noglob
Disable pathname expansion.
-I ignoreeof
Ignore EOF's from input when interactive.
-i interactive
Force the shell to behave interactively.
-m monitor
Turn on job control (set automatically when interactive).
-n noexec
If not interactive, read commands but do not execute them. This
is useful for checking the syntax of shell scripts.
-p privileged
Turn on privileged mode. This mode is enabled on startup if ei-
ther the effective user or group id is not equal to the real user
or group id. Turning this mode off sets the effective user and
group ids to the real user and group ids. Also on interactive
shells and when enabled, this mode sources /etc/suid_profile (in-
stead of ~/.profile) after /etc/profile and ignores the contents
of the ENV variable.
-s stdin
Read commands from standard input (set automatically if no file
arguments are present). This option has no effect when set after
the shell has already started running (i.e. with set(1)).
-T asynctraps
When waiting for a child, execute traps immediately. If this op-
tion is not set, traps are executed after the child exits, as
specified in IEEE Std1003.2 (``POSIX'') This nonstandard option
is useful to put guarding shells around childs that block sig-
nals. The surrounding shell may kill the child or it may just re-
turn control to the tty and leave the child alone, like this:
sh -T -c "trap 'exit 1' 2 ; some-blocking-program"
-u nounset
Write a message to standard error when attempting to expand a
variable that is not set, and if the shell is not interactive,
exit immediately. (UNIMPLEMENTED)
-V vi Enable the built-in vi(1) commandline editor (disables -E if it
has been set).
-v verbose
The shell writes its input to standard error as it is read. Use-
ful for debugging.
-x xtrace
Write each command to standard error (preceded by a '+ ') before
it is executed. Useful for debugging.
-c string
Pass the string argument to the shell to be interpreted as input.
Keep in mind that this option only accepts a single string as its
argument, hence multi-word strings must be quoted.
Lexical Structure
The shell reads input in terms of lines from a file and breaks it up into
words at whitespace (blanks and tabs), and at certain sequences of char-
acters that are special to the shell called ``operators''. There are two
types of operators: control operators and redirection operators (their
meaning is discussed later). The following is a list of valid operators:
Control operators:
& && ( ) ; ;; | || \n
Redirection operators:
< > >| << >> <& >& <<-
Quoting
Quoting is used to remove the special meaning of certain characters or
words to the shell, such as operators, whitespace, or keywords. There
are three types of quoting: matched single quotes, matched double quotes,
and backslash.
Single Quotes
Enclosing characters in single quotes preserves the literal mean-
ing of all the characters (except single quotes, making it impos-
sible to put single-quotes in a single-quoted string).
Double Quotes
Enclosing characters within double quotes preserves the literal
meaning of all characters except dollarsign ($), backquote (`),
and backslash (\). The backslash inside double quotes is histor-
ically weird, and serves to quote only the following characters:
$ ` " \ \n. Otherwise it remains literal.
Backslash
A backslash preserves the literal meaning of the following char-
acter, with the exception of \n. A backslash preceding a \n is
treated as a line continuation.
Reserved Words
Reserved words are words that have special meaning to the shell and are
recognized at the beginning of a line and after a control operator. The
following are reserved words:
! { } case do
done elif else esac fi
for if then until while
Aliases
An alias is a name and corresponding value set using the alias(1) builtin
command. Whenever a reserved word may occur (see above), and after
checking for reserved words, the shell checks the word to see if it
matches an alias. If it does, it replaces it in the input stream with its
value. For example, if there is an alias called ``lf'' with the value
``ls -F'', then the input
lf foobar <return>
would become
ls -F foobar <return>
Aliases provide a convenient way for naive users to create shorthands for
commands without having to learn how to create functions with arguments.
They can also be used to create lexically obscure code. This use is dis-
couraged.
Commands
The shell interprets the words it reads according to a language, the
specification of which is outside the scope of this man page (refer to
the BNF in the IEEE Std1003.2 (``POSIX'') document). Essentially though,
a line is read and if the first word of the line (or after a control op-
erator) is not a reserved word, then the shell has recognized a simple
command. Otherwise, a complex command or some other special construct
may have been recognized.
Simple Commands
If a simple command has been recognized, the shell performs the following
actions:
1. Leading words of the form ``name=value'' are stripped off and as-
signed to the environment of the simple command. Redirection opera-
tors and their arguments (as described below) are stripped off and
saved for processing.
2. The remaining words are expanded as described in the section called
``Expansions'', and the first remaining word is considered the com-
mand name and the command is located. The remaining words are con-
sidered the arguments of the command. If no command name resulted,
then the ``name=value'' variable assignments recognized in 1) affect
the current shell.
3. Redirections are performed as described in the next section.
Redirections
Redirections are used to change where a command reads its input or sends
its output. In general, redirections open, close, or duplicate an exist-
ing reference to a file. The overall format used for redirection is:
[n] redir-op file
where redir-op is one of the redirection operators mentioned previously.
The following gives some examples of how these operators can be used.
NOTE: stdin and stdout are commonly used abbreviations for standard input
and standard output, respectively.
[n]> file redirect stdout (or n) to file
[n]>| file same as above, but override the -C option
[n]>> file append stdout (or n) to file
[n]< file redirect stdin (or n) from file
[n1]<&n2 duplicate stdin (or n1) from file descriptor n2
[n]<&- close stdin (or n)
[n1]>&n2 duplicate stdout (or n1) to n2.
[n]>&- close stdout (or n)
The following redirection is often called a ``here-document''.
[n]<< delimiter
here-doc-text...
delimiter
All the text on successive lines up to the delimiter is saved away and
made available to the command on standard input, or file descriptor n if
it is specified. If the delimiter as specified on the initial line is
quoted, then the here-doc-text is treated literally, otherwise the text
is subjected to parameter expansion, command substitution, and arithmetic
expansion (as described in the section on ``Expansions''). If the opera-
tor is ``<<-'' instead of ``<<'', then leading tabs in the here-doc-text
are stripped.
Search and Execution
There are three types of commands: shell functions, builtin commands, and
normal programs -- and the command is searched for (by name) in that or-
der. They each are executed in a different way.
When a shell function is executed, all of the shell positional parameters
(except $0, which remains unchanged) are set to the arguments of the
shell function. The variables which are explicitly placed in the envi-
ronment of the command (by placing assignments to them before the func-
tion name) are made local to the function and are set to the values giv-
en. Then the command given in the function definition is executed. The
positional parameters are restored to their original values when the com-
mand completes. This all occurs within the current shell.
Shell builtins are executed internally to the shell, without spawning a
new process.
Otherwise, if the command name doesn't match a function or builtin, the
command is searched for as a normal program in the filesystem (as de-
scribed in the next section). When a normal program is executed, the
shell runs the program, passing the arguments and the environment to the
program. If the program is not a normal executable file (i.e., if it does
not begin with the "magic number" whose ASCII representation is "#!", so
execve() returns ENOEXEC then) the shell will interpret the program in a
subshell. The child shell will reinitialize itself in this case, so that
the effect will be as if a new shell had been invoked to handle the ad-
hoc shell script, except that the location of hashed commands located in
the parent shell will be remembered by the child.
Note that previous versions of this document and the source code itself
misleadingly and sporadically refer to a shell script without a magic
number as a "shell procedure".
Path Search
When locating a command, the shell first looks to see if it has a shell
function by that name. Then it looks for a builtin command by that name.
If a builtin command is not found, one of two things happen:
1. Command names containing a slash are simply executed without per-
forming any searches.
2. The shell searches each entry in PATH in turn for the command. The
value of the PATH variable should be a series of entries separated
by colons. Each entry consists of a directory name. The current
directory may be indicated implicitly by an empty directory name, or
explicitly by a single period.
Command Exit Status
Each command has an exit status that can influence the behavior of other
shell commands. The paradigm is that a command exits with zero for nor-
mal or success, and non-zero for failure, error, or a false indication.
The man page for each command should indicate the various exit codes and
what they mean. Additionally, the builtin commands return exit codes, as
does an executed shell function.
If a command is terminated by a signal, its exit status is 128 plus the
signal number. Signal numbers are defined in the header file
<sys/signal.h>.
Complex Commands
Complex commands are combinations of simple commands with control opera-
tors or reserved words, together creating a larger complex command. More
generally, a command is one of the following:
simple command
pipeline
list or compound-list
compound command
function definition
Unless otherwise stated, the exit status of a command is that of the last
simple command executed by the command.
Pipelines
A pipeline is a sequence of one or more commands separated by the control
operator |. The standard output of all but the last command is connected
to the standard input of the next command. The standard output of the
last command is inherited from the shell, as usual.
The format for a pipeline is:
[!] command1 [ | command2 ...]
The standard output of command1 is connected to the standard input of
command2. The standard input, standard output, or both of a command is
considered to be assigned by the pipeline before any redirection speci-
fied by redirection operators that are part of the command.
If the pipeline is not in the background (discussed later), the shell
waits for all commands to complete.
If the reserved word ! does not precede the pipeline, the exit status is
the exit status of the last command specified in the pipeline. Other-
wise, the exit status is the logical NOT of the exit status of the last
command. That is, if the last command returns zero, the exit status is
1; if the last command returns greater than zero, the exit status is ze-
ro.
Because pipeline assignment of standard input or standard output or both
takes place before redirection, it can be modified by redirection. For
example:
$ command1 2>&1 | command2
sends both the standard output and standard error of command1 to the
standard input of command2.
A ; or <newline> terminator causes the preceding AND-OR-list (described
next) to be executed sequentially; a & causes asynchronous execution of
the preceding AND-OR-list.
Note that unlike some other shells, each process in the pipeline is a
child of the invoking shell (unless it is a shell builtin, in which case
it executes in the current shell -- but any effect it has on the environ-
ment is wiped).
Background Commands -- &
If a command is terminated by the control operator ampersand (&), the
shell executes the command asynchronously -- that is, the shell does not
wait for the command to finish before executing the next command.
The format for running a command in background is:
command1 & [command2 & ...]
If the shell is not interactive, the standard input of an asynchronous
command is set to /dev/null.
Lists -- Generally Speaking
A list is a sequence of zero or more commands separated by newlines,
semicolons, or ampersands, and optionally terminated by one of these
three characters. The commands in a list are executed in the order they
are written. If command is followed by an ampersand, the shell starts
the command and immediately proceed onto the next command; otherwise it
waits for the command to terminate before proceeding to the next one.
Short-Circuit List Operators
``&&'' and ``||'' are AND-OR list operators. ``&&'' executes the first
command, and then executes the second command if the exit status of the
first command is zero. ``||'' is similar, but executes the second com-
mand if the exit status of the first command is nonzero. ``&&'' and
``||'' both have the same priority.
Flow-Control Constructs -- if, while, for, case
The syntax of the if command is
if list
then list
[ elif list
then list ] ...
[ else list ]
fi
The syntax of the while command is
while list
do list
done
The two lists are executed repeatedly while the exit status of the first
list is zero. The until command is similar, but has the word until in
place of while, which causes it to repeat until the exit status of the
first list is zero.
The syntax of the for command is
for variable in word...
do list
done
The words are expanded, and then the list is executed repeatedly with the
variable set to each word in turn. do and done may be replaced with
``{'' and ``}''.
The syntax of the break and continue command is
break [ num ]
continue [ num ]
Break terminates the num innermost for or while loops. Continue contin-
ues with the next iteration of the innermost loop. These are implemented
as builtin commands.
The syntax of the case command is
case word in
pattern) list ;;
esac
The pattern can actually be one or more patterns (see Shell Patterns de-
scribed later), separated by ``|'' characters.
Grouping Commands Together
Commands may be grouped by writing either
(list)
or
{ list; }
The first of these executes the commands in a subshell. Builtin commands
grouped into a (list) will not affect the current shell. The second form
does not fork another shell so is slightly more efficient. Grouping com-
mands together this way allows you to redirect their output as though
they were one program:
{ echo -n "hello"; echo " world"; } > greeting
Functions
The syntax of a function definition is
name ( ) command
A function definition is an executable statement; when executed it in-
stalls a function named name and returns an exit status of zero. The
command is normally a list enclosed between ``{'' and ``}''.
Variables may be declared to be local to a function by using a local com-
mand. This should appear as the first statement of a function, and the
syntax is
local [ variable | - ] ...
Local is implemented as a builtin command.
When a variable is made local, it inherits the initial value and exported
and readonly flags from the variable with the same name in the surround-
ing scope, if there is one. Otherwise, the variable is initially unset.
The shell uses dynamic scoping, so that if you make the variable x local
to function f, which then calls function g, references to the variable x
made inside g will refer to the variable x declared inside f, not to the
global variable named x.
The only special parameter than can be made local is ``-''. Making ``-''
local any shell options that are changed via the set command inside the
function to be restored to their original values when the function re-
turns.
The syntax of the return command is
return [ exitstatus ]
It terminates the currently executing function. Return is implemented as
a builtin command.
Variables and Parameters
The shell maintains a set of parameters. A parameter denoted by a name
is called a variable. When starting up, the shell turns all the environ-
ment variables into shell variables. New variables can be set using the
form
name=value
Variables set by the user must have a name consisting solely of alphabet-
ics, numerics, and underscores - the first of which must not be numeric.
A parameter can also be denoted by a number or a special character as ex-
plained below.
Positional Parameters
A positional parameter is a parameter denoted by a number (n > 0). The
shell sets these initially to the values of its commandline arguments
that follow the name of the shell script. The set(1) builtin can also be
used to set or reset them.
Special Parameters
A special parameter is a parameter denoted by one of the following spe-
cial characters. The value of the parameter is listed next to its char-
acter.
* Expands to the positional parameters, starting from one. When
the expansion occurs within a double-quoted string it expands to
a single field with the value of each parameter separated by the
first character of the IFS variable, or by a <space> if IFS is
unset.
@ Expands to the positional parameters, starting from one. When
the expansion occurs within double-quotes, each positional param-
eter expands as a separate argument. If there are no positional
parameters, the expansion of @ generates zero arguments, even
when @ is double-quoted. What this basically means, for example,
is if $1 is ``abc'' and $2 is ``def ghi'', then "$@" expands to
the two arguments:
"abc" "def ghi"
# Expands to the number of positional parameters.
? Expands to the exit status of the most recent pipeline.
- (hyphen) Expands to the current option flags (the single-letter
option names concatenated into a string) as specified on invoca-
tion, by the set builtin command, or implicitly by the shell.
$ Expands to the process ID of the invoked shell. A subshell re-
tains the same value of $ as its parent.
! Expands to the process ID of the most recent background command
executed from the current shell. For a pipeline, the process ID
is that of the last command in the pipeline.
0 (zero) Expands to the name of the shell or shell script.
Word Expansions
This clause describes the various expansions that are performed on words.
Not all expansions are performed on every word, as explained later.
Tilde expansions, parameter expansions, command substitutions, arithmetic
expansions, and quote removals that occur within a single word expand to
a single field. It is only field splitting or pathname expansion that
can create multiple fields from a single word. The single exception to
this rule is the expansion of the special parameter @ within double-
quotes, as was described above.
The order of word expansion is:
1. Tilde Expansion, Parameter Expansion, Command Substitution, Arith-
metic Expansion (these all occur at the same time).
2. Field Splitting is performed on fields generated by step (1) unless
the IFS variable is null.
3. Pathname Expansion (unless set -f is in effect).
4. Quote Removal.
The $ character is used to introduce parameter expansion, command substi-
tution, or arithmetic evaluation.
Tilde Expansion (substituting a user's home directory)
A word beginning with an unquoted tilde character (~) is subjected to
tilde expansion. All the characters up to a slash (/) or the end of the
word are treated as a username and are replaced with the user's home di-
rectory. If the username is missing (as in ~/foobar), the tilde is re-
placed with the value of the HOME variable (the current user's home di-
rectory).
Parameter Expansion
The format for parameter expansion is as follows:
${expression}
where expression consists of all characters until the matching }. Any }
escaped by a backslash or within a quoted string, and characters in em-
bedded arithmetic expansions, command substitutions, and variable expan-
sions, are not examined in determining the matching }.
The simplest form for parameter expansion is:
${parameter}
The value, if any, of parameter is substituted.
The parameter name or symbol can be enclosed in braces, which are option-
al except for positional parameters with more than one digit or when pa-
rameter is followed by a character that could be interpreted as part of
the name. If a parameter expansion occurs inside double-quotes:
1. Pathname expansion is not performed on the results of the expansion.
2. Field splitting is not performed on the results of the expansion,
with the exception of @.
In addition, a parameter expansion can be modified by using one of the
following formats.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion
of word is substituted; otherwise, the value of parameter is sub-
stituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the expan-
sion of word is assigned to parameter. In all cases, the final
value of parameter is substituted. Only variables, not position-
al parameters or special parameters, can be assigned in this way.
${parameter:?[word]}
Indicate Error if Null or Unset. If parameter is unset or null,
the expansion of word (or a message indicating it is unset if
word is omitted) is written to standard error and the shell exits
with a nonzero exit status. Otherwise, the value of parameter is
substituted. An interactive shell need not exit.
${parameter:+word}
Use Alternate Value. If parameter is unset or null, null is sub-
stituted; otherwise, the expansion of word is substituted.
In the parameter expansions shown previously, use of the colon in
the format results in a test for a parameter that is unset or
null; omission of the colon results in a test for a parameter
that is only unset.
${#parameter}
String Length. The length in characters of the value of parame-
ter.
The following four varieties of parameter expansion provide for
substring processing. In each case, pattern matching notation
(see Shell Patterns), rather than regular expression notation, is
used to evaluate the patterns. If parameter is * or @, the re-
sult of the expansion is unspecified. Enclosing the full parame-
ter expansion string in double-quotes does not cause the follow-
ing four varieties of pattern characters to be quoted, whereas
quoting characters within the braces has this effect.
${parameter%word}
Remove Smallest Suffix Pattern. The word is expanded to produce
a pattern. The parameter expansion then results in parameter,
with the smallest portion of the suffix matched by the pattern
deleted.
${parameter%%word}
Remove Largest Suffix Pattern. The word is expanded to produce a
pattern. The parameter expansion then results in parameter, with
the largest portion of the suffix matched by the pattern deleted.
${parameter#word}
Remove Smallest Prefix Pattern. The word is expanded to produce
a pattern. The parameter expansion then results in parameter,
with the smallest portion of the prefix matched by the pattern
deleted.
${parameter##word}
Remove Largest Prefix Pattern. The word is expanded to produce a
pattern. The parameter expansion then results in parameter, with
the largest portion of the prefix matched by the pattern deleted.
Command Substitution
Command substitution allows the output of a command to be substituted in
place of the command name itself. Command substitution occurs when the
command is enclosed as follows:
$(command)
or (``backquoted'' version):
`command`
The shell expands the command substitution by executing command in a sub-
shell environment and replacing the command substitution with the stan-
dard output of the command, removing sequences of one or more <newline>s
at the end of the substitution. (Embedded <newline>s before the end of
the output are not removed; however, during field splitting, they may be
translated into <space>s, depending on the value of IFS and quoting that
is in effect.)
Arithmetic Expansion
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value. The format for arithmetic expan-
sion is as follows:
$((expression))
The expression is treated as if it were in double-quotes, except that a
double-quote inside the expression is not treated specially. The shell
expands all tokens in the expression for parameter expansion, command
substitution, and quote removal.
Next, the shell treats this as an arithmetic expression and substitutes
the value of the expression.
White Space Splitting (Field Splitting)
After parameter expansion, command substitution, and arithmetic expansion
the shell scans the results of expansions and substitutions that did not
occur in double-quotes for field splitting and multiple fields can re-
sult.
The shell treats each character of the IFS as a delimiter and use the de-
limiters to split the results of parameter expansion and command substi-
tution into fields.
Pathname Expansion (File Name Generation)
Unless the -f flag is set, file name generation is performed after word
splitting is complete. Each word is viewed as a series of patterns, sep-
arated by slashes. The process of expansion replaces the word with the
names of all existing files whose names can be formed by replacing each
pattern with a string that matches the specified pattern. There are two
restrictions on this: first, a pattern cannot match a string containing a
slash, and second, a pattern cannot match a string starting with a period
unless the first character of the pattern is a period. The next section
describes the patterns used for both Pathname Expansion and the case(1)
command.
Shell Patterns
A pattern consists of normal characters, which match themselves, and
meta-characters. The meta-characters are ``!'', ``*'', ``?'', and
``[''. These characters lose their special meanings if they are quoted.
When command or variable substitution is performed and the dollar sign or
back quotes are not double quoted, the value of the variable or the out-
put of the command is scanned for these characters and they are turned
into meta-characters.
An asterisk (``*'') matches any string of characters. A question mark
matches any single character. A left bracket (``['') introduces a charac-
ter class. The end of the character class is indicated by a ``]''; if
the ``]'' is missing then the ``['' matches a ``['' rather than introduc-
ing a character class. A character class matches any of the characters
between the square brackets. A range of characters may be specified us-
ing a minus sign. The character class may be complemented by making an
exclamation point the first character of the character class.
To include a ``]'' in a character class, make it the first character
listed (after the ``!'', if any). To include a minus sign, make it the
first or last character listed.
Builtins
This section lists the builtin commands which are builtin because they
need to perform some operation that can't be performed by a separate pro-
cess. In addition to these, there are several other commands that may be
builtin for efficiency (e.g. printf(1), echo(1), test(1), etc).
: A null command that returns a 0 (true) exit value.
. file The commands in the specified file are read and executed by the
shell. If file contains any `/' characters, it is used as is.
Otherwise, the shell searches the PATH for the file. If it is
not found in the PATH, it is sought in the current working direc-
tory.
alias [name[=string] ...]
If name=string is specified, the shell defines the alias ``name''
with value ``string''. If just ``name'' is specified, the value
of the alias ``name'' is printed. With no arguments, the alias
builtin prints the names and values of all defined aliases (see
unalias).
bg [job] ...
Continue the specified jobs (or the current job if no jobs are
given) in the background.
command command arg ...
Execute the specified builtin command. (This is useful when you
have a shell function with the same name as a builtin command.)
cd [directory]
Switch to the specified directory (default $HOME). If an entry
for CDPATH appears in the environment of the cd command or the
shell variable CDPATH is set and the directory name does not be-
gin with a slash, then the directories listed in CDPATH will be
searched for the specified directory. The format of CDPATH is
the same as that of PATH. In an interactive shell, the cd command
will print out the name of the directory that it actually
switched to if this is different from the name that the user
gave. These may be different either because the CDPATH mechanism
was used or because a symbolic link was crossed.
eval string ...
Concatenate all the arguments with spaces. Then re-parse and ex-
ecute the command.
exec [command arg ...]
Unless command is omitted, the shell process is replaced with the
specified program (which must be a real program, not a shell
builtin or function). Any redirections on the exec command are
marked as permanent, so that they are not undone when the exec
command finishes.
exit [exitstatus]
Terminate the shell process. If exitstatus is given it is used
as the exit status of the shell; otherwise the exit status of the
preceding command is used.
export name ...
The specified names are exported so that they will appear in the
environment of subsequent commands. The only way to un-export a
variable is to unset it. The shell allows the value of a vari-
able to be set at the same time it is exported by writing
export name=value
With no arguments the export command lists the names of all ex-
ported variables.
fc [-e editor] [first [last]]
fc -l [-nr] [first [last]]
fc -s [old=new] [first]
The fc builtin lists, or edits and re-executes, commands previ-
ously entered to an interactive shell.
-e editor
Use the editor named by editor to edit the commands. The
editor string is a command name, subject to search via
the PATH variable. The value in the FCEDIT variable is
used as a default when -e is not specified. If FCEDIT is
null or unset, the value of the EDITOR variable is used.
If EDITOR is null or unset, ed(1) is used as the editor.
-l (ell)
List the commands rather than invoking an editor on them.
The commands are written in the sequence indicated by the
first and last operands, as affected by -r, with each
command preceded by the command number.
-n Suppress command numbers when listing with -l.
-r Reverse the order of the commands listed (with -l) or
edited (with neither -l nor -s).
-s Re-execute the command without invoking an editor.
first
last Select the commands to list or edit. The number of pre-
vious commands that can be accessed are determined by the
value of the HISTSIZE variable. The value of first or
last or both are one of the following:
[+]number
A positive number representing a command number; command
numbers can be displayed with the -l option.
-number
A negative decimal number representing the command that
was executed number of commands previously. For example,
-1 is the immediately previous command.
string A string indicating the most recently entered command
that begins with that string. If the old=new operand is
not also specified with -s, the string form of the first
operand cannot contain an embedded equal sign.
The following environment variables affect the execution of fc:
FCEDIT Name of the editor to use.
HISTSIZE
The number of previous commands that are accessible.
fg [job]
Move the specified job or the current job to the foreground.
getopts optstring var
The POSIX getopts command. The getopts command deprecates the
older getopt command. The first argument should be a series of
letters, each possibly followed by a colon which indicates that
the option takes an argument. The specified variable is set to
the parsed option. The index of the next argument is placed into
the shell variable OPTIND. If an option takes an argument, it is
placed into the shell variable OPTARG. If an invalid option is
encountered, var is set to '?'. It returns a false value (1)
when it encounters the end of the options.
hash -rv command ...
The shell maintains a hash table which remembers the locations of
commands. With no arguments whatsoever, the hash command prints
out the contents of this table. Entries which have not been
looked at since the last cd command are marked with an asterisk;
it is possible for these entries to be invalid.
With arguments, the hash command removes the specified commands
from the hash table (unless they are functions) and then locates
them. With the -v option, hash prints the locations of the com-
mands as it finds them. The -r option causes the hash command to
delete all the entries in the hash table except for functions.
jobid [job]
Print the process id's of the processes in the job. If the job
argument is omitted, use the current job.
jobs This command lists out all the background processes which are
children of the current shell process.
pwd Print the current directory. The builtin command may differ from
the program of the same name because the builtin command remem-
bers what the current directory is rather than recomputing it
each time. This makes it faster. However, if the current direc-
tory is renamed, the builtin version of pwd will continue to
print the old name for the directory.
read [ -p prompt ] [ -t timeout ] [ -e ] variable ...
The prompt is printed if the -p option is specified and the stan-
dard input is a terminal. Then a line is read from the standard
input. The trailing newline is deleted from the line and the
line is split as described in the section on word splitting
above, and the pieces are assigned to the variables in order. If
there are more pieces than variables, the remaining pieces (along
with the characters in IFS that separated them) are assigned to
the last variable. If there are more variables than pieces, the
remaining variables are assigned the null string.
If the -t option is specified the timeout elapses before any in-
put is supplied, the read command will return without assigning
any values. The timeout value may optionally be followed by one
of 's', 'm' or 'h' to explicitly specify seconds, minutes or or
hours. If none is supplied, 's' is assumed.
The -e option causes any backslashes in the input to be treated
specially. If a backslash is followed by a newline, the back-
slash and the newline will be deleted. If a backslash is fol-
lowed by any other character, the backslash will be deleted and
the following character will be treated as though it were not in
IFS, even if it is.
readonly name ...
The specified names are marked as read only, so that they cannot
be subsequently modified or unset. The shell allows the value of
a variable to be set at the same time it is marked read only by
writing using the following form
readonly name=value
With no arguments the readonly command lists the names of all
read only variables.
set [ { -options | +options | -- } ] arg ...
The set command performs three different functions.
With no arguments, it lists the values of all shell variables.
If options are given, it sets the specified option flags, or
clears them as described in the section called ``Argument List
Processing''.
The third use of the set command is to set the values of the
shell's positional parameters to the specified args. To change
the positional parameters without changing any options, use
``--'' as the first argument to set. If no args are present, the
set command will clear all the positional parameters (equivalent
to executing ``shift $#''.
setvar variable value
Assigns value to variable. (In general it is better to write
variable=value rather than using setvar. Setvar is intended to
be used in functions that assign values to variables whose names
are passed as parameters.)
shift [n]
Shift the positional parameters n times. A shift sets the value
of $1 to the value of $2, the value of $2 to the value of $3, and
so on, decreasing the value of $# by one. If there are zero po-
sitional parameters, shifting doesn't do anything.
trap [action] signal ...
Cause the shell to parse and execute action when any of the spec-
ified signals are received. The signals are specified by signal
number. Action may be null or omitted; the former causes the
specified signal to be ignored and the latter causes the default
action to be taken. When the shell forks off a subshell, it re-
sets trapped (but not ignored) signals to the default action.
The trap command has no effect on signals that were ignored on
entry to the shell.
type [name] ...
Interpret each name as a command and print the resolution of the
command search. Possible resolutions are: shell keyword, alias,
shell builtin, command, tracked alias and not found. For aliases
the alias expansion is printed; for commands and tracked aliases
the complete pathname of the command is printed.
ulimit [-HSacdflmnust] [limit]
Set or display resource limits (see getrlimit(2)). If ``limit''
is specified, the named resource will be set; otherwise the cur-
rent resource value will be displayed.
If ``-H'' is specified, the hard limits will be set or displayed.
While everybody is allowed to reduce a hard limit, only the supe-
ruser can increase it. Option ``-S'' specifies the soft limits
instead. When displaying limits, only one of ``-S'' or ``-H''
can be given. The default is to display the soft limits, and to
set both, the hard and the soft limits.
Option ``-a'' requests to display all resources. The parameter
``limit'' is not acceptable in this mode.
The remaining options specify which resource value is to be dis-
played or modified. They are mutually exclusive.
-c coredumpsize
The maximal size of core dump files, in 512-byte blocks.
-d datasize
The maximal size of the data segment of a process, in
kilobytes.
-f filesize
The maximal size of a file, in 512-byte blocks. This is
the default.
-l lockedmem
The maximal size of memory that can be locked by a pro-
cess, in kilobytes.
-m memoryuse
The maximal resident set size of a process, in kilobytes.
-n nofiles
The maximal number of descriptors that could be opened by
a process.
-s stacksize
The maximal size of the stack segment, in kilobytes.
-t time
The maximal amount of CPU time to be used by each pro-
cess, in seconds.
-u userproc
The maximal number of simultaneous processes for this us-
er ID.
umask [mask]
Set the value of umask (see umask(2)) to the specified octal
value. If the argument is omitted, the umask value is printed.
unalias [-a] [name]
If ``name'' is specified, the shell removes that alias. If
``-a'' is specified, all aliases are removed.
unset name ...
The specified variables and functions are unset and unexported.
If a given name corresponds to both a variable and a function,
both the variable and the function are unset.
wait [job]
Wait for the specified job to complete and return the exit status
of the last process in the job. If the argument is omitted, wait
for all jobs to complete and the return an exit status of zero.
Commandline Editing
When sh is being used interactively from a terminal, the current command
and the command history (see fc in Builtins) can be edited using vi-mode
commandline editing. This mode uses commands similar to a subset of
those described in the vi man page. The command 'set -o vi' enables vi-
mode editing and places sh into vi insert mode. With vi-mode enabled, sh
can be switched between insert mode and command mode by typing <ESC>.
Hitting <return> while in command mode will pass the line to the shell.
Similarly, the 'set -o emacs' command can be used to enable a subset of
emacs-style commandline editing features.
SEE ALSO
expr(1), test(1)
HISTORY
A sh command appeared in AT&T System V.1 UNIX.
4th Berkeley Distribution May 5, 1995 1
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