Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
SH(1)			FreeBSD	General	Commands Manual			 SH(1)

NAME
     sh	-- command interpreter (shell)

SYNOPSIS
     sh	[-/+abCEefIimnpsTuVvx] [-/+o longname] [-c string] [arg	...]

DESCRIPTION
     The sh utility is the standard command interpreter	for the	system.	 The
     current version of	sh is in the process of	being changed to conform with
     the IEEE Std 1003.2 (``POSIX.2'') specification for the shell.  This ver-
     sion has many features which make it appear similar in some respects to
     the Korn shell, but it is not a Korn shell	clone like pdksh(1).  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
     tutorial nor 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 started 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
     builtin history and line editing capabilities.  It	incorporates many fea-
     tures to aid interactive use and has the advantage	that the interpreta-
     tive 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, which can	be executed directly by	the
     shell.

   Invocation
     If	no arguments are present and if	the standard input of the shell	is
     connected to a terminal (or if the	-i option is set), the shell is	con-
     sidered an	interactive shell.  An interactive shell generally prompts
     before each command and handles programming and command errors differ-
     ently (as described below).  When first starting, the shell inspects
     argument 0, and if	it begins with a dash (-), the shell is	also consid-
     ered 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 then .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 then 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.	 The user can set the ENV variable to some
     file by placing the following line	in the file .profile in	the home
     directory,	substituting for .shinit the filename desired:

	   ENV=$HOME/.shinit; export ENV

     The first non-option argument specified on	the command line will be
     treated 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 long name,
     with the exception	of -c and -/+o.	 These long names are provided next to
     the single	letter options in the descriptions below.  The long name for
     an	option may be specified	as an argument to the -/+o option of sh(1).
     Once the shell is running,	the long name for an option may	be specified
     as	an argument to the -/+o	option of the set builtin command (described
     later in the section called Builtin Commands).  Introducing an option
     with a dash (-) enables the option, while using a plus (+)	disables the
     option.  A	``--'' or plain	``-'' will stop	option processing and will
     force the remaining words on the command line to be treated as arguments.
     The -/+o and -c options do	not have long names.  They take	arguments and
     are described after the single letter options.

     -a	allexport
	     Flag variables for	export when assignments	are made to them.

     -b	notify
	     Enable asynchronous notification of background job	completion.
	     (UNIMPLEMENTED)

     -C	noclobber
	     Do	not overwrite existing files with ``>''.  (UNIMPLEMENTED)

     -E	emacs
	     Enable the	builtin	emacs(1) command line editor (disables the -V
	     option if it has been set).

     -e	errexit
	     Exit immediately if any untested command fails in non-interactive
	     mode.  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 in interactive mode.

     -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
	     either 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.  When this mode is
	     enabled for interactive shells, the file /etc/suid_profile	is
	     sourced instead of	~/.profile after /etc/profile is sourced, and
	     the contents of the ENV variable are ignored.

     -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. when set with the set
	     command).

     -T	asynctraps
	     When waiting for a	child, execute traps immediately.  If this
	     option is not set,	traps are executed after the child exits, as
	     specified in IEEE Std 1003.2 (``POSIX.2'')	This nonstandard
	     option is useful for putting guarding shells around children that
	     block signals.  The surrounding shell may kill the	child or it
	     may just return 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	builtin	vi(1) command line 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	(preceded by ``+ '') to	standard error before
	     it	is executed.  Useful for debugging.

     The -c option may be used to pass its string argument to the shell	to be
     interpreted as input.  Keep in mind that this option only accepts a sin-
     gle string	as its argument, hence multi-word strings must be quoted.

     The -/+o option takes as its only argument	the long name of an option to
     be	enabled	or disabled.  For example, the following two invocations of sh
     both enable the builtin emacs(1) command line editor:

	   set -E
	   set -o emacs

   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 called ``operators'', which	are special to the shell.  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.  It remains literal unless it precedes the follow-
	     ing characters, which it serves to	quote:
		   $	 `     "     \	   \n

     Backslash
	     A backslash preserves the literal meaning of the following	char-
	     acter, with the exception of the newline character	(\n).  A back-
	     slash preceding a newline 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 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

     would become

	   ls -F foobar

     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 Std 1003.2 (``POSIX.2'') document).  Essentially
     though, a line is read and	if the first word of the line (or after	a con-
     trol operator) is not a reserved word, then the shell has recognized a
     simple command.  Otherwise, a complex command or some other special con-
     struct 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
	  assigned to the environment of the simple command.  Redirection
	  operators 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
	  Word 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

     The `redir-op' is one of the redirection operators	mentioned previously.
     The following gives some examples of how these operators can be used.
     Note that stdin and stdout	are commonly used abbreviations	for standard
     input and standard	output respectively.

	   [n]>	file	 redirect stdout (or file descriptor n)	to file

	   [n]>| file	 same as above,	but override the -C option

	   [n]>> file	 append	stdout (or file	descriptor n) to file

	   [n]<	file	 redirect stdin	(or file descriptor n) from file

	   [n]<> file	 redirect stdin	(or file descriptor n) to and from
			 file

	   [n1]<&n2	 duplicate stdin (or file descriptor n1) from file
			 descriptor n2

	   [n]<&-	 close stdin (or file descriptor n)

	   [n1]>&n2	 duplicate stdout (or file descriptor n1) to file
			 descriptor n2

	   [n]>&-	 close stdout (or file descriptor 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 Word Expansions).  If the oper-
     ator 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.  The command is searched for (by name) in	that order.
     The three types of	commands are all 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
     given.  Then the command given in the function definition is executed.
     The positional parameters are restored to their original values when the
     command completes.	 This all occurs within	the current shell.

     Shell builtin commands are	executed internally to the shell, without
     spawning a	new process.

     Otherwise,	if the command name does not match a function or builtin com-
     mand, the command is searched for as a normal program in the filesystem
     (as described 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
     "#!", resulting in	an ENOEXEC return value	from execve(2))	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
     zero.

     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
     below in the section called Short-Circuit List Operators) to be executed
     sequentially; an ``&'' causes asynchronous	execution of the preceding
     AND-OR-list.

     Note that unlike some other shells, sh executes each process in the pipe-
     line as a child of	the sh process.	 Shell builtin commands	are the	excep-
     tion to this rule.	 They are executed in the current shell, although they
     do	not affect its environment when	used in	pipelines.

   Background Commands (&)
     If	a command is terminated	by the control operator	ampersand (&), the
     shell executes the	command	asynchronously;	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.	 The do	and done commands may be
     replaced with ``{'' and ``}''.

     The syntax	of the break and continue commands is:
	   break [num]
	   continue [num]

     The break command terminates the num innermost for	or while loops.	 The
     continue command continues	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
     described later), separated by ``|'' characters.

   Grouping Commands Together
     Commands may be grouped by	writing	either

	   (list)

     or

	   { list; }

     The first form executes the commands in a subshell.  Note that builtin
     commands thus executed do not affect the current shell.  The second form
     does not fork another shell, so it	is slightly more efficient.  Grouping
     commands together this way	allows the user	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
     installs 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 the local
     command.  This should appear as the first statement of a function,	and
     the syntax	is:

	   local [variable ...]	[-]

     The local command 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	the variable x is made 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 causes any shell options that are changed via the set command
     inside the	function to be restored	to their original values when the
     function returns.

     The syntax	of the return command is

	   return [exitstatus]

     It	terminates the currently executing function.  The return command 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	letter of a variable name must
     not be numeric.  A	parameter can also be denoted by a number or a special
     character as explained below.

   Positional Parameters
     A positional parameter is a parameter denoted by a	number greater than
     zero.  The	shell sets these initially to the values of its	command	line
     arguments that follow the name of the shell script.  The set builtin com-
     mand 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
	     retains 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 the -f option is in effect).

     4.	  Quote	Removal.

     The ``$'' character is used to introduce parameter	expansion, command
     substitution, 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
     directory.	 If the	username is missing (as	in ~/foobar), the tilde	is
     replaced with the value of	the HOME variable (the current user's home
     directory).

   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 charac-
     ters in embedded arithmetic expansions, command substitutions, and	vari-
     able expansions, 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
     optional except for positional parameters with more than one digit	or
     when parameter 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	the special parameter @.

     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 posi-
	     tional 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 one	of the special parameters * or @, the
     result of the expansion is	unspecified.  Enclosing	the full parameter
     expansion string in double-quotes does not	cause the following four vari-
     eties 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	the 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 newlines at
     the end of	the substitution.  Embedded newlines before the	end of the
     output are	not removed; however, during field splitting, they may be
     translated	into spaces depending on the value of IFS and the 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
     result.

     The shell treats each character of	the IFS	as a delimiter and uses	the
     delimiters	to split the results of	parameter expansion and	command	sub-
     stitution into fields.

   Pathname Expansion (File Name Generation)
     Unless the	-f option 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 com-
     mand.

   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 output 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 character
     class.  The end of	the character class is indicated by a ``]''; if	the
     ``]'' is missing then the ``['' matches a ``['' rather than introducing a
     character class.  A character class matches any of	the characters between
     the square	brackets.  A range of characters may be	specified using	a
     minus sign.  The character	class may be complemented by making an excla-
     mation 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 ``-'', make it the	first
     or	last character listed.

   Builtin Commands
     This section lists	the commands which are builtin because they need to
     perform some operation that cannot	be performed by	a separate process.
     In	addition to these, builtin versions of the printf(1) and test(1) com-
     mands are provided	for efficiency.

     :	     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 ...]

     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 com-
	     mand 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 cmd [arg ...]
	     Execute the specified builtin command, cmd.  This is useful when
	     the user wishes to	override a shell function with the same	name
	     as	a builtin command.

     cd	[directory]
	     Switch to the specified directory,	or to the directory specified
	     in	the HOME environment variable if no directory is specified.
	     If	directory is not found as a subdirectory of the	current	direc-
	     tory (and does not	begin with ``/'', ``./'', or ``../''), then
	     the directories listed in the CDPATH variable 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.

     chdir   A synonym for the cd builtin command.

     echo [-en]	string
	     Print string to the standard output with a	newline	appended.

	     -n	     Suppress the output of the	trailing newline.

	     -e	     Process C-style backslash escape sequences.  echo under-
		     stands the	following character escapes:

		     \a	     Alert (ring the terminal bell)

		     \b	     Backspace

		     \c	     Suppress the trailing newline (this has the side-
			     effect of truncating the line if it is not	the
			     last character)

		     \e	     The ESC character (ASCII 0x1b)

		     \f	     Formfeed

		     \n	     Newline

		     \r	     Carriage return

		     \t	     Horizontal	tab

		     \v	     Vertical tab

		     \\	     Literal backslash

		     \0nnn   (Zero) The	character whose	octal value is nnn

		     If	string is not enclosed in quotes then the backslash
		     itself must be escaped with a backslash to	protect	it
		     from the shell. For example

			   $ echo -e "a\vb"
			   a
			    b
			   $ echo -e a\\vb
			   a
			    b
			   $ echo -e "a\\b"
			   a\b
			   $ echo -e a\\\\b
			   a\b

     eval string ...
	     Concatenate all the arguments with	spaces.	 Then re-parse and
	     execute 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 command or	function).  Any	redirections on	the exec com-
	     mand 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 as	it is exported by writing

		   export name=value

	     With no arguments the export command lists	the names of all
	     exported variables.

     fc	[-e editor] [first [last]]

     fc	-l [-nr] [first	[last]]

     fc	-s [old=new] [first]
	     The fc builtin command lists, or edits and	re-executes, commands
	     previously	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:

		     [+]num  A positive	number representing a command number;
			     command numbers can be displayed with the -l
			     option.

		     -num    A negative	decimal	number representing the	com-
			     mand that was executed num	of commands previ-
			     ously.  For example, -1 is	the immediately	previ-
			     ous 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 con-
			     tain 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(1) 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 each specified command
	     from the hash table (unless they are functions) and then locates
	     it.  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 specified 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 path of the current directory.  The builtin command may
	     differ from the program of	the same name because the builtin com-
	     mand remembers what the current directory is rather than recom-
	     puting it each time.  This	makes it faster.  However, if the cur-
	     rent directory is renamed,	the builtin version of pwd(1) will
	     continue to print the old name for	the directory.

     read [-p prompt] [-t timeout] [-er] 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 White	Space
	     Splitting (Field 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 sepa-
	     rated them) are assigned to the last variable.  If	there are more
	     variables than pieces, the	remaining variables are	assigned the
	     null string.

	     Backslashes are treated specially,	unless the -r option is	speci-
	     fied.  If a backslash is followed by a newline, the backslash and
	     the newline will be deleted.  If a	backslash is followed 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.

	     If	the -t option is specified and the timeout elapses before any
	     input 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	hours.	If none	is supplied, ``s'' is assumed.

	     The -e option exists only for backward compatibility with older
	     scripts.

     readonly name ...
	     Each specified name is marked as read only, so that it cannot be
	     subsequently modified or unset.  The shell	allows the value of a
	     variable to be set	at the same time as it is marked read only by
	     using the following form:

		   readonly name=value

	     With no arguments the readonly command lists the names of all
	     read only variables.

     set [-/+abCEefIimnpTuVvx] [-/+o longname] [-c string] [-- arg ...]
	     The set command performs three different functions:

	     With no arguments,	it lists the values of all shell variables.

	     If	options	are given, either in short form	or using the long
	     ``-/+o longname'' form, it	sets or	clears the specified options
	     as	described in the section called	Argument List Processing.

	     If	the ``--'' option is specified,	set will replace the shell's
	     positional	parameters with	the subsequent arguments.  If no argu-
	     ments follow the ``--'' option, all the positional	parameters
	     will be cleared, which is equivalent to executing the command
	     ``shift $#''.  The	``--'' flag may	be ommitted when specifying
	     arguments to be used as positional	replacement parameters.	 This
	     is	not recommended, because the first argument may	begin with a
	     dash (-) or a plus	(+), which the set command will	interpret as a
	     request to	enable or disable options.

     setvar variable value
	     Assigns the specified value to the	specified variable.  Setvar is
	     intended to be used in functions that assign values to variables
	     whose names are passed as parameters.  In general it is better to
	     write

		   variable=value
	     rather than using setvar.

     shift [n]
	     Shift the positional parameters n times, or once if n is not
	     specified.	 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 positional parameters, shifting
	     does not do anything.

     trap [action] signal ...
	     Cause the shell to	parse and execute action when any specified
	     signal is received.  The signals are specified by signal number.
	     The action	may be null or omitted;	the former causes the speci-
	     fied signal to be ignored and the latter causes the default
	     action to be taken.  When the shell forks off a subshell, it
	     resets 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, 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 [-HSabcdflmnust] [limit]
	     Set or display resource limits (see getrlimit(2)).	 If limit is
	     specified,	the named resource will	be set;	otherwise the current
	     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.  The -S option 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 causes the ulimit command 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.

	     -b	sbsize
		     The maximum size of socket	buffer usage, in bytes.

	     -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.

	     -l	lockedmem
		     The maximal size of memory	that can be locked by a
		     process, 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
		     process, in seconds.

	     -u	userproc
		     The maximal number	of simultaneous	processes for this
		     user ID.

     umask [mask]
	     Set the file creation mask	(see umask(2)) to the octal value
	     specified by mask.	 If the	argument is omitted, the current mask
	     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 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	Builtin	Commands) can be edited	using
     vi-mode command line editing.  This mode uses commands similar to a sub-
     set of those described in the vi man page.	 The command ``set -o vi'' (or
     ``set -V'') 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'' (or ``set -E'') command can be used to
     enable a subset of	emacs-style command line editing features.

SEE ALSO
     builtin(1), echo(1), expr(1), printf(1), pwd(1), test(1)

HISTORY
     A sh command appeared in AT&T System V Release 1 UNIX.

FreeBSD	10.1			  May 5, 1995			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | HISTORY

Want to link to this manual page? Use this URL:
<http://www.freebsd.org/cgi/man.cgi?query=sh&sektion=1&manpath=FreeBSD+4.5-RELEASE>

home | help