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

FreeBSD Manual Pages


home | help
SH(1)			  BSD General Commands Manual			 SH(1)

     sh	-- command interpreter (shell)

     sh	[-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
	[+o option_name] [command_file [argument ...]]
     sh	-c [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
	[+o option_name] command_string	[command_name [argument	...]]
     sh	-s [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
	[+o option_name] [argument ...]

     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 POSIX
     1003.2 and	1003.2a	specifications 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 (see ksh(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 or a complete
     specification of the shell.

     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.

     If	no arguments are present and if	the standard input of the shell	is
     connected to a terminal (or if the	-i flag	is set), and the -c option is
     not present, the shell is considered an interactive shell.	 An interac-
     tive shell	generally prompts before each command and handles programming
     and command errors	differently (as	described below).  When	first start-
     ing, the shell inspects argument 0, and if	it begins with a dash `-', the
     shell is also considered a	login shell.  This is normally done automati-
     cally 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 vari-
     able to some file,	place the following line in your .profile of your home

	   ENV=$HOME/.shinit; export ENV

     substituting for ".shinit"	any filename you wish.	Since the ENV file is
     read for every invocation of the shell, including shell scripts and non-
     interactive shells, the following paradigm	is useful for restricting com-
     mands in the ENV file to interactive invocations.	Place commands within
     the "case"	and "esac" below (these	commands are described later):

	   case	$- in *i*)
		 # commands for	interactive use	only

     If	command	line 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.

   Argument List Processing
     All of the	single letter options have a corresponding name	that can be
     used as an	argument to the	-o option.  The	set -o name is provided	next
     to	the single letter option in the	description below.  Specifying a dash
     "-" turns the option on, while using a plus "+" disables the option.  The
     following options can be set from the command line	or with	the set	built-
     in	(described later).

	   -a allexport	    Export all variables assigned to.

	   -c		    Read commands from the command_string operand in-
			    stead of from the standard input.  Special parame-
			    ter	0 will be set from the command_name operand
			    and	the positional parameters ($1, $2, etc.)  set
			    from the remaining argument	operands.

	   -C noclobber	    Don't overwrite existing files with	">".

	   -e errexit	    If not interactive,	exit immediately if any
			    untested command fails.  The exit status of	a com-
			    mand is considered to be explicitly	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.

	   -n noexec	    If not interactive,	read commands but do not exe-
			    cute them.	This is	useful for checking the	syntax
			    of shell scripts.

	   -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.

	   -v verbose	    The	shell writes its input to standard error as it
			    is read.  Useful for debugging.

	   -x xtrace	    Write each command to standard error (preceded by
			    a `+ ') before it is executed.  Useful for debug-

	   -q quietprofile  If the -v or -x options have been set, do not ap-
			    ply	them when reading initialization files,	these
			    being /etc/profile,	.profile, and the file speci-
			    fied by the	ENV environment	variable.

	   -I ignoreeof	    Ignore EOFs	from input when	interactive.

	   -i interactive   Force the shell to behave interactively.

	   -m monitor	    Turn on job	control	(set automatically when	inter-

	   -s stdin	    Read commands from standard	input (set automati-
			    cally if no	file arguments are present).  This op-
			    tion has no	effect when set	after the shell	has
			    already started running (i.e. with set).

	   -V vi	    Enable the built-in	vi(1) command line editor
			    (disables -E if it has been	set).  (See the
			    Command Line Editing section below.)

	   -E emacs	    Enable the built-in	emacs style command line edi-
			    tor	(disables -V if	it has been set).  (See	the
			    Command Line Editing section below.)

	   -b notify	    Enable asynchronous	notification of	background job
			    completion.	 (Not implemented.)

	      cdprint	    Make an interactive	shell always print the new di-
			    rectory name when changed by the cd	command.

	      tabcomplete   Enables filename completion	in the command line
			    editor.  Typing a tab character will extend	the
			    current input word to match	a filename.  If	more
			    than one filename matches it is only extended to
			    be the common prefix.  Typing a second tab charac-
			    ter	will list all the matching names.  One of the
			    editing modes, either -E or	-V, must be enabled
			    for	this to	work.

   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).  Following is	a list of operators:

	   Control operators:
		 & && (	) ; ;; | || <newline>

	   Redirection operators:
		 < > >|	<< >> <& >& <<-	<>

     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.

     A backslash preserves the literal meaning of the following	character,
     with the exception	of <newline>.  A backslash preceding a <newline> is
     treated as	a line continuation.

   Single Quotes
     Enclosing characters in single quotes preserves the literal meaning of
     all the characters	(except	single quotes, making it impossible to put
     single-quotes in a	single-quoted string).

   Double Quotes
     Enclosing characters within double	quotes preserves the literal meaning
     of	all characters except dollar sign ($), backquote (`), and backslash
     (\).  The backslash inside	double quotes is historically weird, and
     serves to quote only the following	characters:
	   $ ` " \ <newline>.
     Otherwise it remains literal.

   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:

	   !	   elif	   fi	   while   case
	   else	   for	   then	   {	   }
	   do	   done	   until   if	   esac

     Their meaning is discussed	later.

     An	alias is a name	and corresponding value	set using the alias built-in
     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-

     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 POSIX 1003.2 document).  Essentially though, a line	is
     read and if the first word	of the line (or	after a	control	operator) 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

	   1.	Leading	words of the form "name=value" are stripped off	and
		assigned to the	environment of the simple command.  Redirect-
		ion 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 "Expansions", and the first remaining word is consid-
		ered the command name and the command is located.  The remain-
		ing words are considered the arguments of the command.	If no
		command	name resulted, then the	"name=value" variable assign-
		ments recognized in item 1 affect the current shell.

	   3.	Redirections are performed as described	in the next section.

     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.
     Following is a list of the	possible redirections.	The [n]	is an optional
     number, as	in `3' (not `[3]'), that refers	to a file descriptor.

	   [n]>	file   Redirect	standard output	(or n) to file.

	   [n]>| file  Same, but override the -C option.

	   [n]>> file  Append standard output (or n) to	file.

	   [n]<	file   Redirect	standard input (or n) from file.

	   [n1]<&n2    Duplicate standard input	(or n1)	from file descriptor

	   [n]<&-      Close standard input (or	n).

	   [n1]>&n2    Duplicate standard output (or n1) to n2.

	   [n]>&-      Close standard output (or n).

	   [n]<> file  Open file for reading and writing on standard input (or

     The following redirection is often	called a "here-document".

	   [n]<< delimiter
		 here-doc-text ...

     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 operator
     is	"<<-" instead of "<<", then leading tabs in the	here-doc-text are

   Search and Execution
     There are three types of commands:	shell functions, built-in commands,
     and normal	programs -- and	the command is searched	for (by	name) in that
     order.  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
     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 built-ins are executed internally to	the shell, without spawning a
     new process.

     Otherwise,	if the command name doesn't match a function or	built-in, the
     command is	searched for as	a normal program in the	file system (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(2)	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	com-
     mands 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 built-in command by that
     name.  If a built-in 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	built-in commands return exit codes,
     as	does an	executed shell function.

     If	a command consists entirely of variable	assignments then the exit sta-
     tus of the	command	is that	of the last command substitution if any, oth-
     erwise 0.

   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:

     +o	 simple	command

     +o	 pipeline

     +o	 list or compound-list

     +o	 compound command

     +o	 function definition

     Unless otherwise stated, the exit status of a command is that of the last
     simple command executed by	the command.

     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

     Because pipeline assignment of standard input or standard output or both
     takes place before	redirection, it	can be modified	by redirection.	 For

	   $ 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 built-in, 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 com-
     mand, and then executes the second	command	if and only if the exit	status
     of	the first command is zero.  "||" is similar, but executes the second
     command if	and only if the	exit status of the first command is nonzero.
     "&&" and "||" both	have the same priority.	 Note that these operators are
     left-associative, so "true	|| echo	bar && echo baz" writes	"baz" and
     nothing else.  This is not	the way	it works in C.	Also, if you forget
     the left-hand side	(for example when continuing lines but forgetting to
     use a backslash) it defaults to a true statement.	This behavior is not
     useful and	should not be relied upon.

   Flow-Control	Constructs -- if, while, for, case
     The syntax	of the if command is

	   if list
	   then	list
	   [ elif list
	   then	   list	] ...
	   [ else list ]

     The syntax	of the while command is

	   while list
	   do	list

     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

     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	built-in commands.

     The syntax	of the case command is

	   case	word in
	   pattern) list ;;

     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; }

     The first of these	executes the commands in a subshell.  Built-in com-
     mands grouped into	a (list) will not affect the current shell.  The sec-
     ond form does not fork another shell so is	slightly more efficient.
     Grouping commands together	this way allows	you to redirect	their output
     as	though they were one program:

	   { echo -n " hello " ; echo "	world" ; } > greeting

     Note that "}" must	follow a control operator (here, ";") so that it is
     recognized	as a reserved word and not as another command argument.

     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 built-in command.

     When a variable is	made local, it inherits	the initial value and exported
     and read-only 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	that can be made local is "-".	Making "-" lo-
     cal causes	any shell options that are changed via the set command inside
     the function to be	restored to their original values when the function

     The syntax	of the return command is

	   return [exitstatus]

     It	terminates the currently executing function.  Return is	implemented as
     a built-in	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


     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 command line arguments
     that follow the name of the shell script.	The set	built-in 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-

     *		  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 posi-
		  tional parameter expands as a	separate argument.  If there
		  are no positional parameters,	the expansion of @ generates
		  zero arguments, even when @ is double-quoted.	 What this ba-
		  sically 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 op-
		  tion names concatenated into a string) as specified on invo-
		  cation, by the set built-in command, or implicitly by	the

     $		  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 com-
		  mand 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-

   Parameter Expansion
     The format	for parameter expansion	is as follows:


     where expression consists of all characters until the matching "}".  Any
     "}" escaped by a backslash	or within a quoted string, and characters in
     embedded arithmetic expansions, command substitutions, and	variable ex-
     pansions, are not examined	in determining the matching "}".

     The simplest form for parameter expansion is:


     The value,	if any,	of parameter is	substituted.

     The parameter name	or symbol can be enclosed in braces, which are op-
     tional 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 rules for @.

     In	addition, a parameter expansion	can be modified	by using one of	the
     following formats.	 If the	":" is omitted in the following	modifiers,
     then the expansion	is applied only	to unset parameters, not null ones.

     ${parameter:-word}	   Use Default Values.	If parameter is	unset or null,
			   the expansion of word is substituted; otherwise,
			   the value of	parameter is substituted.

     ${parameter:=word}	   Assign Default Values.  If parameter	is unset or
			   null, the expansion of word is assigned to parame-
			   ter.	 In all	cases, the final value of parameter is
			   substituted.	 Only variables, not positional	param-
			   eters 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 writ-
			   ten to standard error and the shell exits with a
			   nonzero exit	status.	 Otherwise, the	value of pa-
			   rameter is substituted.  An interactive shell need
			   not exit.

     ${parameter:+word}	   Use Alternative Value.  If parameter	is unset or
			   null, null is substituted; otherwise, the expansion
			   of word is substituted.

     ${#parameter}	   String Length.  The length in characters of the
			   value of parameter.

     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	result of the expansion	is un-
     specified.	 Enclosing the full parameter expansion	string in double-
     quotes does not cause the following four varieties	of pattern characters
     to	be quoted, whereas quoting characters within the braces	has this ef-

     ${parameter%word}	   Remove Smallest Suffix Pattern.  The	word is	ex-
			   panded to produce a pattern.	 The parameter expan-
			   sion	then results in	parameter, with	the smallest
			   portion of the suffix matched by the	pattern

     ${parameter%%word}	   Remove Largest Suffix Pattern.  The word is ex-
			   panded to produce a pattern.	 The parameter expan-
			   sion	then results in	parameter, with	the largest
			   portion of the suffix matched by the	pattern

     ${parameter#word}	   Remove Smallest Prefix Pattern.  The	word is	ex-
			   panded to produce a pattern.	 The parameter expan-
			   sion	then results in	parameter, with	the smallest
			   portion of the prefix matched by the	pattern

     ${parameter##word}	   Remove Largest Prefix Pattern.  The word is ex-
			   panded to produce a pattern.	 The parameter expan-
			   sion	then results in	parameter, with	the largest
			   portion of the prefix matched by the	pattern

   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:


     or	("backquoted" version):


     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:


     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.

     Arithmetic	expressions use	a syntax similar to that of the	C language,
     and are evaluated using the `intmax_t' data type (this is an extension to
     POSIX, which requires only	`long' arithmetic).  Shell variables may be
     referenced	by name	inside an arithmetic expression, without needing a "$"

   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-

     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.

     Non-whitespace characters in IFS are treated strictly as parameter	termi-
     nators.  So adjacent non-whitespace IFS characters	will produce empty pa-

     If	IFS is unset it	is assumed to contain space, tab, and newline.

   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 com-

   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-

     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 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 mi-
     nus sign.	The character class may	be complemented	by making an exclama-
     tion 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.

     This section lists	the built-in commands which are	built-in because they
     need to perform some operation that can't be performed by a separate
     process.  In addition to these, there are several other commands that may
     be	built in 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

     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 built-in 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 [-p] [-v] [-V] command [arg ...]
	    Execute the	specified command but ignore shell functions when
	    searching for it.  (This is	useful when you	have a shell function
	    with the same name as a built-in command.)

	    -p	   search for command using a PATH that	guarantees to find all
		   the standard	utilities.

	    -V	   Do not execute the command but search for the command and
		   print the resolution	of the command search.	This is	the
		   same	as the type built-in.

	    -v	   Do not execute the command but search for the command and
		   print the absolute pathname of utilities, the name for
		   built-ins or	the expansion of aliases.

     cd	[-P] [directory	[replace]]
	    Switch to the specified directory (default $HOME).	If replace is
	    specified, then the	new directory name is generated	by replacing
	    the	first occurrence of directory in the current directory name
	    with replace.  If directory	is `-',	then the current working di-
	    rectory is changed to the previous current working directory as
	    set	in OLDPWD.  Otherwise 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 begin with a slash,	or its first
	    (or	only) component	isn't dot or dot dot, then the directories
	    listed in CDPATH will be searched for the specified	directory.
	    The	format of CDPATH is the	same as	that of	PATH.

	    The	-P option instructs the	shell to update	PWD with the specified
	    physical directory path and	change to that directory.  This	is the

	    When the directory changes,	the variable OLDPWD is set to the
	    working directory before the change.

	    Some shells	also support a -L option, which	instructs the shell to
	    update PWD with the	logical	path and to change the current direc-
	    tory accordingly.  This is not supported.

	    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 exe-
	    cute 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
	    built-in 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 ...

     export -p
	    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 variable
	    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.  With the	-p option specified the	output will be
	    formatted suitably for non-interactive use.

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

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

     fc	-s [old=new] [first]
	    The	fc built-in 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 com-
		   mand	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.


	    last   Select the commands to list or edit.	 The number of previ-
		   ous 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:

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

			  A negative decimal number representing the command
			  that was executed number of commands previously.
			  For example, -1 is the immediately previous command.

		   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	oper-
		   and 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, not to be confused with the Bell	Labs
	    -derived getopt(1).

	    The	first argument should be a series of letters, each of which
	    may	be optionally followed by a colon to indicate that the option
	    requires an	argument.  The variable	specified is set to the	parsed

	    The	getopts	command	deprecates the older getopt(1) utility due to
	    its	handling of arguments containing whitespace.

	    The	getopts	built-in may be	used to	obtain options and their argu-
	    ments from a list of parameters.  When invoked, getopts places the
	    value of the next option from the option string in the list	in the
	    shell variable specified by	var and	its index in the shell vari-
	    able OPTIND.  When the shell is invoked, OPTIND is initialized to
	    1.	For each option	that requires an argument, the getopts built-
	    in will place it in	the shell variable OPTARG.  If an option is
	    not	allowed	for in the optstring, then OPTARG will be unset.

	    optstring is a string of recognized	option letters (see
	    getopt(3)).	 If a letter is	followed by a colon, the option	is ex-
	    pected to have an argument which may or may	not be separated from
	    it by whitespace.  If an option character is not found where ex-
	    pected, getopts will set the variable var to a "?";	getopts	will
	    then unset OPTARG and write	output to standard error.  By specify-
	    ing	a colon	as the first character of optstring all	errors will be

	    A nonzero value is returned	when the last option is	reached.  If
	    there are no remaining arguments, getopts will set var to the spe-
	    cial option, "--", otherwise, it will set var to "?".

	    The	following code fragment	shows how one might process the	argu-
	    ments for a	command	that can take the options [a] and [b], and the
	    option [c],	which requires an argument.

		  while	getopts	abc: f
			  case $f in
			  a | b)  flag=$f;;
			  c)	  carg=$OPTARG;;
			  \?)	  echo $USAGE; exit 1;;
		  shift	$(expr $OPTIND - 1)

	    This code will accept any of the following as equivalent:

		  cmd -acarg file file
		  cmd -a -c arg	file file
		  cmd -carg -a file file
		  cmd -a -carg -- file file

     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.

     inputrc file
	    Read the file to set keybindings as	defined	by editrc(5).

     jobid [job]
	    Print the process id's of the processes in the job.	 If the	job
	    argument is	omitted, the current job is used.

     jobs   This command lists out all the background processes	which are
	    children of	the current shell process.

     pwd [-LP]
	    Print the current directory.  If -L	is specified the cached	value
	    (initially set from	PWD) is	checked	to see if it refers to the
	    current directory; if it does the value is printed.	 Otherwise the
	    current directory name is found using getcwd(3).  The environment
	    variable PWD is set	to the printed value.

	    The	default	is pwd -L, but note that the built-in cd command
	    doesn't currently support the -L option and	will cache (almost)
	    the	absolute path.	If cd is changed, pwd may be changed to	de-
	    fault to pwd -P.

	    If the current directory is	renamed	and replaced by	a symlink to
	    the	same directory,	or the initial PWD value followed a symbolic
	    link, then the cached value	may not	be the absolute	path.

	    The	built-in command may differ from the program of	the same name
	    because the	program	will use PWD and the built-in uses a sepa-
	    rately cached value.

     read [-p prompt] [-r] 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.  The read built-in will
	    indicate success unless EOF	is encountered on input, in which case
	    failure is returned.

	    By default,	unless the -r option is	specified, the backslash "\"
	    acts as an escape character, causing the following character to be
	    treated literally.	If a backslash is followed by a	newline, the
	    backslash and the newline will be deleted.

     readonly name ...

     readonly -p
	    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

		  readonly name=value

	    With no arguments the readonly command lists the names of all read
	    only variables.  With the -p option	specified the output will be
	    formatted suitably for non-interactive use.

     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

	    The	third use of the set command is	to set the values of the
	    shell's positional parameters to the specified arguments.  To
	    change the positional parameters without changing any options, use
	    "--" as the	first argument to set.	If no arguments	are present,
	    the	set command will clear all the positional parameters (equiva-
	    lent 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 posi-
	    tional parameters, shift does nothing.

     trap [-l]

     trap [action] signal ...
	    Cause the shell to parse and execute action	when any of the	speci-
	    fied signals are received.	The signals are	specified by signal
	    number or as the name of the signal.  If signal is 0 or its	equiv-
	    alent, EXIT, the action is executed	when the shell exits.  action
	    may	be null, which cause the specified signals to be ignored.
	    With action	omitted	or set to `-' the specified signals are	set to
	    their default action.  When	the shell forks	off a subshell,	it re-
	    sets trapped (but not ignored) signals to the default action.  On
	    non-interactive shells, the	trap command has no effect on signals
	    that were ignored on entry to the shell.  On interactive shells,
	    the	trap command will catch	or reset signals ignored on entry.
	    Issuing trap with option -l	will print a list of valid signal
	    names.  trap without any arguments cause it	to write a list	of
	    signals and	their associated action	to the standard	output in a
	    format that	is suitable as an input	to the shell that achieves the
	    same trapping results.



	    List trapped signals and their corresponding action

		  trap -l

	    Print a list of valid signals

		  trap '' INT QUIT tstp	30

	    Ignore signals INT QUIT TSTP USR1

		  trap date INT

	    Print date upon receiving signal INT

     type [name	...]
	    Interpret each name	as a command and print the resolution of the
	    command search.  Possible resolutions are: shell keyword, alias,
	    shell built-in, 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 [-H	| -S] [-a | -tfdscmlpnv	[value]]
	    Inquire about or set the hard or soft limits on processes or set
	    new	limits.	 The choice between hard limit (which no process is
	    allowed to violate,	and which may not be raised once it has	been
	    lowered) and soft limit (which causes processes to be signaled but
	    not	necessarily killed, and	which may be raised) is	made with
	    these flags:

	    -H		set or inquire about hard limits

	    -S		set or inquire about soft limits.  If neither -H nor
			-S is specified, the soft limit	is displayed or	both
			limits are set.	 If both are specified,	the last one

	    The	limit to be interrogated or set, then, is chosen by specifying
	    any	one of these flags:

	    -a		show all the current limits

	    -b		show or	set the	limit on the socket buffer size	of a
			process	(in bytes)

	    -t		show or	set the	limit on CPU time (in seconds)

	    -f		show or	set the	limit on the largest file that can be
			created	(in 512-byte blocks)

	    -d		show or	set the	limit on the data segment size of a
			process	(in kilobytes)

	    -s		show or	set the	limit on the stack size	of a process
			(in kilobytes)

	    -c		show or	set the	limit on the largest core dump size
			that can be produced (in 512-byte blocks)

	    -m		show or	set the	limit on the total physical memory
			that can be in use by a	process	(in kilobytes)

	    -l		show or	set the	limit on how much memory a process can
			lock with mlock(2) (in kilobytes)

	    -p		show or	set the	limit on the number of processes this
			user can have at one time

	    -n		show or	set the	limit on the number of files a process
			can have open at once

	    -v		show or	set the	limit on how large a process address
			space can be

	    If none of these is	specified, it is the limit on file size	that
	    is shown or	set.  If value is specified, the limit is set to that
	    number; otherwise the current limit	is displayed.

	    Limits of an arbitrary process can be displayed or set using the
	    sysctl(8) utility.

     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 then return an	exit status of zero.

   Command Line	Editing
     When sh is	being used interactively from a	terminal, the current command
     and the command history (see fc in	Built-ins) can be edited using emacs-
     mode or vi-mode command-line editing.  The	command	`set -o	emacs' enables
     emacs-mode	editing.  The command `set -o vi' enables vi-mode editing and
     places the	current	shell process into vi insert mode.  (See the Argument
     List Processing section above.)

     The vi mode uses commands similar to a subset of those described in the
     vi(1) man page.  With vi-mode enabled, sh can be switched between insert
     mode and command mode.  It's similar to vi(1): pressing the <ESC> key
     will throw	you into command VI command mode.  Pressing the	<return> key
     while in command mode will	pass the line to the shell.

     The emacs mode uses commands similar to a subset available	in the
     emacs(1) editor.  With emacs-mode enabled,	special	keys can be used to
     modify the	text in	the buffer using the control key.

     sh	uses the editline(3) library.

     HOME	Set automatically by login(1) from the user's login directory
		in the password	file (passwd(5)).  This	environment variable
		also functions as the default argument for the cd built-in.

     PATH	The default search path	for executables.  See the above	sec-
		tion Path Search.

     CDPATH	The search path	used with the cd built-in.

     LANG	The string used	to specify localization	information that al-
		lows users to work with	different culture-specific and lan-
		guage conventions.  See	nls(7).

     MAIL	The name of a mail file, that will be checked for the arrival
		of new mail.  Overridden by MAILPATH.

     MAILCHECK	The frequency in seconds that the shell	checks for the arrival
		of mail	in the files specified by the MAILPATH or the MAIL
		file.  If set to 0, the	check will occur at each prompt.

     MAILPATH	A colon	":" separated list of file names, for the shell	to
		check for incoming mail.  This environment setting overrides
		the MAIL setting.  There is a maximum of 10 mailboxes that can
		be monitored at	once.

     PS1	The primary prompt string, which defaults to "$	 ", unless you
		are the	superuser, in which case it defaults to	"#  ".

     PS2	The secondary prompt string, which defaults to ">  ".

     PS4	Output before each line	when execution trace (set -x) is en-
		abled, defaults	to "+  ".

     IFS	Input Field Separators.	 This is normally set to <space>,
		<tab>, and <newline>.  See the White Space Splitting section
		for more details.

     TERM	The default terminal setting for the shell.  This is inherited
		by children of the shell, and is used in the history editing

     HISTSIZE	The number of lines in the history buffer for the shell.



     Errors that are detected by the shell, such as a syntax error, will cause
     the shell to exit with a non-zero exit status.  If	the shell is not an
     interactive shell,	the execution of the shell file	will be	aborted.  Oth-
     erwise the	shell will return the exit status of the last command exe-
     cuted, or if the exit built-in is used with a numeric argument, it	will
     return the	argument.

     csh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1),
     editline(3), getopt(3), editrc(5),	passwd(5), environ(7), nls(7),

     A sh command appeared in Version 1	AT&T UNIX.  It was, however, unmain-
     tainable so we wrote this one.

     Setuid shell scripts should be avoided at all costs, as they are a	sig-
     nificant security risk.

     PS1, PS2, and PS4 should be subject to parameter expansion	before being

     The characters generated by filename completion should probably be	quoted
     to	ensure that the	filename is still valid	after the input	line has been

BSD				October	4, 2011				   BSD


Want to link to this manual page? Use this URL:

home | help