


      SH(1)                       (UNIX 5.2)                      SH(1)



      NAME
           sh, rshell - shell, the standard/restricted command
           programming language

      SYNTAX
           sh [ -acefhiknrstuvx ] [ args ]
           rshell [ -acefhiknrstuvx ] [ args ]

      DESCRIPTION
           Sh is a command programming language that executes commands
           read from a terminal or a file.  Rshell is a restricted
           version of the standard command interpreter sh; it is used
           to set up login names and execution environments whose
           capabilities are more controlled than those of the standard
           shell.  See Invocation below for the meaning of arguments to
           the shell.

         Definitions
           A blank is a tab or a space.  A name is a sequence of
           letters, digits, or underscores beginning with a letter or
           underscore.  A parameter is a name, a digit, or any of the
           characters *, @, #, ?, -, $, and !.

         Commands
           A simple-command is a sequence of non-blank words separated
           by blanks.  The first word specifies the name of the command
           to be executed.  Except as specified below, the remaining
           words are passed as arguments to the invoked command.  The
           command name is passed as argument 0 (see exec(2)).  The
           value of a simple-command is its exit status if it
           terminates normally, or (octal) 200+status if it terminates
           abnormally (see signal(2) for a list of status values).

           A pipeline is a sequence of one or more commands separated
           by | (or, for historical compatibility, by ^).  The standard
           output of each command but the last is connected by a
           pipe(2) to the standard input of the next command.  Each
           command is run as a separate process; the shell waits for
           the last command to terminate.  The exit status of a
           pipeline is the exit status of the last command.

           A list is a sequence of one or more pipelines separated by
           ;, &, &&, or ||, and optionally terminated by ; or &.  Of
           these four symbols, ; and & have equal precedence, which is
           lower than that of && and ||.  The symbols && and || also
           have equal precedence.  A semicolon (;) causes sequential
           execution of the preceding pipeline; an ampersand (&) causes
           asynchronous execution of the preceding pipeline (i.e., the
           shell does not wait for that pipeline to finish).  The
           symbol && (||) causes the list following it to be executed
           only if the preceding pipeline returns a zero (non-zero)
           exit status.  An arbitrary number of new-lines may appear in



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           a list, instead of semicolons, to delimit commands.

           A command is either a simple-command or one of the
           following.  Unless otherwise stated, the value returned by a
           command is that of the last simple-command executed in the
           command.

           for name [ in word ... ] do list done
                Each time a for command is executed, name is set to the
                next word taken from the in word list.  If in word ...
                is omitted, then the for command executes the do list
                once for each positional parameter that is set (see
                Parameter Substitution below).  Execution ends when
                there are no more words in the list.
           case word in [ pattern [ | pattern ] ... ) list ;; ] ... esac
                A case command executes the list associated with the
                first pattern that matches word.  The form of the
                patterns is the same as that used for file-name
                generation (see File Name Generation) except that a
                slash, a leading dot, or a dot immediately following a
                slash need not be matched explicitly.
           if list then list [ elif list then list ] ... [ else list ] fi
                The list following if is executed and, if it returns a
                zero exit status, the list following the first then is
                executed.  Otherwise, the list following elif is
                executed and, if its value is zero, the list following
                the next then is executed.  Failing that, the else list
                is executed.  If no else list or then list is executed,
                then the if command returns a zero exit status.
           while list do list done
                A while command repeatedly executes the while list and,
                if the exit status of the last command in the list is
                zero, executes the do list; otherwise the loop
                terminates.  If no commands in the do list are
                executed, then the while command returns a zero exit
                status; until may be used in place of while to negate
                the loop termination test.
           (list)
                Execute list in a sub-shell.
           {list;}
                list is simply executed.
           name () {list;}
                Define a function which is referenced by name. The body
                of the function is the list of commands between { and
                }.  Execution of functions is described below (see
                Execution).

           The following words are only recognized as the first word of
           a command and when not quoted:

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




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         Comments
           A word beginning with # causes that word and all the
           following characters up to a new-line to be ignored.

         Command Substitution
           The standard output from a command enclosed in a pair of
           grave accents (``) may be used as part or all of a word;
           trailing new-lines are removed.

         Parameter Substitution
           The character $ is used to introduce substitutable
           parameters.  There are two types of parameters, positional
           and keyword.  If parameter is a digit, it is a positional
           parameter.  Positional parameters may be assigned values by
           set.  Keyword parameters (also known as variables) may be
           assigned values by writing:

                name=value [ name=value ] ...

           Pattern-matching is not performed on value.  There cannot be
           a function and a variable with the same name.

           ${parameter}
                The value, if any, of the parameter is substituted.
                The braces are required only when parameter is followed
                by a letter, digit, or underscore that is not to be
                interpreted as part of its name.  If parameter is * or
                @, all the positional parameters, starting with $1, are
                substituted (separated by spaces).  Parameter $0 is set
                from argument zero when the shell is invoked.
           ${parameter:-word}
                If parameter is set and is non-null, substitute its
                value; otherwise substitute word.
           ${parameter:=word}
                If parameter is not set or is null set it to word; the
                value of the parameter is substituted.  Positional
                parameters may not be assigned to in this way.
           ${parameter:?word}
                If parameter is set and is non-null, substitute its
                value; otherwise, print word and exit from the shell.
                If word is omitted, the message ``parameter null or not
                set'' is printed.
           ${parameter:+word}
                If parameter is set and is non-null, substitute word;
                otherwise substitute nothing.

           In the above, word is not evaluated unless it is to be used
           as the substituted string, so that, in the following
           example, pwd is executed only if d is not set or is null:

                echo ${d:-`pwd`}




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           If the colon (:) is omitted from the above expressions, the
           shell only checks whether parameter is set or not.

           The following parameters are automatically set by the shell:
                #    The number of positional parameters in decimal.
                -    Flags supplied to the shell on invocation or by
                     the set command.
                ?    The decimal value returned by the last
                     synchronously executed command.
                $    The process number of this shell.
                !    The process number of the last background command
                     invoked.

           The following parameters are used by the shell:
                HOME The default argument (home directory) for the cd
                     command.
                PATH The search path for commands (see Execution
                     below).  The user may not change PATH if executing
                     under rshell.
                CDPATH
                     The search path for the cd command.
                MAIL If this parameter is set to the name of a mail
                     file and the MAILPATH parameter is not set, the
                     shell informs the user of the arrival of mail in
                     the specified file.
                MAILCHECK
                     This parameter specifies how often (in seconds)
                     the shell will check for the arrival of mail in
                     the files specified by the MAILPATH or MAIL
                     parameters.  The default value is 600 seconds (10
                     minutes).  If set to 0, the shell will check
                     before each prompt.
                MAILPATH
                     A colon (:) separated list of file names.  If this
                     parameter is set, the shell informs the user of
                     the arrival of mail in any of the specified files.
                     Each file name can be followed by % and a message
                     that will be printed when the modification time
                     changes.  The default message is you have mail.
                PS1  Primary prompt string, by default ``$ ''.
                PS2  Secondary prompt string, by default ``> ''.
                IFS  Internal field separators, normally space, tab,
                     and new-line.
                SHELL
                     When the shell is invoked, it scans the
                     environment (see Environment below) for this name.
                     If it is found and there is an 'r' in the file
                     name part of its value, the shell becomes a
                     restricted shell.

           The shell gives default values to PATH, PS1, PS2, MAILCHECK
           and IFS.  HOME and MAIL are set by login(1).



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         Blank Interpretation
           After parameter and command substitution, the results of
           substitution are scanned for internal field separator
           characters (those found in IFS) and split into distinct
           arguments where such characters are found.  Explicit null
           arguments ("" or '') are retained.  Implicit null arguments
           (those resulting from parameters that have no values) are
           removed.

         File Name Generation
           Following substitution, each command word is scanned for the
           characters *, ?, and [.  If one of these characters appears
           the word is regarded as a pattern.  The word is replaced
           with alphabetically sorted file names that match the
           pattern.  If no file name is found that matches the pattern,
           the word is left unchanged.  The character . at the start of
           a file name or immediately following a /, as well as the
           character / itself, must be matched explicitly.

                *    Matches any string, including the null string.
                ?    Matches any single character.
                [...]
                     Matches any one of the enclosed characters.  A
                     pair of characters separated by - matches any
                     character lexically between the pair, inclusive.
                     If the first character following the opening ``[''
                     is a ``!'' any character not enclosed is matched.

         Quoting
           The following characters have a special meaning to the shell
           and cause termination of a word unless quoted:

                ;  &  (  )  |  ^  <  >  new-line  space  tab

           A character may be quoted (i.e., made to stand for itself)
           by preceding it with a \.  The pair \new-line is ignored.
           All characters enclosed between a pair of single quote marks
           (''), except a single quote, are quoted.  Inside double
           quote marks (""), parameter and command substitution occurs
           and \ quotes the characters \, `, ", and $.  "$*" is
           equivalent to "$1 $2 ...", whereas "$@" is equivalent to
           "$1" "$2" ....

         Prompting
           When used interactively, the shell prompts with the value of
           PS1 before reading a command.  If at any time a new-line is
           typed and further input is needed to complete a command, the
           secondary prompt (i.e., the value of PS2) is issued.

         Input/Output
           Before a command is executed, its input and output may be
           redirected using a special notation interpreted by the



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           shell.  The following may appear anywhere in a simple-
           command or may precede or follow a command and are not
           passed on to the invoked command; substitution occurs before
           word or digit is used:

           <word         Use file word as standard input (file
                         descriptor 0).
           >word         Use file word as standard output (file
                         descriptor 1).  If the file does not exist it
                         is created; otherwise, it is truncated to zero
                         length.
           >>word        Use file word as standard output.  If the file
                         exists output is appended to it (by first
                         seeking to the end-of-file); otherwise, the
                         file is created.
           <<[-]word     The shell input is read up to a line that is
                         the same as word, or to an end-of-file.  The
                         resulting document becomes the standard input.
                         If any character of word is quoted, no
                         interpretation is placed upon the characters
                         of the document; otherwise, parameter and
                         command substitution occurs, (unescaped)
                         \new-line is ignored, and \ must be used to
                         quote the characters \, $, `, and the first
                         character of word.  If - is appended to <<,
                         all leading tabs are stripped from word and
                         from the document.
           <&digit       Use the file associated with file descriptor
                         digit as standard input.  Similarly for the
                         standard output using >&digit.
           <&-           The standard input is closed.  Similarly for
                         the standard output using >&-.

           If any of the above is preceded by a digit, the file
           descriptor which will be associated with the file is that
           specified by the digit (instead of the default 0 or 1).  For
           example:

                ... 2>&1

           associates file descriptor 2 with the file currently
           associated with file descriptor 1.

           The order in which redirections are specified is
           significant.  The shell evaluates redirections left-to-
           right.  For example:

                ... 1>xxx 2>&1

           first associates file descriptor 1 with file xxx.  It
           associates file descriptor 2 with the file associated with
           file descriptor 1 (i.e. xxx).  If the order of redirections



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           were reversed, file descriptor 2 would be associated with
           the terminal (assuming file descriptor 1 had been) and file
           descriptor 1 would be associated with file xxx.

           If a command is followed by & the default standard input for
           the command is the empty file /dev/null.  Otherwise, the
           environment for the execution of a command contains the file
           descriptors of the invoking shell as modified by
           input/output specifications.

           Redirection of output is not allowed in the restricted
           shell.

         Environment
           The environment (see environ(5)) is a list of name-value
           pairs that is passed to an executed program in the same way
           as a normal argument list.  The shell interacts with the
           environment in several ways.  On invocation, the shell scans
           the environment and creates a parameter for each name found,
           giving it the corresponding value.  If the user modifies the
           value of any of these parameters or creates new parameters,
           none of these affects the environment unless the export
           command is used to bind the shell's parameter to the
           environment (see also set -a).  A parameter may be removed
           from the environment with the unsetcommand.  The environment
           seen by any executed command is thus composed of any
           unmodified name-value pairs originally inherited by the
           shell, minus any pairs removed by unset, plus any
           modifications or additions, all of which must be noted in
           export commands.

           The environment for any simple-command may be augmented by
           prefixing it with one or more assignments to parameters.
           Thus:

                TERM=450 cmd                  and
                (export TERM; TERM=450; cmd)

           are equivalent (as far as the execution of cmd is
           concerned).

           If the -k flag is set, all keyword arguments are placed in
           the environment, even if they occur after the command name.
           The following first prints a=b c and c:

                echo a=b c
                set -k
                echo a=b c

         Signals
           The INTERRUPT and QUIT signals for an invoked command are
           ignored if the command is followed by &; otherwise signals



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           have the values inherited by the shell from its parent, with
           the exception of signal 11 (but see also the trap command
           below).

         Execution
           Each time a command is executed, the above substitutions are
           carried out.  If the command name matches one of the Special
           Commands listed below, it is executed in the shell process.
           If the command name does not match a Special Command, but
           matches the name of a defined function, the function is
           executed in the shell process (note how this differs from
           the execution of shell procedures).  The positional
           parameters $1, $2, ....  are set to the arguments of the
           function.  If the command name matches neither a Special
           Command nor the name of a defined function, a new process is
           created and an attempt is made to execute the command via
           exec(2).

           The shell parameter PATH defines the search path for the
           directory containing the command.  Alternative directory
           names are separated by a colon (:).  The default path is
           :/bin:/usr/bin (specifying the current directory, /bin, and
           /usr/bin, in that order).  Note that the current directory
           is specified by a null path name, which can appear
           immediately after the equal sign or between the colon
           delimiters anywhere else in the path list.  If the command
           name contains a / the search path is not used; such commands
           will not be executed by the restricted shell.  Otherwise,
           each directory in the path is searched for an executable
           file.  If the file has execute permission but is not an
           a.out file, it is assumed to be a file containing shell
           commands.  A sub-shell is spawned to read it.  A
           parenthesized command is also executed in a sub-shell.

           The location in the search path where a command was found is
           remembered by the shell (to help avoid unnecessary execs
           later).  If the command was found in a relative directory,
           its location must be re-determined whenever the current
           directory changes.  The shell forgets all remembered
           locations whenever the PATH variable is changed or the hash
           -r command is executed (see below).

         Special Commands
           Input/output redirection is now permitted for these
           commands.  File descriptor 1 is the default output location.

           :    No effect; the command does nothing.  A zero exit code
                is returned.
           . file
                Read and execute commands from file and return.  The
                search path specified by PATH is used to find the
                directory containing file.



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           break [ n ]
                Exit from the enclosing for or while loop, if any.  If
                n is specified break n levels.
           continue [ n ]
                Resume the next iteration of the enclosing for or while
                loop.  If n is specified resume at the n-th enclosing
                loop.
           cd [ arg ]
                Change the current directory to arg.  The shell
                parameter HOME is the default arg.  The shell parameter
                CDPATH defines the search path for the directory
                containing arg.  Alternative directory names are
                separated by a colon (:).  The default path is <null>
                (specifying the current directory).  Note that the
                current directory is specified by a null path name,
                which can appear immediately after the equal sign or
                between the colon delimiters anywhere else in the path
                list.  If arg begins with a / the search path is not
                used.  Otherwise, each directory in the path is
                searched for arg.  The cd command may not be executed
                by rshell.
           echo [ arg ... ]
                Echo arguments.
           eval [ arg ... ]
                The arguments are read as input to the shell and the
                resulting command(s) executed.
           exec [ arg ... ]
                The command specified by the arguments is executed in
                place of this shell without creating a new process.
                Input/output arguments may appear and, if no other
                arguments are given, cause the shell input/output to be
                modified.
           exit [ n ]
                Causes a shell to exit with the exit status specified
                by n.  If n is omitted the exit status is that of the
                last command executed (an end-of-file will also cause
                the shell to exit.)
           export [ name ... ]
                The given names are marked for automatic export to the
                environment of subsequently-executed commands.  If no
                arguments are given, a list of all names that are
                exported in this shell is printed.  Function names may
                not be exported.
           hash [ -r ] [ name ... ]
                For each name, the location in the search path of the
                command specified by name is determined and remembered
                by the shell.  The -r option causes the shell to forget
                all remembered locations.  If no arguments are given,
                information about remembered commands is presented.
                Hits is the number of times a command has been invoked
                by the shell process.  Cost is a measure of the work
                required to locate a command in the search path.  There



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                are certain situations which require that the stored
                location of a command be recalculated.  Commands for
                which this will be done are indicated by an asterisk
                (*) adjacent to the hits information.  Cost will be
                incremented when the recalculation is done.
           newgrp [ arg ... ]
                Equivalent to exec newgrp arg ....  See newgrp(1) for
                usage and description.
           pwd  Print the current working directory.  See pwd(1) for
                usage and description.
           read [ name ... ]
                One line is read from the standard input and the first
                word is assigned to the first name, the second word to
                the second name, etc., with leftover words assigned to
                the last name.  The return code is 0 unless an end-of-
                file is encountered.
           readonly [ name ... ]
                The given names are marked readonly and the values of
                the these names may not be changed by subsequent
                assignment.  If no arguments are given, a list of all
                readonly names is printed.
           return [ n ]
                Causes a function to exit with the return value
                specified by n.  If n is omitted, the return status is
                that of the last command executed.
           set [ --aefhkntuvx [ arg ... ] ]
                -a   Mark variables which are modified or created for
                     export.
                -e   Exit immediately if a command exits with a non-
                     zero exit status.
                -f   Disable file name generation
                -h   Locate and remember function commands as functions
                     are defined (function commands are normally
                     located when the function is executed).
                -k   All keyword arguments are placed in the
                     environment for a command, not just those that
                     precede the command name.
                -n   Read commands but do not execute them.
                -t   Exit after reading and executing one command.
                -u   Treat unset variables as an error when
                     substituting.
                -v   Print shell input lines as they are read.
                -x   Print commands and their arguments as they are
                     executed.
                --   Do not change any of the flags; useful in setting
                     $1 to -.
                Using + rather than - causes these flags to be turned
                off.  These flags can also be used upon invocation of
                the shell.  The current set of flags may be found in
                $-.  The remaining arguments are positional parameters
                and are assigned, in order, to $1, $2, ....  If no
                arguments are given the values of all names are



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                printed.
           shift [ n ]
                The positional parameters from $n+1 ...  are renamed $1
                ....  If n is not given, it is assumed to be 1.
           test
                Evaluate conditional expressions. See test(1) for usage
                and description.
           times
                Print the accumulated user and system times for
                processes run from the shell.
           trap [ arg ] [ n ] ...
                The command arg is to be read and executed when the
                shell receives signal(s) n.  (Note that arg is scanned
                once when the trap is set and once when the trap is
                taken.) Trap commands are executed in order of signal
                number.  Any attempt to set a trap on a signal that was
                ignored on entry to the current shell is ineffective.
                An attempt to trap on signal 11 (memory fault) produces
                an error.  If arg is absent all trap(s) n are reset to
                their original values.  If arg is the null string this
                signal is ignored by the shell and by the commands it
                invokes.  If n is 0 the command arg is executed on exit
                from the shell.  The trap command with no arguments
                prints a list of commands associated with each signal
                number.
           type [ name ... ]
                For each name, indicate how it would be interpreted if
                used as a command name.
           ulimit [ -fp ] [ n ]
                imposes a size limit of n
                -f   imposes a size limit of n blocks on files written
                     by child processes (files of any size may be
                     read).  With no argument, the current limit is
                     printed.
                -p   changes the pipe size to n (UNIX/RT only).
                If no option is given, -f is assumed.
           umask [ nnn ]
                The user file-creation mask is set to nnn (see
                umask(2)).  If nnn is omitted, the current value of the
                mask is printed.
           unset [ name ... ]
                For each name, remove the corresponding variable or
                function.  The variables PATH, PS1, PS2, MAILCHECK and
                IFS cannot be unset.
           wait [ n ]
                Wait for the specified process and report its
                termination status.  If n is not given all currently
                active child processes are waited for and the return
                code is zero.

         Invocation
           If the shell is invoked through exec(2) and the first



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           character of argument zero is -, commands are initially read
           from /etc/profile and from $HOME/.profile, if such files
           exist.  Thereafter, commands are read as described below,
           which is also the case when the shell is invoked as /bin/sh.
           The flags below are interpreted by the shell on invocation
           only; Note that unless the -c or -s flag is specified, the
           first argument is assumed to be the name of a file
           containing commands, and the remaining arguments are passed
           as positional parameters to that command file:

           -c string If the -c flag is present commands are read from
                     string.
           -s        If the -s flag is present or if no arguments
                     remain commands are read from the standard input.
                     Any remaining arguments specify the positional
                     parameters.  Shell output (except for Special
                     Commands) is written to file descriptor 2.
           -i        If the -i flag is present or if the shell input
                     and output are attached to a terminal, this shell
                     is interactive.  In this case TERMINATE is ignored
                     (so that kill 0 does not kill an interactive
                     shell) and INTERRUPT is caught and ignored (so
                     that wait is interruptible).  In all cases, QUIT
                     is ignored by the shell.
           -r        If the -r flag is present the shell is a
                     restricted shell.

           The remaining flags and arguments are described under the
           set command above.

         Rshell Only
           Rshell is used to set up login names and execution
           environments whose capabilities are more controlled than
           those of the standard shell.  The actions of rshell are
           identical to those of sh, except that the following are
           disallowed:
                changing directory (see cd(1)),
                setting the value of $PATH,
                specifying path or command names containing /,
                redirecting output (> and >>).

           The restrictions above are enforced after .profile is
           interpreted.

           When a command to be executed is found to be a shell
           procedure, rshell invokes sh to execute it.  Thus, it is
           possible to provide to the end-user shell procedures that
           have access to the full power of the standard shell, while
           imposing a limited menu of commands; this scheme assumes
           that the end-user does not have write and execute
           permissions in the same directory.




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           The net effect of these rules is that the writer of the
           .profile has complete control over user actions, by
           performing guaranteed setup actions and leaving the user in
           an appropriate directory (probably not the login directory).

           The system administrator often sets up a directory of
           commands (i.e., /usr/rbin) that can be safely invoked by
           rshell.  Some systems also provide a restricted editor red.















































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      EXIT STATUS
           Errors detected by the shell, such as syntax errors, cause
           the shell to return a non-zero exit status.  If the shell is
           being used non-interactively execution of the shell file is
           abandoned.  Otherwise, the shell returns the exit status of
           the last command executed (see also the exit command above).

      FILES
           /etc/profile
           $HOME/.profile
           /tmp/sh*
           /dev/null

      SEE ALSO
           acctcom(1), cd(1), echo(1), env(1), login(1), newgrp(1),
           pwd(1), test(1), umask(1).
           dup(2), exec(2), fork(2), pipe(2), signal(2), ulimit(2),
           umask(2), wait(2), a.out(4), profile(4), environ(5).

      CAVEATS
           If a command is executed, and a command with the same name
           is installed in a directory in the search path before the
           directory where the original command was found, the shell
           will continue to exec the original command.  Use the hash
           command to correct this situation.

           If you move the current directory or one above it, pwd may
           not give the correct response.  Use the cd command with a
           full path name to correct this situation.


























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