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

     sh -- command interpreter (shell)

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

     sh is the standard command interpreter for the system.  The current ver-
     sion(1,3,5) of sh is in(1,8) the process of being changed to conform with the POSIX
     1003.2 and 1003.2a specifications for the shell.  This version(1,3,5) has many
     features which make it appear similar in(1,8) some respects to the Korn shell,
     but it is not a Korn shell clone (see ksh(1)).  Only features designated
     by POSIX, plus a few Berkeley extensions, are being incorporated into
     this shell.  This man(1,5,7) page is not intended to be a tutorial or a complete
     specification of the shell.

     The shell is a command that reads lines from either a file(1,n) or the termi-
     nal, interprets them, and generally executes other commands.  It is the
     program that is running when a user logs into the system (although a user
     can select(2,7,2 select_tut) 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(1,8) addition to data storage, along with
     built in(1,8) history(1,3,n,1 builtins) 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(1,n) and the file(1,n) can be executed directly by
     the shell.

     If no args are present and if(3,n) the standard input of the shell is con-
     nected to a terminal (or if(3,n) the -i flag is set(7,n,1 builtins)), and the -c option is not
     present, 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(3,n) it begins with a dash `-', the
     shell is also considered a login(1,3,5) shell.  This is normally done automati-
     cally by the system when the user first logs in.  A login(1,3,5) shell first
     reads commands from the files /etc/profile and .profile if(3,n) they exist.
     If the environment variable ENV is set(7,n,1 builtins) on entry to a shell, or is set(7,n,1 builtins) in(1,8)
     the .profile of a login(1,3,5) shell, the shell next reads commands from the
     file(1,n) named(5,8) in(1,8) ENV.  Therefore, a user should place commands that are to
     be executed only at login(1,3,5) time(1,2,n) in(1,8) the .profile file(1,n), and commands that
     are executed for every shell inside the ENV file.  To set(7,n,1 builtins) the ENV vari-
     able to some file(1,n), place the following line in(1,8) your .profile of your home

           ENV=$HOME/.shinit; export ENV

     substituting for ``.shinit'' any filename you wish.  Since the ENV file(1,n)
     is read(2,n,1 builtins) for every invocation of the shell, including shell scripts and
     non-interactive shells, the following paradigm is useful for restricting
     commands in(1,8) the ENV file(1,n) to interactive invocations.  Place commands
     within the ``case'' and ``esac'' below (these commands are described

           case $- in(1,8) *i*)
                 # commands for interactive use only

     If command line arguments besides the options have been specified, then
     the shell treats the first argument as the name of a file(1,n) from which to
     read(2,n,1 builtins) commands (a shell script), and the remaining arguments are set(7,n,1 builtins) as
     the positional parameters of the shell ($1, $2, etc).  Otherwise, the
     shell reads commands from its standard input.

   Argument List Processing
     All of the single letter options have a corresponding name that can be
     used as an argument to the -o option.  The set(7,n,1 builtins) -o name is provided next
     to the single letter option in(1,8) the description below.  Specifying a dash
     ``-'' turns the option on, while using a plus ``+'' disables the option.
     The following options can be set(7,n,1 builtins) from the command line or with the set(7,n,1 builtins)
     builtin (described later).

           -a allexport     Export all variables assigned to.

           -c               Read commands from the command_string operand
                            instead of from the standard input.  Special
                            parameter 0 will be set(7,n,1 builtins) from the command_name op-
                            erand and the positional parameters ($1, $2, etc.)
                            set(7,n,1 builtins) from the remaining argument operands.

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

           -e errexit       If not interactive, exit(3,n,1 builtins) immediately if(3,n) any
                            untested command fails.  The exit(3,n,1 builtins) status of a com-
                            mand is considered to be explicitly tested if(3,n) the
                            command is used to control an if(3,n), elif, while, or
                            until; or if(3,n) the command is the left hand operand
                            of an ``&&'' or ``||'' operator.

           -f noglob        Disable pathname expansion.

           -n noexec        If not interactive, read(2,n,1 builtins) commands but do not exe-
                            cute them.  This is useful for checking the syntax
                            of shell scripts.

           -u nounset       Write a message to standard error(8,n) when attempting
                            to expand a variable that is not set(7,n,1 builtins), and if(3,n) the
                            shell is not interactive, exit(3,n,1 builtins) immediately.

           -v verbose       The shell writes its input to standard error(8,n) as it
                            is read.  Useful for debugging.

           -x xtrace        Write each command to standard error(8,n) (preceded by
                            a `+ ') before it is executed.  Useful for debug-

           -q quietprofile  If the -v or -x options have been set(7,n,1 builtins), do not
                            apply them when reading initialization files,
                            these being /etc/profile, .profile, and the file(1,n)
                            specified by the ENV environment variable.

           -I ignoreeof     Ignore EOF's from input when interactive.

           -i interactive   Force the shell to behave interactively.

           -m monitor       Turn on job control (set(7,n,1 builtins) automatically when inter-

           -s stdin         Read commands from standard input (set(7,n,1 builtins) automati-
                            cally if(3,n) no file(1,n) arguments are present).  This
                            option has no effect when set(7,n,1 builtins) after the shell has
                            already started running (i.e. with set(7,n,1 builtins)).

           -V vi            Enable the built-in vi(1) command line editor
                            (disables -E if(3,n) it has been set(7,n,1 builtins)).  (See the
                            Command Line Editing section below.)

           -E emacs         Enable the built-in emacs style command line edi-
                            tor (disables -V if(3,n) it has been set(7,n,1 builtins)).  (See the
                            Command Line Editing section below.)

           -b notify        Enable asynchronous notification of background job
                            completion.  (UNIMPLEMENTED for 4.4alpha)

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

   Lexical Structure
     The shell reads input in(1,8) terms of lines from a file(1,n) and breaks it up into
     words at whitespace (blanks and tabs), and at certain sequences of char-
     acters that are special to the shell called ``operators''.  There are two
     types of operators: control operators and redirection operators (their
     meaning is discussed later).  Following is a list of operators:

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

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

     Quoting is used to remove the special meaning of certain characters or
     words to the shell, such as operators, whitespace, or keywords.  There
     are three types of quoting: matched single quotes, matched double quotes,
     and backslash.

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

   Single Quotes
     Enclosing characters in(1,8) single quotes preserves the literal meaning of
     all the characters (except single quotes, making it impossible to put
     single-quotes in(1,8) a single-quoted string(3,n)).

   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, and
     serves to quote only the following characters:
           $ ` " \ <newline>.
     Otherwise it remains literal.

   Reserved Words
     Reserved words are words that have special meaning to the shell and are
     recognized at the beginning of a line and after a control operator.  The
     following are reserved words:

           !       elif    fi      while   case
           else    for     then    {       }
           do      done    until   if(3,n)      esac

     Their meaning is discussed later.

     An alias is a name and corresponding value set(7,n,1 builtins) using the alias builtin
     command.  Whenever a reserved word may occur (see above), and after
     checking for reserved words, the shell checks the word to see if(3,n) it
     matches an alias.  If it does, it replaces it in(1,8) the input stream with
     its value.  For example, if(3,n) there is an alias called ``lf'' with the
     value ``ls -F'', then the input:

           lf foobar <return>

     would become

           ls -F foobar <return>

     Aliases provide a convenient way for naive users(1,5) to create shorthands for
     commands without having to learn how to create functions with arguments.
     They can also be used to create lexically obscure code.  This use is dis-

     The shell interprets the words it reads according to a language, the
     specification of which is outside the scope of this man(1,5,7) page (refer to
     the BNF in(1,8) the POSIX 1003.2 document).  Essentially though, a line is
     read(2,n,1 builtins) and if(3,n) the first word of the line (or after a control operator) is
     not a reserved word, then the shell has recognized a simple command.
     Otherwise, a complex command or some other special construct may have
     been recognized.

   Simple Commands
     If a simple command has been recognized, the shell performs the following

           1.   Leading words of the form ``name=value'' are stripped off and
                assigned to the environment of the simple command.  Redirect-
                ion operators and their arguments (as described below) are
                stripped off and saved for processing.

           2.   The remaining words are expanded as described in(1,8) the section
                called ``Expansions'', and the first remaining word is consid-
                ered the command name and the command is located.  The remain-
                ing words are considered the arguments of the command.  If no
                command name resulted, then the ``name=value'' variable
                assignments recognized in(1,8) item 1 affect the current shell.

           3.   Redirections are performed as described in(1,8) the next section.

     Redirections are used to change where a command reads its input or sends
     its output.  In general, redirections open(2,3,n), close(2,7,n), or duplicate an exist-
     ing reference to a file.  The overall format used for redirection is:

           [n] redir-op file(1,n)

     where redir-op is one of the redirection operators mentioned previously.
     Following is a list of the possible redirections.  The [n] is an optional
     number, as in(1,8) `3' (not `[3]'), that refers to a file(1,n) descriptor.

           [n]> file(1,n)   Redirect standard output (or n) to file.

           [n]>| file(1,n)  Same, but override the -C option.

           [n]>> file(1,n)  Append standard output (or n) to file.

           [n]< file(1,n)   Redirect standard input (or n) from file.

           [n1]<&n2    Duplicate standard input (or n1) from file(1,n) descriptor

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

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

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

           [n]<> file(1,n)  Open file(1,n) for reading and writing on standard input (or

     The following redirection is often called a ``here-document''.

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

     All the text on successive lines up to the delimiter is saved away and
     made available to the command on standard input, or file(1,n) descriptor n if(3,n)
     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(1,8) the section on ``Expansions'').  If the opera-
     tor is ``<<-'' instead of ``<<'', then leading tabs in(1,8) the here-doc-text
     are stripped.

   Search and Execution
     There are three types of commands: shell functions, builtin commands, and
     normal programs -- and the command is searched for (by name) in(1,8) that
     order.  They each are executed in(1,8) a different way.

     When a shell function is executed, all of the shell positional parameters
     (except $0, which remains unchanged) are set(7,n,1 builtins) to the arguments of the
     shell function.  The variables which are explicitly placed in(1,8) 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(7,n,1 builtins) to the values
     given.  Then the command given in(1,8) 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 builtins are executed internally to the shell, without spawning a
     new process.

     Otherwise, if(3,n) the command name doesn't match a function or builtin, the
     command is searched for as a normal program in(1,8) the file(1,n) system (as
     described in(1,8) 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(1,n) (i.e., if(3,n) it
     does not begin with the "magic(4,5) number" whose ASCII representation is
     "#!", so execve(2) returns ENOEXEC then) the shell will interpret the
     program in(1,8) a subshell.  The child shell will reinitialize itself in(1,8) this
     case, so that the effect will be as if(3,n) a new shell had been invoked to
     handle the ad-hoc shell script, except that the location of hashed com-
     mands located in(1,8) the parent shell will be remembered by the child.

     Note that previous versions of this document and the source code itself
     misleadingly and sporadically refer to a shell script without a magic(4,5)
     number as a "shell procedure".

   Path Search
     When locating a command, the shell first looks to see if(3,n) it has a shell
     function by that name.  Then it looks for a builtin command by that name.
     If a builtin command is not found, one of two things happen:

     1.   Command names containing a slash are simply executed without per-
          forming any searches.

     2.   The shell searches each entry in(1,8) PATH in(1,8) 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(3,n,1 builtins) 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(8,n), or a false indication.
     The man(1,5,7) page for each command should indicate the various exit(3,n,1 builtins) codes and
     what they mean.  Additionally, the builtin commands return exit(3,n,1 builtins) codes, as
     does an executed shell function.

     If a command consists entirely of variable assignments then the exit(3,n,1 builtins) sta-
     tus of the command is that of the last command substitution if(3,n) any, oth-
     erwise 0.

   Complex Commands
     Complex commands are combinations of simple commands with control opera-
     tors or reserved words, together creating a larger complex command.  More
     generally, a command is one of the following:

        simple command


        list or compound-list

        compound command

        function definition

     Unless otherwise stated, the exit(3,n,1 builtins) status of a command is that of the last
     simple command executed by the command.

     A pipeline is a sequence of one or more commands separated by the control
     operator |.  The standard output of all but the last command is connected
     to the standard input of the next command.  The standard output of the
     last command is inherited from the shell, as usual.

     The format for a pipeline is:

           [!] command1 [| command2 ...]

     The standard output of command1 is connected to the standard input of
     command2.  The standard input, standard output, or both of a command is
     considered to be assigned by the pipeline before any redirection speci-
     fied by redirection operators that are part of the command.

     If the pipeline is not in(1,8) the background (discussed later), the shell
     waits for all commands to complete.

     If the reserved word ! does not precede the pipeline, the exit(3,n,1 builtins) status is
     the exit(3,n,1 builtins) status of the last command specified in(1,8) the pipeline.  Other-
     wise, the exit(3,n,1 builtins) status is the logical NOT of the exit(3,n,1 builtins) status of the last
     command.  That is, if(3,n) the last command returns zero, the exit(3,n,1 builtins) status is
     1; if(3,n) the last command returns greater than zero, the exit(3,n,1 builtins) 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(8,n) of command1 to the
     standard input of command2.

     A ; or <newline> terminator causes the preceding AND-OR-list (described
     next) to be executed sequentially; a & causes asynchronous execution of
     the preceding AND-OR-list.

     Note that unlike some other shells, each process in(1,8) the pipeline is a
     child of the invoking shell (unless it is a shell builtin, in(1,8) which case
     it executes in(1,8) the current shell -- but any effect it has on the environ-
     ment is wiped).

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

     The format for running a command in(1,8) background is:

           command1 & [command2 & ...]

     If the shell is not interactive, the standard input of an asynchronous
     command is set(7,n,1 builtins) 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(1,8) a list are executed in(1,8) the order they
     are written.  If command is followed by an ampersand, the shell starts
     the command and immediately proceed onto the next command; otherwise it
     waits for the command to terminate before proceeding to the next one.

   Short-Circuit List Operators
     ``&&'' and ``||'' are AND-OR list operators.  ``&&'' executes the first
     command, and then executes the second command if(3,n) and only if(3,n) the exit(3,n,1 builtins)
     status of the first command is zero.  ``||'' is similar, but executes the
     second command if(3,n) and only if(3,n) the exit(3,n,1 builtins) status of the first command is
     nonzero.  ``&&'' and ``||'' both have the same priority.  Note that these
     operators are left-associative, so ``true || echo(1,3x,1 builtins) bar && echo(1,3x,1 builtins) baz''
     writes ``baz'' and nothing else.  This is not the way it works in(1,8) C.

   Flow-Control Constructs -- if(3,n), while, for, case
     The syntax of the if(3,n) command is

           if(3,n) 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(3,n,1 builtins) status of the first
     list is zero.  The until command is similar, but has the word until in(1,8)
     place of while, which causes it to repeat until the exit(3,n,1 builtins) status of the
     first list is zero.

     The syntax of the for command is

           for variable in(1,8) word ...
           do   list

     The words are expanded, and then the list is executed repeatedly with the
     variable set(7,n,1 builtins) to each word in(1,8) turn.  do and done may be replaced with
     ``{'' and ``}''.

     The syntax of the break and continue command is

           break [ num ]
           continue [ num ]

     Break terminates the num innermost for or while loops.  Continue contin-
     ues with the next iteration of the innermost loop.  These are implemented
     as builtin commands.

     The syntax of the case command is

           case word in(1,8)
           pattern) list ;;

     The pattern can actually be one or more patterns (see Shell Patterns
     described later), separated by ``|'' characters.

   Grouping Commands Together
     Commands may be grouped by writing either



           { list; }

     The first of these executes the commands in(1,8) a subshell.  Builtin commands
     grouped into a (list) will not affect the current shell.  The second form
     does not fork another shell so is slightly more efficient.  Grouping com-
     mands together this way allows you to redirect their output as though
     they were one program:

           { echo(1,3x,1 builtins) -n " hello " ; echo(1,3x,1 builtins) " world" ; } > greeting

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

     The syntax of a function definition is

           name () command

     A function definition is an executable statement; when executed it
     installs a function named(5,8) name and returns an exit(3,n,1 builtins) status of zero.  The
     command is normally a list enclosed between ``{'' and ``}''.

     Variables may be declared to be local to a function by using a local com-
     mand.  This should appear as the first statement of a function, and the
     syntax is

           local [variable | -] ...

     Local is implemented as a builtin command.

     When a variable is made local, it inherits the initial value and exported
     and readonly flags from the variable with the same name in(1,8) the surround-
     ing scope, if(3,n) there is one.  Otherwise, the variable is initially unset.
     The shell uses dynamic scoping, so that if(3,n) you make the variable x local
     to function f, which then calls function g, references to the variable x
     made inside g will refer to the variable x declared inside f, not to the
     global variable named(5,8) x.

     The only special parameter that can be made local is ``-''.  Making ``-''
     local any shell options that are changed via the set(7,n,1 builtins) command inside the
     function to be restored to their original values when the function

     The syntax of the return command is

           return [exitstatus]

     It terminates the currently executing function.  Return is implemented as
     a builtin command.

   Variables and Parameters
     The shell maintains a set(7,n,1 builtins) 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(7,n,1 builtins) using the


     Variables set(7,n,1 builtins) by the user must have a name consisting solely of alphabet-
     ics, numerics, and underscores - the first of which must not be numeric.
     A parameter can also be denoted by a number or a special character as
     explained below.

   Positional Parameters
     A positional parameter is a parameter denoted by a number (n > 0).  The
     shell sets these initially to the values of its command line arguments
     that follow the name of the shell script.  The set(7,n,1 builtins) builtin can also be
     used to set(7,n,1 builtins) or reset(1,7,1 tput) them.

   Special Parameters
     A special parameter is a parameter denoted by one of the following spe-
     cial characters.  The value of the parameter is listed next to its char-

     *            Expands to the positional parameters, starting from one.
                  When the expansion occurs within a double-quoted string(3,n) 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(3,n) IFS is unset.

     @            Expands to the positional parameters, starting from one.
                  When the expansion occurs within double-quotes, each posi-
                  tional parameter expands as a separate argument.  If there
                  are no positional parameters, the expansion of @ generates
                  zero arguments, even when @ is double-quoted.  What this
                  basically means, for example, is if(3,n) $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(3,n,1 builtins) status of the most recent pipeline.

     - (Hyphen.)  Expands to the current option flags (the single-letter
                  option names concatenated into a string(3,n)) as specified on
                  invocation, by the set(7,n,1 builtins) builtin command, or implicitly by the

     $            Expands to the process ID of the invoked shell.  A subshell
                  retains the same value of $ as its parent.

     !            Expands to the process ID of the most recent background com-
                  mand executed from the current shell.  For a pipeline, the
                  process ID is that of the last command in(1,8) the pipeline.

     0 (Zero.)    Expands to the name of the shell or shell script.

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

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

     The order of word expansion is:

     1.   Tilde Expansion, Parameter Expansion, Command Substitution, Arith-
          metic Expansion (these all occur at the same time(1,2,n)).

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

     3.   Pathname Expansion (unless set(7,n,1 builtins) -f is in(1,8) effect).

     4.   Quote Removal.

     The $ character is used to introduce parameter expansion, command substi-
     tution, or arithmetic evaluation.

   Tilde Expansion (substituting a user's home directory)
     A word beginning with an unquoted tilde character (~) is subjected to
     tilde expansion.  All the characters up to a slash (/) or the end of the
     word are treated as a username and are replaced with the user's home
     directory.  If the username is missing (as in(1,8) ~/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(3,n), and charac-
     ters in(1,8) embedded arithmetic expansions, command substitutions, and vari-
     able expansions, are not examined in(1,8) determining the matching ``}''.

     The simplest form for parameter expansion is:


     The value, if(3,n) any, of parameter is substituted.

     The parameter name or symbol can be enclosed in(1,8) 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 @.

     In addition, a parameter expansion can be modified by using one of the
     following formats.

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

     ${parameter:=word}    Assign Default Values.  If parameter is unset or
                           null, the expansion of word is assigned to parame-
                           ter.  In all cases, the final value of parameter is
                           substituted.  Only variables, not positional param-
                           eters or special parameters, can be assigned in(1,8)
                           this way.

     ${parameter:?[word]}  Indicate Error if(3,n) Null or Unset.  If parameter is
                           unset or null, the expansion of word (or a message
                           indicating it is unset if(3,n) word is omitted) is writ-
                           ten to standard error(8,n) and the shell exits with a
                           nonzero exit(3,n,1 builtins) status.  Otherwise, the value of
                           parameter is substituted.  An interactive shell
                           need not exit.

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

     In the parameter expansions shown previously, use of the colon in(1,8) the
     format results in(1,8) a test for a parameter that is unset or null; omission
     of the colon results in(1,8) a test for a parameter that is only unset.

     ${#parameter}         String Length.  The length in(1,8) characters of the
                           value of parameter.

     The following four varieties of parameter expansion provide for substring
     processing.  In each case, pattern matching notation (see Shell
     Patterns), rather than regular expression notation, is used to evaluate
     the patterns.  If parameter is * or @, the result of the expansion is
     unspecified.  Enclosing the full parameter expansion string(3,n) in(1,8) double-
     quotes does not cause the following four varieties of pattern characters
     to be quoted, whereas quoting characters within the braces has this

     ${parameter%word}     Remove Smallest Suffix Pattern.  The word is
                           expanded to produce a pattern.  The parameter
                           expansion then results in(1,8) parameter, with the
                           smallest portion of the suffix matched by the pat-
                           tern deleted.

     ${parameter%%word}    Remove Largest Suffix Pattern.  The word is
                           expanded to produce a pattern.  The parameter
                           expansion then results in(1,8) parameter, with the
                           largest portion of the suffix matched by the pat-
                           tern deleted.

     ${parameter#word}     Remove Smallest Prefix Pattern.  The word is
                           expanded to produce a pattern.  The parameter
                           expansion then results in(1,8) parameter, with the
                           smallest portion of the prefix matched by the pat-
                           tern deleted.

     ${parameter##word}    Remove Largest Prefix Pattern.  The word is
                           expanded to produce a pattern.  The parameter
                           expansion then results in(1,8) parameter, with the
                           largest portion of the prefix matched by the pat-
                           tern deleted.

   Command Substitution
     Command substitution allows the output of a command to be substituted in(1,8)
     place of the command name itself.  Command substitution occurs when the
     command is enclosed as follows:


     or (``backquoted'' version(1,3,5)):


     The shell expands the command substitution by executing command in(1,8) a sub-
     shell environment and replacing the command substitution with the stan-
     dard output of the command, removing sequences of one or more <newline>s
     at the end of the substitution.  (Embedded <newline>s before the end of
     the output are not removed; however, during field splitting, they may be
     translated into <space>s, depending on the value of IFS and quoting that
     is in(1,8) 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(3,n) it were in(1,8) double-quotes, except that a
     double-quote inside the expression is not treated specially.  The shell
     expands all tokens in(1,8) the expression for parameter expansion, command
     substitution, and quote removal.

     Next, the shell treats this as an arithmetic expression and substitutes
     the value of the expression.

   White Space Splitting (Field Splitting)
     After parameter expansion, command substitution, and arithmetic expansion
     the shell scans the results of expansions and substitutions that did not
     occur in(1,8) double-quotes for field splitting and multiple fields can

     The shell treats each character of the IFS as a delimiter and use the
     delimiters to split(1,n) the results of parameter expansion and command sub-
     stitution into fields.

   Pathname Expansion (File Name Generation)
     Unless the -f flag is set(7,n,1 builtins), file(1,n) 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(3,n) that matches the specified pattern.  There are two
     restrictions on this: first, a pattern cannot match a string(3,n) containing a
     slash, and second, a pattern cannot match a string(3,n) 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(3,n) 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(3,n) of characters.  A question mark
     matches any single character.  A left bracket (``['') introduces a char-
     acter class.  The end of the character class is indicated by a (``]'');
     if(3,n) the ``]'' is missing then the ``['' matches a ``['' rather than intro-
     ducing a character class.  A character class matches any of the charac-
     ters 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(1,8) a character class, make it the first character
     listed (after the ``!'', if(3,n) any).  To include a minus sign, make it the
     first or last character listed.

     This section lists the builtin commands which are builtin because they
     need to perform some operation that can't be performed by a separate
     process.  In addition to these, there are several other commands that may
     be builtin for efficiency (e.g.  printf(1,3,1 builtins)(1), echo(1,3x,1 builtins)(1), test(1), etc).

     :      A null command that returns a 0 (true) exit(3,n,1 builtins) value.

     . file(1,n)
            The commands in(1,8) the specified file(1,n) are read(2,n,1 builtins) and executed by the

     alias [name[=string(3,n) ...]]
            If name=string(3,n) is specified, the shell defines the alias name with
            value string(3,n).  If just name is specified, the value of the alias
            name is printed.  With no arguments, the alias builtin prints the
            names and values of all defined aliases (see unalias).

     bg [job] ...
            Continue the specified jobs (or the current job if(3,n) no jobs are
            given) in(1,8) the background.

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

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

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

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

     cd [directory [replace]]
            Switch to the specified directory (default $HOME).  If replace is
            specified, then the new directory name is generated by replacing
            the first occurrence of directory in(1,8) the current directory name
            with replace.  Otherwise if(3,n) an entry for CDPATH appears in(1,8) the
            environment of the cd command or the shell variable CDPATH is set(7,n,1 builtins)
            and the directory name does not begin with a slash, then the
            directories listed in(1,8) CDPATH will be searched for the specified
            directory.  The format of CDPATH is the same as that of PATH.  In
            an interactive shell, the cd command will print out the name of
            the directory that it actually switched to if(3,n) 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(1,2)
            was crossed.

     eval string(3,n) ...
            Concatenate all the arguments with spaces.  Then re-parse and exe-
            cute the command.

     exec(3,n,1 builtins) [command arg ...]
            Unless command is omitted, the shell process is replaced with the
            specified program (which must be a real program, not a shell
            builtin or function).  Any redirections on the exec(3,n,1 builtins) command are
            marked as permanent, so that they are not undone when the exec(3,n,1 builtins)
            command finishes.

     exit(3,n,1 builtins) [exitstatus]
            Terminate the shell process.  If exitstatus is given it is used as
            the exit(3,n,1 builtins) status of the shell; otherwise the exit(3,n,1 builtins) status of the
            preceding command is used.

     export name ...

     export -p
            The specified names are exported so that they will appear in(1,8) 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(7,n,1 builtins) at the same time(1,2,n) it is exported by writing

                  export name=value

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

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

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

     fc -s [old=new] [first]
            The fc builtin lists, or edits and re-executes, commands previ-
            ously entered to an interactive shell.

            -e editor
                   Use the editor named(5,8) by editor to edit the commands.  The
                   editor string(3,n) is a command name, subject to search via the
                   PATH variable.  The value in(1,8) 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(1,8) the sequence indicated by the
                   first and last operands, as affected by -r, with each com-
                   mand preceded by the command number.

            -n     Suppress command numbers when listing with -l.

            -r     Reverse the order of the commands listed (with -l) or
                   edited (with neither -l nor -s).

            -s     Re-execute the command without invoking an editor.


            last   Select the commands to list or edit.  The number of previ-
                   ous commands that can be accessed are determined by the
                   value of the HISTSIZE variable.  The value of first or last
                   or both are one of the following:

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

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

                   A string(3,n) indicating the most recently entered command that
                   begins with that string.  If the old=new operand is not
                   also specified with -s, the string(3,n) form of the first oper-
                   and cannot contain an embedded equal sign.

            The following environment variables affect the execution of fc:

            FCEDIT    Name of the editor to use.

            HISTSIZE  The number of previous commands that are accessible.

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

     getopts optstring var
            The POSIX getopts command, not to be confused with the Bell Labs
            -derived getopt(1,3,3 Getopt::Std)(1).

            The first argument should be a series of letters, each of which
            may be optionally followed by a colon to indicate that the option
            requires an argument.  The variable specified is set(7,n,1 builtins) to the parsed

            The getopts command deprecates the older getopt(1,3,3 Getopt::Std)(1) utility due to
            its handling of arguments containing whitespace.

            The getopts builtin may be used to obtain options and their argu-
            ments from a list of parameters.  When invoked, getopts places the
            value of the next option from the option string(3,n) in(1,8) the list in(1,8) the
            shell variable specified by var and its index in(1,8) the shell vari-
            able OPTIND.  When the shell is invoked, OPTIND is initialized to
            1.  For each option that requires an argument, the getopts builtin
            will place it in(1,8) the shell variable OPTARG.  If an option is not
            allowed for in(1,8) the optstring, then OPTARG will be unset.

            optstring is a string(3,n) of recognized option letters (see
            getopt(1,3,3 Getopt::Std)(3)).  If a letter is followed by a colon, the option is
            expected to have an argument which may or may not be separated
            from it by white space.  If an option character is not found where
            expected, getopts will set(7,n,1 builtins) the variable var to a ``?''; getopts
            will then unset OPTARG and write(1,2) output to standard error.  By
            specifying a colon as the first character of optstring all errors
            will be ignored.

            A nonzero value is returned when the last option is reached.  If
            there are no remaining arguments, getopts will set(7,n,1 builtins) var to the spe-
            cial option, ``--'', otherwise, it will set(7,n,1 builtins) var to ``?''.

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

                  while getopts abc: f
                          case $f in(1,8)
                          a | b)  flag=$f;;
                          c)      carg=$OPTARG;;
                          \?)     echo(1,3x,1 builtins) $USAGE; exit(3,n,1 builtins) 1;;
                  shift `expr(1,3,n) $OPTIND - 1`

            This code will accept(2,8) any of the following as equivalent:

                  cmd -acarg file(1,n) file(1,n)
                  cmd -a -c arg file(1,n) file(1,n)
                  cmd -carg -a file(1,n) file(1,n)
                  cmd -a -carg -- file(1,n) file(1,n)

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

            With arguments, the hash command removes the specified commands
            from the hash table (unless they are functions) and then locates
            them.  With the -v option, hash prints the locations of the com-
            mands as it finds them.  The -r option causes the hash command to
            delete all the entries in(1,8) the hash table except for functions.

     inputrc file(1,n)
            Read the file(1,n) to set(7,n,1 builtins) keybindings as defined by editrc(5).

     jobid [job]
            Print the process id's of the processes in(1,8) the job.  If the job
            argument is omitted, the current job is used.

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

     pwd(1,n,1 builtins) [-LP]
            Print the current directory.  If -L is specified the cached value
            (initially set(7,n,1 builtins) from PWD) is checked to see if(3,n) it refers to the
            current directory, if(3,n) it does the value is printed.  Otherwise the
            current directory name is found using getcwd(3).  The environment
            variable PWD is set(7,n,1 builtins) to printed value.

            The default is pwd(1,n,1 builtins) -L, but note that the builtin cd command
            doesn't currently support -L or -P and will cache (almost) the
            absolute path.  If cd is changed, pwd(1,n,1 builtins) may be changed to default to
            pwd(1,n,1 builtins) -P.

            If the current directory is renamed and replaced by a symlink to
            the same directory, or the initial PWD value followed a symbolic
            link(1,2), then the cached value may not be the absolute path.

            The builtin command may differ from the program of the same name
            because the program will use PWD and the builtin uses a separately
            cached value.

     read(2,n,1 builtins) [-p prompt] [-r] variable [...]
            The prompt is printed if(3,n) the -p option is specified and the stan-
            dard input is a terminal.  Then a line is read(2,n,1 builtins) from the standard
            input.  The trailing newline is deleted from the line and the line
            is split(1,n) as described in(1,8) the section on word splitting above, and
            the pieces are assigned to the variables in(1,8) order.  If there are
            more pieces than variables, the remaining pieces (along with the
            characters in(1,8) IFS that separated them) are assigned to the last
            variable.  If there are more variables than pieces, the remaining
            variables are assigned the null string.  The read(2,n,1 builtins) builtin will
            indicate success unless EOF is encountered on input, in(1,8) which case
            failure is returned.

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

     readonly name ...

     readonly -p
            The specified names are marked as read(2,n,1 builtins) only, so that they cannot
            be subsequently modified or unset.  The shell allows the value of
            a variable to be set(7,n,1 builtins) at the same time(1,2,n) it is marked read(2,n,1 builtins) only by

                  readonly name=value

            With no arguments the readonly command lists the names of all read(2,n,1 builtins)
            only variables.  With the -p option specified the output will be
            formatted suitably for non-interactive use.

     set(7,n,1 builtins) [{ -options | +options | -- }] arg ...
            The set(7,n,1 builtins) command performs three different functions.

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

            If options are given, it sets the specified option flags, or
            clears them as described in(1,8) the section called Argument List

            The third use of the set(7,n,1 builtins) command is to set(7,n,1 builtins) the values of the
            shell's positional parameters to the specified args.  To change
            the positional parameters without changing any options, use ``--''
            as the first argument to set.  If no args are present, the set(7,n,1 builtins)
            command will clear(1,3x,3x clrtobot) all the positional parameters (equivalent to
            executing ``shift $#''.)

     setvar variable value
            Assigns value to variable.  (In general it is better to write(1,2)
            variable=value rather than using setvar.  setvar is intended to be
            used in(1,8) functions that assign values to variables whose names are
            passed as parameters.)

     shift [n]
            Shift the positional parameters n times.  A shift sets the value
            of $1 to the value of $2, the value of $2 to the value of $3, and
            so on, decreasing the value of $# by one.  If there are zero posi-
            tional parameters, shift does nothing.

     trap [-l]

     trap [action] signal(2,7) ...
            Cause the shell to parse and execute action when any of the speci-
            fied signals are received.  The signals are specified by signal(2,7)
            number or as the name of the signal.  If signal(2,7) is 0, the action
            is executed when the shell exits.  action may be null, which cause
            the specified signals to be ignored.  With action omitted or set(7,n,1 builtins)
            to `-' the specified signals are set(7,n,1 builtins) to their default action.
            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.  Issu-
            ing trap with option -l will print a list of valid signal(2,7) names.
            trap without any arguments cause it to write(1,2) a list of signals and
            their associated action to the standard output in(1,8) a format that is
            suitable as an input to the shell that achieves the same trapping



            List trapped signals and their corresponding action

                  trap -l

            Print a list of valid signals

                  trap '' INT QUIT tstp 30

            Ignore signals INT QUIT TSTP USR1

                  trap date INT

            Print date upon receiving signal(2,7) INT

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

     ulimit [-H | -S] [-a | -tfdscmlpn [value]]
            Inquire about or set(7,n,1 builtins) the hard or soft limits on processes or set(7,n,1 builtins)
            new limits.  The choice between hard limit (which no process is
            allowed to violate, and which may not be raised once it has been
            lowered) and soft limit (which causes processes to be signaled but
            not necessarily killed, and which may be raised) is made with
            these flags:

            -H          set(7,n,1 builtins) or inquire about hard limits

            -S          set(7,n,1 builtins) or inquire about soft limits.  If neither -H nor
                        -S is specified, the soft limit is displayed or both
                        limits are set.  If both are specified, the last one

            The limit to be interrogated or set(7,n,1 builtins), then, is chosen by specifying
            any one of these flags:

            -a          show all the current limits

            -b          show or set(7,n,1 builtins) the limit on the socket(2,7,n) buffer size of a
                        process (in(1,8) bytes)

            -t          show or set(7,n,1 builtins) the limit on CPU time(1,2,n) (in(1,8) seconds)

            -f          show or set(7,n,1 builtins) the limit on the largest file(1,n) that can be
                        created (in(1,8) 512-byte blocks)

            -d          show or set(7,n,1 builtins) the limit on the data segment size of a
                        process (in(1,8) kilobytes)

            -s          show or set(7,n,1 builtins) the limit on the stack size of a process
                        (in(1,8) kilobytes)

            -c          show or set(7,n,1 builtins) the limit on the largest core dump size
                        that can be produced (in(1,8) 512-byte blocks)

            -m          show or set(7,n,1 builtins) the limit on the total physical memory
                        that can be in(1,8) use by a process (in(1,8) kilobytes)

            -l          show or set(7,n,1 builtins) the limit on how much memory a process can
                        lock with mlock(2) (in(1,8) kilobytes)

            -p          show or set(7,n,1 builtins) the limit on the number of processes this
                        user can have at one time(1,2,n)

            -n          show or set(7,n,1 builtins) the limit on the number of files a process
                        can have open(2,3,n) at once

            If none of these is specified, it is the limit on file(1,n) size that
            is shown or set.  If value is specified, the limit is set(7,n,1 builtins) to that
            number; otherwise the current limit is displayed.

            Limits of an arbitrary process can be displayed or set(7,n,1 builtins) using the
            sysctl(2,5,8)(8) utility.

     umask [mask]
            Set the value of umask (see umask(2)) to the specified octal
            value.  If the argument is omitted, the umask value is printed.

     unalias [-a] [name]
            If name is specified, the shell removes that alias.  If -a is
            specified, all aliases are removed.

     unset name ...
            The specified variables and functions are unset and unexported.
            If a given name corresponds to both a variable and a function,
            both the variable and the function are unset.

     wait [job]
            Wait for the specified job to complete and return the exit(3,n,1 builtins) status
            of the last process in(1,8) the job.  If the argument is omitted, wait
            for all jobs to complete and then return an exit(3,n,1 builtins) status of zero.

   Command Line Editing
     When sh is being used interactively from a terminal, the current command
     and the command history(1,3,n,1 builtins) (see fc in(1,8) Builtins) can be edited using emacs-
     mode or vi-mode command-line editing.  The command `set(7,n,1 builtins) -o emacs' enables
     emacs-mode editing.  The command `set(7,n,1 builtins) -o vi' enables vi-mode editing and
     places sh into vi insert mode.  (See the Argument List Processing section

     The vi mode uses commands similar to a subset of those described in(1,8) the
     vi(1) man(1,5,7) page.  With vi-mode enabled, sh can be switched between insert
     mode and command mode.  It's similar to vi: typing <ESC> will throw you
     into command VI command mode.  Hitting <return> while in(1,8) command mode
     will pass the line to the shell.

     The emacs mode uses commands similar to a subset available in(1,8) the emacs
     editor.  With emacs-mode enabled, special keys can be used to modify the
     text in(1,8) the buffer using the control key.

     sh uses the editline(3) library.

     Errors that are detected by the shell, such as a syntax error(8,n), will cause
     the shell to exit(3,n,1 builtins) with a non-zero exit(3,n,1 builtins) status.  If the shell is not an
     interactive shell, the execution of the shell file(1,n) will be aborted.  Oth-
     erwise the shell will return the exit(3,n,1 builtins) status of the last command exe-
     cuted, or if(3,n) the exit(3,n,1 builtins) builtin is used with a numeric argument, it will
     return the argument.

     HOME       Set automatically by login(1,3,5)(1) from the user's login(1,3,5) directory
                in(1,8) the password file(1,n) (passwd(1,5)(5)).  This environment variable
                also functions as the default argument for the cd builtin.

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

     CDPATH     The search path used with the cd builtin.

     LANG       The string(3,n) used to specify localization information that
                allows users(1,5) to work with different culture-specific and lan-
                guage conventions.  See nls(7).

     MAIL       The name of a mail(1,8) file(1,n), that will be checked for the arrival
                of new mail.  Overridden by MAILPATH.

     MAILCHECK  The frequency in(1,8) seconds that the shell checks for the arrival
                of mail(1,8) in(1,8) the files specified by the MAILPATH or the MAIL
                file.  If set(7,n,1 builtins) to 0, the check will occur at each prompt.

     MAILPATH   A colon ``:'' separated list of file(1,n) names, for the shell to
                check for incoming mail.  This environment setting overrides
                the MAIL setting.  There is a maximum of 10 mailboxes that can
                be monitored at once.

     PS1        The primary prompt string(3,n), which defaults to ``$  '', unless
                you are the superuser, in(1,8) which case it defaults to ``#  ''.

     PS2        The secondary prompt string(3,n), which defaults to ``>  ''.

     PS4        Output before each line when execution trace(3x,n,3x _nc_tracebits) (set(7,n,1 builtins) -x) is
                enabled, defaults to ``+  ''.

     IFS        Input Field Separators.  This is normally set(7,n,1 builtins) to <space>,
                <tab>, and <newline>.  See the White Space Splitting section
                for more details.

     TERM       The default terminal setting for the shell.  This is inherited
                by children of the shell, and is used in(1,8) the history(1,3,n,1 builtins) editing

     HISTSIZE   The number of lines in(1,8) the history(1,3,n,1 builtins) buffer for the shell.



     csh(1), echo(1,3x,1 builtins)(1), getopt(1,3,3 Getopt::Std)(1), ksh(1), login(1,3,5)(1), printf(1,3,1 builtins)(1), test(1),
     editline(3), getopt(1,3,3 Getopt::Std)(3), editrc(5), passwd(1,5)(5), environ(7), nls(7),

     A sh command appeared in(1,8) Version 1 AT&T UNIX.  It was, however, unmain-
     tainable so we wrote this one.

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

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

BSD                             April 17, 2004                             BSD

References for this manual (incoming links)