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SH(1)			  BSD General Commands Manual			 SH(1)

     sh — command interpreter (shell)

     sh [-/+abCEefIimnPpTuVvx] [-/+o longname] [script [arg ...]]
     sh [-/+abCEefIimnPpTuVvx] [-/+o longname] -c string [name [arg ...]]
     sh [-/+abCEefIimnPpTuVvx] [-/+o longname] -s [arg ...]

     The sh utility is the standard command interpreter for the system.	 The
     current version of sh is in the process of being changed to conform with
     the IEEE Std 1003.2 (“POSIX.2”) specification for the shell.  This ver‐
     sion has many features which make it appear similar in some respects to
     the Korn shell, but it is not a Korn shell clone like pdksh.  Only fea‐
     tures 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.

     The shell is a command that reads lines from either a file or the termi‐
     nal, interprets them, and generally executes other commands.  It is the
     program that is started when a user logs into the system, although a user
     can select a different shell with the chsh(1) command.  The shell imple‐
     ments a language that has flow control constructs, a macro facility that
     provides a variety of features in addition to data storage, along with
     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, which can be executed directly by the

     If no arguments are present and if the standard input of the shell is
     connected to a terminal (or if the -i option is set), the shell is con‐
     sidered an interactive shell.  An interactive shell generally prompts
     before each command and handles programming and command errors differ‐
     ently (as described below).  When first starting, the shell inspects
     argument 0, and if it begins with a dash (‘-’), the shell is also consid‐
     ered a login shell.  This is normally done automatically by the system
     when the user first logs in.  A login shell first reads commands from the
     files /etc/profile and then .profile in a user's home directory, 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

	   ENV=$HOME/.shinit; export ENV

     The first non-option argument specified on the command line will be
     treated as the name of a file from which to read commands (a shell
     script), and the remaining arguments are set as the positional parameters
     of the shell ($1, $2, etc.).  Otherwise, the shell reads commands from
     its standard input.

     Unlike older versions of sh the ENV script is only sourced on invocation
     of interactive shells.  This closes a well-known, and sometimes easily
     exploitable security hole related to poorly thought out ENV scripts.

   Argument List Processing
     All of the single letter options to sh have a corresponding long name,
     with the exception of -c and -/+o.	 These long names are provided next to
     the single letter options in the descriptions below.  The long name for
     an option may be specified as an argument to the -/+o option of sh.  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 explic‐
	     itly tested if the command is part of the list used to control an
	     if, elif, while, or until; if the command is the left hand oper‐
	     and of an “&&” or “||” operator; or if the command is a pipeline
	     preceded by the ! operator.  If a shell function is executed and
	     its exit status is explicitly tested, all commands of the func‐
	     tion are considered to be tested as well.

     -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 command).

     -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, a positional parameter or the special parameter ! that
	     is not set, and if the shell is not interactive, exit immedi‐

     -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.  Use‐
	     ful for debugging.

     -x xtrace
	     Write each command (preceded by the value of the PS4 variable) to
	     standard error before it is executed.  Useful for debugging.

     The -c option causes the commands to be read from the string operand
     instead of from the standard input.  Keep in mind that this option only
     accepts a single string as its argument, hence multi-word strings must be

     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
     The shell reads input in terms of lines from a file and breaks it up into
     words at whitespace (blanks and tabs), and at certain sequences of char‐
     acters called “operators”, which are special to the shell.	 There are two
     types of operators: control operators and redirection operators (their
     meaning is discussed later).  The following is a list of valid operators:

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

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

     The character ‘#’ introduces a comment if used at the beginning of a
     word.  The word starting with ‘#’ and the rest of the line are ignored.

     ASCII NUL characters (character code 0) are not allowed in shell input.

     Quoting is used to remove the special meaning of certain characters or
     words to the shell, such as operators, whitespace, keywords, or alias

     There are three types of quoting: matched single quotes, matched double
     quotes, and backslash.

     Single Quotes
	     Enclosing characters in single quotes preserves the literal mean‐
	     ing of all the characters (except single quotes, making it impos‐
	     sible to put single-quotes in a single-quoted string).

     Double Quotes
	     Enclosing characters within double quotes preserves the literal
	     meaning of all characters except dollar sign (‘$’), 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 char‐
	     acter, with the exception of the newline character (‘\n’).	 A
	     backslash preceding a newline is treated as a line continuation.

     Keywords or 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 keywords:

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

     An alias is a name and corresponding value set using the alias built-in
     command.  Whenever a keyword may occur (see above), and after checking
     for keywords, 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 exam‐
     ple, if there is an alias called “lf” with the value “ls -F”, then the

	   lf foobar

     would become

	   ls -F foobar

     Aliases provide a convenient way for naive users to create shorthands for
     commands without having to learn how to create functions with arguments.
     They can also be used to create lexically obscure code.  This use is dis‐

     An alias name may be escaped in a command line, so that it is not
     replaced by its alias value, by using quoting characters within or adja‐
     cent to the alias name.  This is most often done by prefixing an alias
     name with a backslash to execute a function, built-in, or normal program
     with the same name.  See the Quoting subsection.

     The shell interprets the words it reads according to a language, the
     specification of which is outside the scope of this man page (refer to
     the BNF in the IEEE Std 1003.2 (“POSIX.2”) document).  Essentially
     though, a line is read and if the first word of the line (or after a con‐
     trol operator) is not a keyword, 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.  Redirection operators and
	  their arguments (as described below) are stripped off and saved for

     2.	  The remaining words are expanded as described in the section called
	  Word Expansions, and the first remaining word is considered the com‐
	  mand name and the command is located.	 The remaining words are con‐
	  sidered the arguments of the command.	 If no command name resulted,
	  then the “name=value” variable assignments recognized in 1) affect
	  the current shell.

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

     Redirections are used to change where a command reads its input or sends
     its output.  In general, redirections open, close, or duplicate an exist‐
     ing reference to a file.  The overall format used for redirection is:

	   [n] redir-op file

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

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

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

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

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

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

	   [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 oper‐
     ator 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.  The command is searched for (by name) in that
     order.  The three types of commands are all executed in a different way.

     When a shell function is executed, all of the shell positional parameters
     (except $0, which remains unchanged) are set to the arguments of the
     shell function.  The variables which are explicitly placed in the envi‐
     ronment of the command (by placing assignments to them before the func‐
     tion name) are made local to the function and are set to the values
     given.  Then the command given in the function definition is executed.
     The positional parameters are restored to their original values when the
     command completes.	 This all occurs within the current shell.

     Shell built-in commands are executed internally to the shell, without
     spawning a new process.  There are two kinds of built-in commands: regu‐
     lar and special.  Assignments before special builtins persist after they
     finish executing and assignment errors, redirection errors and certain
     operand errors cause a script to be aborted.  Both regular and special
     builtins can affect the shell in ways normal programs cannot.

     Otherwise, if the command name does not match a function or built-in com‐
     mand, 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
     child (see the description of the hash built-in command below).

     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 the PATH environment variable 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 is terminated by a signal, its exit status is 128 plus the
     signal number.  Signal numbers are defined in the header file

   Complex Commands
     Complex commands are combinations of simple commands with control opera‐
     tors or keywords, together creating a larger complex command.  More gen‐
     erally, 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.

     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 con‐
     nected 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.

     Note that unlike some other shells, sh executes each process in a pipe‐
     line with more than one command in a subshell environment and as a child
     of the sh process.

     If the pipeline is not in the background (discussed later), the shell
     waits for all commands to complete.

     If the keyword ! 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 com‐
     mand.  That is, if the last command returns zero, the exit status is 1;
     if the last command returns greater than zero, the exit status is zero.

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

	   command1 2>&1 | command2

     sends both the standard output and standard error of command1 to the
     standard input of command2.

     A ‘;’ or newline terminator causes the preceding AND-OR-list (described
     below in the section called Short-Circuit List Operators) to be executed
     sequentially; an ‘&’ causes asynchronous execution of the preceding AND-

   Background Commands (&)
     If a command is terminated by the control operator ampersand (‘&’), the
     shell executes the command asynchronously; the shell does not wait for
     the command to finish before executing the next command.

     The format for running a command in background is:

	   command1 & [command2 & ...]

     If the shell is not interactive, the standard input of an asynchronous
     command is set to /dev/null.

   Lists (Generally Speaking)
     A list is a sequence of zero or more commands separated by newlines,
     semicolons, or ampersands, and optionally terminated by one of these
     three characters.	The commands in a list are executed in the order they
     are written.  If command is followed by an ampersand, the shell starts
     the command and immediately proceeds 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 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

     If in and the following words are omitted, in "$@" is used instead.  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 special 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.  The exit code of the case
     command is the exit code of the last command executed in the list or zero
     if no patterns were matched.

   Grouping Commands Together
     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

     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 surround‐
     ing scope, if there is one.  Otherwise, the variable is initially unset.
     The shell uses dynamic scoping, so that if the variable x is made local
     to function f, which then calls function g, references to the variable x
     made inside g will refer to the variable x declared inside f, not to the
     global variable named x.

     The only special parameter that 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 current executional scope, returning from the previous
     nested function, sourced script, or shell instance, in that order.	 The
     return command is implemented as a special 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 letter of a variable name must
     not be numeric.  A parameter can also be denoted by a number or a special
     character as explained below.

   Positional Parameters
     A positional parameter is a parameter denoted by a number greater than
     zero.  The shell sets these initially to the values of its command line
     arguments that follow the name of the shell script.  The set built-in
     command can also be used to set or reset them.

   Special Parameters
     A special parameter is a parameter denoted by a special one-character
     name.  The special parameters recognized by the sh shell of FreeBSD are
     shown in the following list, exactly as they would appear in input typed
     by the user or in the source of a shell script.

     $*	     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 param‐
	     eter expands as a separate argument.  If there are no positional
	     parameters, the expansion of @ generates zero arguments, even
	     when @ is double-quoted.  What this basically means, for example,
	     is if $1 is “abc” and $2 is “def ghi”, then "$@" expands to the
	     two arguments:

		   "abc"   "def ghi"

     $#	     Expands to the number of positional parameters.

     $?	     Expands to the exit status of the most recent pipeline.

     $-	     (hyphen) Expands to the current option flags (the single-letter
	     option names concatenated into a string) as specified on invoca‐
	     tion, by the set 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 script if passed on the
	     command line, the name operand if given (with -c) or otherwise
	     argument 0 passed to the shell.

   Word Expansions
     This clause describes the various expansions that are performed on words.
     Not all expansions are performed on every word, as explained later.

     Tilde expansions, parameter expansions, command substitutions, arithmetic
     expansions, and quote removals that occur within a single word expand to
     a single field.  It is only field splitting or pathname expansion that
     can create multiple fields from a single word.  The single exception to
     this rule is the expansion of the special parameter @ within double-
     quotes, as was described above.

     The order of word expansion is:

     1.	  Tilde Expansion, Parameter Expansion, Command Substitution, Arith‐
	  metic Expansion (these all occur at the same time).

     2.	  Field Splitting is performed on fields generated by step (1) unless
	  the IFS variable is null.

     3.	  Pathname Expansion (unless the -f option is in effect).

     4.	  Quote Removal.

     The ‘$’ character is used to introduce parameter expansion, command sub‐
     stitution, or arithmetic expansion.

   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
     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 sub‐

	     Assign Default Values.  If parameter is unset or null, the expan‐
	     sion of word is assigned to parameter.  In all cases, the final
	     value of parameter is substituted.	 Quoting inside word does not
	     prevent field splitting or pathname expansion.  Only variables,
	     not positional parameters or special parameters, can be assigned
	     in this way.

	     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 sub‐
	     stituted; otherwise, the expansion of word is substituted.

     In the parameter expansions shown previously, use of the colon in the
     format results in a test for a parameter that is unset or null; omission
     of the colon results in a test for a parameter that is only unset.

	     String Length.  The length in characters of the value of

     The following four varieties of parameter expansion provide for substring
     processing.  In each case, pattern matching notation (see Shell
     Patterns), rather than regular expression notation, is used to evaluate
     the patterns.  If parameter is one of the special parameters * or @, the
     result of the expansion is unspecified.  Enclosing the full parameter
     expansion string in double-quotes does not cause the following four vari‐
     eties of pattern characters to be quoted, whereas quoting characters
     within the braces has this effect.

	     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
     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 sub‐
     shell environment and replacing the command substitution with the stan‐
     dard output of the command, removing sequences of one or more newlines at
     the end of the substitution.  Embedded newlines before the end of the
     output are not removed; however, during field splitting, they may be
     translated into spaces depending on the value of IFS and the quoting that
     is in effect.

   Arithmetic Expansion
     Arithmetic expansion provides a mechanism for evaluating an arithmetic
     expression and substituting its value.  The format for arithmetic expan‐
     sion is as follows:


     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, arithmetic expansion and quote removal.

     The allowed expressions are a subset of C expressions, summarized below.

	   Values     All values are of type intmax_t.

	   Constants  Decimal, octal (starting with 0) and hexadecimal (start‐
		      ing with 0x) integer constants.

	   Variables  Shell variables can be read and written and contain
		      integer constants.

	   Unary operators
		      ! ~ + -

	   Binary operators
		      * / % + - << >> < <= > >= == != & ^ | && ||

	   Assignment operators
		      = += -= *= /= %= <<= >>= &= ^= |=

	   Short-circuit evaluation
		      The && and || operators always evaluate both sides.
		      This is a bug.

     The result of the expression is substituted in decimal.

   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

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

   Pathname Expansion (File Name Generation)
     Unless the -f option is set, file name generation is performed after word
     splitting is complete.  Each word is viewed as a series of patterns, sep‐
     arated by slashes.	 The process of expansion replaces the word with the
     names of all existing files whose names can be formed by replacing each
     pattern with a string that matches the specified pattern.	There are two
     restrictions on this: first, a pattern cannot match a string containing a
     slash, and second, a pattern cannot match a string starting with a period
     unless the first character of the pattern is a period.  The next section
     describes the patterns used for both Pathname Expansion and the case com‐

   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
     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
     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, built-in versions of essential utilities are pro‐
     vided 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.  The return command may be used to return to the . com‐
	     mand's caller.  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

     [	     A built-in equivalent of test(1).

     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.	Also see the
	     Aliases subsection.

     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 directo‐
	     ries listed in the CDPATH variable will be searched for the spec‐
	     ified 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 sym‐
	     bolic 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 ...]]

     command [-v | -V] [utility]
	     The first form of invocation executes 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.

	     If the -v option is specified, utility is not executed but a
	     description of its interpretation by the shell is printed.	 For
	     ordinary commands the output is the path name; for shell built-in
	     commands, shell functions and keywords only the name is written.
	     Aliases are printed as “alias name=value”.

	     The -V option is identical to -v except for the output.  It
	     prints “utility is description” where description is either the
	     path name to utility, a special shell builtin, a shell builtin, a
	     shell function, a shell keyword or an alias for value.

     echo [-e | -n] [string ...]
	     Print a space-separated list of the arguments to the standard
	     output and append a newline character.

	     -n	     Suppress the output of the trailing newline.

	     -e	     Process C-style backslash escape sequences.  The echo
		     command understands the following character escapes:

		     \a	     Alert (ring the terminal bell)

		     \b	     Backspace

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

		     \e	     The ESC character (ASCII 0x1b)

		     \f	     Formfeed

		     \n	     Newline

		     \r	     Carriage return

		     \t	     Horizontal tab

		     \v	     Vertical tab

		     \\	     Literal backslash

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

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

			   $ echo -e "a\vb"
			   $ 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 com‐
	     mand are marked as permanent, so that they are not undone when
	     the exec command finishes.

     exit [exitstatus]
	     Terminate the shell process.  If exitstatus is given it is used
	     as the exit status of the shell; otherwise the exit status of the
	     preceding command is used.	 The exit status should be an integer
	     between 0 and 255.

     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 vari‐
	     able to be set at the same time as it is exported by writing

		   export name=value

	     With no arguments the export command lists the names of all
	     exported variables.  If the -p option is specified, the exported
	     variables are printed as “export name=value” lines, suitable for
	     re-input to the shell.

     false   A null command that returns a non-zero (false) exit value.

     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 pre‐
		     vious commands that can be accessed are determined by the
		     value of the HISTSIZE variable.  The value of first or
		     last or both are one of the following:

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

		     -num    A negative decimal number representing the com‐
			     mand that was executed num of commands previ‐
			     ously.  For example, -1 is the immediately previ‐
			     ous command.

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

	     The following environment variables affect the execution of fc:

	     FCEDIT    Name of the editor to use for history editing.

	     HISTSIZE  The number of previous commands that are accessible.

     fg [job]
	     Move the specified job or the current job to the foreground.

     getopts optstring var
	     The POSIX getopts command.	 The getopts command deprecates the
	     older getopt(1) command.  The first argument should be a series
	     of letters, each possibly followed by a colon which indicates
	     that the option takes an argument.	 The specified variable is set
	     to the parsed option.  The index of the next argument is placed
	     into the shell variable OPTIND.  If an option takes an argument,
	     it is placed into the shell variable OPTARG.  If an invalid
	     option is encountered, var is set to ‘?’.	It returns a false
	     value (1) when it encounters the end of the options.

     hash [-rv] [command ...]
	     The shell maintains a hash table which remembers the locations of
	     commands.	With no arguments whatsoever, the hash command prints
	     out the contents of this table.  Entries which have not been
	     looked at since the last cd command are marked with an asterisk;
	     it is possible for these entries to be invalid.

	     With arguments, the hash command removes each specified command
	     from the hash table (unless they are functions) and then locates
	     it.  With the -v option, hash prints the locations of the com‐
	     mands as it finds them.  The -r option causes the hash command to
	     delete all the entries in the hash table except for functions.

     jobid [job]
	     Print the process IDs of the processes in the specified job.  If
	     the job argument is omitted, use the current job.

     jobs [-lps] [job ...]
	     Print information about the specified jobs, or all jobs if no job
	     argument is given.	 The information printed includes job ID, sta‐
	     tus and command name.

	     If the -l option is specified, the PID of each job is also
	     printed.  If the -p option is specified, only the process IDs for
	     the process group leaders are printed, one per line.  If the -s
	     option is specified, only the PIDs of the job commands are
	     printed, one per line.

     local [variable ...] [-]
	     See the Functions subsection.

     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 stan‐
	     dard input is a terminal.	Then a line is read from the standard
	     input.  The trailing newline is deleted from the line and the
	     line is split as described in the section on White Space
	     Splitting (Field Splitting) above, and the pieces are assigned to
	     the variables in order.  If there are more pieces than variables,
	     the remaining pieces (along with the characters in IFS that sepa‐
	     rated them) are assigned to the last variable.  If there are more
	     variables than pieces, the remaining variables are assigned the
	     null string.

	     Backslashes are treated specially, unless the -r option is speci‐
	     fied.  If a backslash is followed by a newline, the backslash and
	     the newline will be deleted.  If a backslash is followed by any
	     other character, the backslash will be deleted and the following
	     character will be treated as though it were not in IFS, even if
	     it is.

	     If the -t option is specified and the timeout elapses before a
	     complete line of input is supplied, the read command will return
	     an exit status of 1 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 sup‐
	     plied, ‘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 read-
	     only variables are printed as “readonly name=value” lines, suit‐
	     able for re-input to the shell.

     return [exitstatus]
	     See the Functions subsection.

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

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

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

	     If the “--” option is specified, set will replace the shell's
	     positional parameters with the subsequent arguments.  If no argu‐
	     ments follow the “--” option, all the positional parameters will
	     be cleared, which is equivalent to executing the command “shift
	     $#”.  The “--” flag may be omitted when specifying arguments to
	     be used as positional replacement parameters.  This is not recom‐
	     mended, 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.  The
	     setvar command is intended to be used in functions that assign
	     values to variables whose names are passed as parameters.	In
	     general it is better to write “variable=value” rather than using

     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.

     test    A built-in equivalent of test(1).

     times   Print the amount of time spent executing the shell and its chil‐
	     dren.  The first output line shows the user and system times for
	     the shell itself, the second one contains the user and system
	     times for the children.

     trap [action] signal ...

     trap -l
	     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 an empty string or a dash (‘-’); the former causes the
	     specified signal to be ignored and the latter causes the default
	     action to be taken.  Omitting the action is another way to
	     request the default action, for compatibility reasons this usage
	     is not recommended though.	 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.

	     Option -l causes the trap command to display a list of valid sig‐
	     nal names.

     true    A null command that returns a 0 (true) exit value.

     type [name ...]
	     Interpret each name as a command and print the resolution of the
	     command search.  Possible resolutions are: shell keyword, alias,
	     special shell builtin, shell builtin, command, tracked alias and
	     not found.	 For aliases the alias expansion is printed; for com‐
	     mands and tracked aliases the complete pathname of the command is

     ulimit [-HSabcdflmnpstuvw] [limit]
	     Set or display resource limits (see getrlimit(2)).	 If limit is
	     specified, the named resource will be set; otherwise the current
	     resource value will be displayed.

	     If -H is specified, the hard limits will be set or displayed.
	     While everybody is allowed to reduce a hard limit, only the supe‐
	     ruser can increase it.  The -S option specifies the soft limits
	     instead.  When displaying limits, only one of -S or -H can be
	     given.  The default is to display the soft limits, and to set
	     both the hard and the soft limits.

	     Option -a causes the ulimit command to display all resources.
	     The parameter limit is not acceptable in this mode.

	     The remaining options specify which resource value is to be dis‐
	     played or modified.  They are mutually exclusive.

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

	     -c coredumpsize
		     The maximal size of core dump files, in 512-byte blocks.

	     -d datasize
		     The maximal size of the data segment of a process, in

	     -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.

	     -p pseudoterminals
		     The maximal number of pseudo-terminals for this user ID.

	     -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.

	     -w swapuse
		     The maximum amount of swap space reserved or used for
		     this user ID, in kilobytes.

     umask [-S] [mask]
	     Set the file creation mask (see umask(2)) to the octal or sym‐
	     bolic (see chmod(1)) value specified by mask.  If the argument is
	     omitted, the current mask value is printed.  If the -S option is
	     specified, the output is symbolic, otherwise the output is octal.

     unalias [-a] [name ...]
	     The specified alias names are removed.  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
     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 sub‐
     set of those described in the vi(1) man page.  The command “set -o vi”
     (or “set -V”) enables vi-mode editing and places sh into vi insert mode.
     With vi-mode enabled, sh can be switched between insert mode and command
     mode by typing ⟨ESC⟩.  Hitting ⟨return⟩ while in command mode will pass
     the line to the shell.

     Similarly, the “set -o emacs” (or “set -E”) command can be used to enable
     a subset of emacs-style command line editing features.

     The following environment variables affect the execution of sh:

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

     EDITOR    The fallback editor used with the fc built-in.  If not set, the
	       default editor is ed(1).

     FCEDIT    The default editor used with the fc built-in.

     HISTSIZE  The number of previous commands that are accessible.

     HOME      The starting directory of sh.

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

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

     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.

     PATH      The default search path for executables.	 See the Path Search
	       section for details.

     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       The prefix for the trace output (if -x is active).  The default
	       is “+ ”.

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

     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 builtin is used with a numeric argument, it will
     return the argument.

     builtin(1), chsh(1), echo(1), ed(1), emacs(1), pwd(1), test(1), vi(1),
     execve(2), getrlimit(2), umask(2), editrc(5)

     A sh command, the Thompson shell, appeared in Version 1 AT&T UNIX.	 It
     was superseded in Version 7 AT&T UNIX by the Bourne shell, which inher‐
     ited the name sh.

     This version of sh was rewritten in 1989 under the BSD license after the
     Bourne shell from AT&T System V Release 4 UNIX.

     This version of sh was originally written by Kenneth Almquist.

     The sh utility does not recognize multibyte characters.

BSD			       October 31, 2010				   BSD

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