*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->FreeBSD man pages -> sh (1)              



NAME    [Toc]    [Back]

     sh -- command interpreter (shell)

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     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 version
 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.  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    [Toc]    [Back]
     The shell is a command that reads lines from either a file or the terminal,
 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 implements
 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 interpretative
 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

   Invocation    [Toc]    [Back]
     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 considered
 an interactive shell.  An interactive shell generally prompts
     before each command and handles programming and command errors differently
 (as described below).  When first starting, the shell inspects
     argument 0, and if it begins with a dash (-), the shell is also considered
 a login shell.  This is normally done automatically by the system
     when the user first logs in.  A login shell first reads commands from the
     files /etc/profile and 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    [Toc]    [Back]
     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.  Once
     the shell is running, the long name for an option may be specified as an
     argument to the -/+o option of the set built-in command (described later
     in the section called Built-in 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.

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

     -E emacs
	     Enable the built-in 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 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.

     -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 physical
	     Change the default for the cd and pwd commands from -L (logical
	     directory layout) to -P (physical directory layout).

     -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

     -T trapsasync
	     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.

     -V vi   Enable the built-in 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.  Useful
 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 single
 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 built-in emacs(1) command line editor:

	   set -E
	   set -o emacs

     If used without an argument, the -o option displays the current option
     settings in a human-readable format.  If +o is used without an argument,
     the current option settings are output in a format suitable for re-input
     into the shell.

   Lexical Structure    [Toc]    [Back]
     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 characters
 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

     Control operators:
		   &	 &&    (     )	   \n
		   ;;	 ;     |     ||

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

   Quoting    [Toc]    [Back]
     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 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 dollarsign ($), backquote (`),
	     and backslash (\).  The backslash inside double quotes is historically
 weird.  It remains literal unless it precedes the following
 characters, which it serves to quote:
		   $	 `     "     \	   \n

	     A backslash preserves the literal meaning of the following character,
 with the exception of the newline character (\n).  A backslash
 preceding a newline is treated as a line continuation.

   Reserved Words    [Toc]    [Back]
     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    [Toc]    [Back]
     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

     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 discouraged.

   Commands    [Toc]    [Back]
     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 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    [Toc]    [Back]
     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.  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 command
 name and the command is located.  The remaining words are considered
 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    [Toc]    [Back]
     Redirections are used to change where a command reads its input or sends
     its output.  In general, redirections open, close, or duplicate an existing
 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

	   [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

     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 operator
 is ``<<-'' instead of ``<<'', then leading tabs in the here-doc-text
     are stripped.

   Search and Execution    [Toc]    [Back]
     There are three types of commands: shell functions, built-in 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 environment
 of the command (by placing assignments to them before the function
 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-in commands are executed internally to the shell, without
     spawning a new process.

     Otherwise, if the command name does not match a function or built-in command,
 the command is searched for as a normal program in the file system
     (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

     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    [Toc]    [Back]
     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 performing
 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    [Toc]    [Back]
     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 normal
 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 is terminated by a signal, its exit status is 128 plus the
     signal number.  Signal numbers are defined in the header file

   Complex Commands    [Toc]    [Back]
     Complex commands are combinations of simple commands with control operators
 or reserved words, together creating a larger complex command.  More
     generally, a command is one of the following:

	   simple command


	   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    [Toc]    [Back]
     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 specified
 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.  Otherwise,
 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
     below in the section called Short-Circuit List Operators) to be executed
     sequentially; an ``&'' causes asynchronous execution of the preceding

     Note that unlike some other shells, sh executes each process in the pipeline
 as a child of the sh process.  Shell built-in commands are the
     exception 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)    [Toc]    [Back]
     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 proceeds onto the next command; otherwise it
     waits for the command to terminate before proceeding to the next one.

   Short-Circuit List Operators    [Toc]    [Back]
     ``&&'' 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 command
 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]

     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.  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 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
     described later), separated by ``|'' characters.

   Grouping Commands Together    [Toc]    [Back]
     Commands may be grouped by writing either



	   { list; }

     The first form executes the commands in a subshell.  Note that built-in
     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    [Toc]    [Back]
     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 built-in 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 surrounding
 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 built-in command.

   Variables and Parameters    [Toc]    [Back]
     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 environment
 variables into shell variables.  New variables can be set using the


     Variables set by the user must have a name consisting solely of alphabetics,
 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    [Toc]    [Back]
     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 built-in
     command can also be used to set or reset them.

   Special Parameters    [Toc]    [Back]
     A special parameter is a parameter denoted by one of the following special
 characters.  The value of the parameter is listed next to its character.

     *	     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

     @	     Expands to the positional parameters, starting from one.  When
	     the expansion occurs within double-quotes, each positional 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 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 invocation,
 by the set built-in 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    [Toc]    [Back]
     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 doublequotes,
 as was described above.

     The order of word expansion is:

     1.   Tilde Expansion, Parameter Expansion, Command Substitution, Arithmetic
 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

   Parameter Expansion    [Toc]    [Back]
     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
 expansions, 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
     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.

	     Use Default Values.  If parameter is unset or null, the expansion
	     of word is substituted; otherwise, the value of parameter is substituted.

	     Assign Default Values.  If parameter is unset or null, the expansion
 of word is assigned to parameter.  In all cases, the final
	     value of parameter is substituted.  Only variables, not positional
 parameters or special parameters, can be assigned in this

	     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.

	     Use Alternate Value.  If parameter is unset or null, null is substituted;
 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.

	     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 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 varieties
 of pattern characters to be quoted, whereas quoting characters
     within the braces has this effect.

	     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

	     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.

	     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

	     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    [Toc]    [Back]
     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 the backquoted version:


     The shell expands the command substitution by executing command in a subshell
 environment and replacing the command substitution with the standard
 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    [Toc]    [Back]
     Arithmetic expansion provides a mechanism for evaluating an arithmetic
     expression and substituting its value.  The format for arithmetic expansion
 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.

   White Space Splitting (Field Splitting)    [Toc]    [Back]
     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

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

   Pathname Expansion (File Name Generation)    [Toc]    [Back]
     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, separated
 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 command.

   Shell Patterns    [Toc]    [Back]
     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

     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 exclamation
 point (!) the first character of the character class.

     To include a ``]'' in a character class, make it the first character
     listed (after the ``!'', if any).	To include a ``-'', make it the first
     or last character listed.

   Built-in Commands    [Toc]    [Back]
     This section lists the commands which are built-in because they need to
     perform some operation that cannot be performed by a separate process.
     In addition to these, a built-in version of the test(1) command is 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 directory.

     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 built-in
	     command prints the names and values of all defined aliases (see
	     unalias).	Alias values are written with appropriate quoting so
	     that they are suitable for re-input to the shell.

     bg [job ...]
	     Continue the specified jobs (or the current job if no jobs are
	     given) in the background.

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

     bind [-aeklrsv] [key [command]]
	     List or alter key bindings for the line editor.  This command is
	     documented in editrc(5).

     cd [-L | -P] [directory]
	     Switch to the specified directory, or to the directory specified
	     in the HOME environment variable if no directory is specified.
	     If directory does not begin with /, ., or .., then the directories
 listed in the CDPATH variable will be searched for the specified
 directory.  If CDPATH is unset, the current directory is
	     searched.	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.

	     If the -P option is specified, .. is handled physically and symbolic
 links are resolved before .. components are processed.  If
	     the -L option is specified, .. is handled logically.  This is the

     chdir   A synonym for the cd built-in command.

     command [-p] [utility [argument ...]]
	     Execute the specified utility as a simple command (see the Simple
	     Commands section).

	     If the -p option is specified, the command search is performed
	     using a default value of PATH that is guaranteed to find all of
	     the standard utilities.

     echo [-e | -n] [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 understands
 the following character escapes:

		     \a      Alert (ring the terminal bell)

		     \b      Backspace

		     \c      Suppress the trailing newline (this has the sideeffect
 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"
			   $ echo -e a\\vb
			   $ echo -e "a\\b"
			   $ echo -e a\\\\b

	     Only one of the -e and -n options may be specified.

     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
	     built-in command 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 [-p] [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 variable
 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.  If the -p option is specified, the exported
	     variables are printed as ``export name=value'' lines, suitable
	     for re-input to the shell.

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

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

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


	     last    Select the commands to list or edit.  The number of previous
 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

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

		     string  A string indicating the most recently entered
			     command that begins with that string.  If the
			     old=new operand is not also specified with -s,
			     the string form of the first operand cannot contain
 an embedded equal sign.

	     The following environment variables affect the execution of fc:

	     FCEDIT  Name of the editor to use.

		     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 commands
 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 [-ls] [job ...]
	     Print information about the specified jobs, or all jobs if no job
	     argument is given.  The information printed includes job ID, status
 and command name.

	     If the -l option is specified, the PID of each job is also
	     printed.  If the -s option is specified, only the PIDs of the
	     jobs are printed, one per line.

     pwd [-L | -P]
	     Print the path of the current directory.  The built-in command
	     may differ from the program of the same name because the built-in
	     command remembers what the current directory is rather than
	     recomputing it each time.	This makes it faster.  However, if the
	     current directory is renamed, the built-in version of pwd(1) will
	     continue to print the old name for the directory.

	     If the -P option is specified, symbolic links are resolved.  If
	     the -L option is specified, the shell's notion of the current
	     directory is printed (symbolic links are not resolved).  This is
	     the default.

     read [-p prompt] [-t timeout] [-er] variable ...
	     The prompt is printed if the -p option is specified and the standard
 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 separated
 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 specified.
  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

     readonly [-p] [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.  If the -p option is specified, the readonly
 variables are printed as ``readonly name=value'' lines,
	     suitable for re-input to the shell.

     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 arguments
 follow the ``--'' option, all the positional parameters
	     will be cleared, which is equivalent to executing the command
	     ``shift $#''.  The ``--'' flag may be omitted 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

	     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 name or number.
	     In addition, the pseudo-signal EXIT may be used to specify an
	     action that is performed when the shell terminates.  The action
	     may be null or omitted; the former causes the specified signal to
	     be ignored and the latter causes the default action to be taken.
	     When the shell forks off a subshell, it 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 built-in 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 [-HSabcdflmnstuv] [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 superuser
 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 displayed
 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

	     -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.

	     -v virtualmem
		     The maximal virtual size of a process, in kilobytes.

     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 [-fv] name ...
	     The specified variables or functions are unset and unexported.
	     If the -v option is specified or no options are given, the name
	     arguments are treated as variable names.  If the -f option is
	     specified, the name arguments are treated as function names.

     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    [Toc]    [Back]
     When sh is being used interactively from a terminal, the current command
     and the command history (see fc in Built-in Commands) can be edited using
     vi-mode command line editing.  This mode uses commands similar to a subset
 of those des

 Similar pages
Name OS Title
sh Tru64 Shell, the standard command language interpreter (POSIX Shell)
Rsh Tru64 The Bourne shell, an interactive command interpreter and command programming language
sh Tru64 The Bourne shell, an interactive command interpreter and command programming language
csh Tru64 C shell command interpreter
source OpenBSD a shell (command interpreter) with C-like syntax
csh OpenBSD a shell (command interpreter) with C-like syntax
suspend OpenBSD a shell (command interpreter) with C-like syntax
bg OpenBSD a shell (command interpreter) with C-like syntax
stop OpenBSD a shell (command interpreter) with C-like syntax
pushd OpenBSD a shell (command interpreter) with C-like syntax
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service