man(1) Manual page archive

     SH(1)                                                       SH(1)

          sh, cd, wait - shell, the standard command programming

          sh [ -acefiknpstuvx ] [ args ]

          Sh is a command programming language that executes commands
          read from a terminal or a file.  See Invocation below for
          the meaning of arguments to the shell.

          A blank is a tab or a space.  A name is a sequence of let-
          ters, digits, or underscores beginning with a letter or
          underscore.  A parameter is a name, a digit, or any of the
          characters *, @, #, ?, -, $, and !.  A word is a sequence of
          characters and quoted strings, surrounded by blanks or new-

          A simple-command is a sequence of words separated by blanks.
          The first word specifies the name of the command to be exe-
          cuted.  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 |.  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 sym-
          bol && (||) 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 a
          list, instead of semicolons, to delimit commands.

     SH(1)                                                       SH(1)

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

          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 pat-
               terns is the same as that used for file-name generation
               (see File Name Generation) except that a slash, a lead-
               ing 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 exe-
               cuted 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 termi-
               nates.  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.
               Execute list in a sub-shell.
               list is simply executed.
          name () command
               Define a function which is referenced by name. The body
               of the function is the command. The most useful form of
               command is a sequence of commands enclosed by { and }.
               Execution of functions is described below (see

          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


     SH(1)                                                       SH(1)

          A word beginning with # causes that word and all the follow-
          ing 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.

               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.
               If parameter is set and is non-null, substitute its
               value; otherwise substitute 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.
               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.
               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 exam-
          ple, pwd is executed only if d is not set or is null:

               echo ${d:-`pwd`}

          If the colon (:) is omitted from the above expressions, the

     SH(1)                                                       SH(1)

          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 syn-
                    chronously executed command.
               $    The process number of this shell.
               !    The process number of the last background command

          The following parameters are used by the shell:
               HOME The default argument (home directory) for the cd
               PATH The search path for commands (see Execution
                    The search path for the cd command.
               MAIL If this parameter is set to the name of a mail
                    file the shell informs the user of the arrival of
                    mail in the specified file.  The file is inspected
                    every three minutes.
                    If this parameter is set to the name of a writable
                    file, the shell appends interactive input to the
                    file, for use by the = command (=(1)).
               PS1  Primary prompt string, by default ``$ ''.
               PS2  Secondary prompt string, by default ``> ''.
               IFS  Internal field separators, normally space, tab,
                    and new-line.

          The shell gives default values to PATH, PS1, PS2 and IFS.
          HOME is set by login(8).

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

        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 directories . and .. (initially or
          after a /) are only matched by patterns beginning with an

     SH(1)                                                       SH(1)

          explicit period.  The character / itself must be matched

               *    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.

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

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

          (The characters { and } need not be quoted inside a ${} con-
          struction.)  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" ....

          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.

          Before a command is executed, its input and output may be
          redirected using a special notation interpreted by the
          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 descrip-
                        tor 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
          >>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.

     SH(1)                                                       SH(1)

          <<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 inter-
                        pretation 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 char-
                        acters \, $, `, and the first character of
          <&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

               ... 2>&1

          associates file descriptor 2 with the file currently associ-
          ated with file descriptor 1.

          The order in which redirections are specified is signifi-
          cant.  The shell evaluates redirections left-to-right.  For

               ... 1>xxx 2>&1

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

          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 or function for each
          name found, giving it the corresponding value.  If the user

     SH(1)                                                       SH(1)

          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 unset command.  The environ-
          ment 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 modifica-
          tions or additions, all of which must be noted in export

          The environment for any simple-command may be augmented by
          prefixing it with one or more assignments to parameters (but
          not functions).  Thus:

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

          are equivalent (as far as the execution of cmd is con-

          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 then c:

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

          The INTERRUPT and QUIT signals for an invoked command are
          ignored if the command is followed by &; otherwise signals
          have the values inherited by the shell from its parent, with
          the exception of signal 11 (but see also the trap command

          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 exe-
          cuted 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

     SH(1)                                                       SH(1)

          :/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 immedi-
          ately after the equal sign or between the colon delimiters
          anywhere else in the path list.  If the command name con-
          tains a / the search path is not used.  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.

        Special Commands
          Input/output redirection is 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.
          builtin [ command ]
               Execute the built-in special command (such as break)
               regardless of functions defined with the same name.
          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
          cd [ arg ]
               Change the current directory to arg. The shell parame-
               ter 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 immedi-
               ately after the equal sign or between the colon delim-
               iters 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.
          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

     SH(1)                                                       SH(1)

          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.
          newgrp [ arg ... ]
               Equivalent to exec newgrp arg ....  See newgrp(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.
          return [ n ]
               Causes a function to exit with the return value speci-
               fied by n. If n is omitted, the return status is that
               of the last command executed.
          set [ --aehknptuvx [ arg ... ] ]
               -a   Mark variables which are modified or created for
               -e   Exit immediately if a command exits with a non-
                    zero exit status.
               -f   Disable file name generation
               -k   All keyword arguments are placed in the environ-
                    ment for a command, not just those that precede
                    the command name.
               -n   Read commands but do not execute them.
               -p   Remove the definitions for all functions imported
                    from the environment, and set IFS to blank, tab
                    and newline.
               -t   Exit after reading and executing one command.
               -u   Treat unset variables as an error when substitut-
               -v   Print shell input lines as they are read.
               -x   Print commands and their arguments as they are
               --   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

     SH(1)                                                       SH(1)

          shift [ n ]
               The positional parameters from $n+1 ...  are renamed $1
               ....  If n is not given, it is assumed to be 1.
               Print the accumulated user and system times for pro-
               cesses 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
          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 and IFS cannot
               be unset.
          wait [ n ]
               Wait for the specified process and report its termina-
               tion status.  If n is not given all currently active
               child processes are waited for and the return code is
          whatis [ name ... ]
               For each name, print the associated value as a parame-
               ter, function, builtin or executable binary as appro-
               priate.  In each case, the value is printed in a form
               that would yield the same value if typed as input to
               the shell itself: parameters are printed as assign-
               ments, functions as their definitions, builtins as
               calls to builtin, and binaries as their full pathnames.

          If the shell is invoked through exec(2) and the first char-
          acter of argument zero is -, commands are initially read
          from $HOME/.profile, if it exists.  Thereafter, commands are
          read as described below, which is also the case when the
          shell is invoked as /bin/sh.  The flags below are inter-
          preted 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

     SH(1)                                                       SH(1)

          arguments are passed as positional parameters to that com-
          mand file:

          -c string If the -c flag is present commands are read from
          -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.

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

          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).


          =(1), cd(1), echo(1), newgrp(1), test(1)
          dup(2), exec(2), fork(2), pipe(2), signal(2), umask(2),
          wait(2), a.out(5), environ(5)

          A function invocation overwrites the arguments of the invok-
          ing shell.