man(1) Manual page archive

     SH(1)                                                       SH(1)

          sh, for, case, if, while, :, ., break, continue, cd, eval,
          exec, exit, export, login, newgrp, read, readonly, set,
          shift, times, trap, umask, wait - command language

          sh [ -ceiknrstuvx ] [ arg ] ...

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

          A simple-command is a sequence of non blank words separated
          by blanks (a blank is a tab or a space).  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 argu-
          ment 0 (see exec(2)). The value of a simple-command is its
          exit status if it terminates normally or 200+status if it
          terminates abnormally (see signal(2) for a list of status

          A pipeline is a sequence of one or more commands separated
          by or. 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.

          A list is a sequence of one or more pipelines separated by
          ;, &, && or oror and optionally terminated by ; or &.  ; and &
          have equal precedence which is lower than that of && and oror,
          && and oror also have equal precedence.  A semicolon causes
          sequential execution; an ampersand causes the preceding
          pipeline to be executed without waiting for it to finish.
          The symbol && (oror) causes the list following to be executed
          only if the preceding pipeline returns a zero (non zero)
          value.  Newlines may appear in a list, instead of semi-
          colons, to delimit commands.

          A command is either a simple-command or one of the follow-
          ing.  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 in the for word list If in word ...  is omit-
               ted then in "$@" is assumed.  Execution ends when there
               are no more words in the list.

     SH(1)                                                       SH(1)

          case word in [pattern [ or 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 genera-

          if list then list [elif list then list] ... [else list] fi
               The list following if is executed and if it returns
               zero the list following then is executed.  Otherwise,
               the list following elif is executed and if its value is
               zero the list following then is executed.  Failing that
               the else list is executed.

          while list [do list] done
               A while command repeatedly executes the while list and
               if its value is zero executes the do list; otherwise
               the loop terminates.  The value returned by a while
               command is that of the last executed command in the do
               list. until may be used in place of while to negate the
               loop termination test.

          ( list )
               Execute list in a subshell.

          { list }
               list is simply executed.

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

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

          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 newlines are removed.

          Parameter substitution.
          The character $ is used to introduce substitutable parame-
          ters.  Positional parameters may be assigned values by set.
          Variables may be set by writing

               name=value [ name=value ] ...

               A parameter is a sequence of letters, digits or under-
               scores (a name), a digit, or any of the characters * @
               # ? - $ !.  The value, if any, of the parameter is sub-
               stituted.  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

     SH(1)                                                       SH(1)

               parameter is a digit then it is a positional parameter.
               If parameter is * or @ then all the positional parame-
               ters, starting with $1, are substituted separated by
               spaces.  $0 is set from argument zero when the shell is

               If parameter is set then substitute its value; other-
               wise substitute word.

               If parameter is not set then set it to word; the value
               of the parameter is then substituted.  Positional
               parameters may not be assigned to in this way.

               If parameter is set then substitute its value; other-
               wise, print word and exit from the shell.  If word is
               omitted then a standard message is printed.

               If parameter is set then substitute word; otherwise
               substitute nothing.

          In the above word is not evaluated unless it is to be used
          as the substituted string.  (So that, for example, echo
          ${d-`pwd`} will only execute pwd if d is unset.)

          The following parameters are automatically set by the shell.

               #    The number of positional parameters in decimal.
               -    Options supplied to the shell on invocation or by
               ?    The value returned by the last executed command in
               $    The process number of this shell.
               !    The process number of the last background command

          The following parameters are used but not set by the shell.

               HOME The default argument (home directory) for the cd
               PATH The search path for commands (see execution).
               MAIL If this variable is set to the name of a mail file
                    then the shell informs the user of the arrival of
                    mail in the specified file.
               PS1  Primary prompt string, by default `$ '.
               PS2  Secondary prompt string, by default `> '.
               IFS  Internal field separators, normally space, tab,
                    and newline.

     SH(1)                                                       SH(1)

          Blank interpretation.
          After parameter and command substitution, any 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
          then 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 pat-
          tern then the word is left unchanged.  The character . at
          the start of a file name or immediately following a /, and
          the character /, must be matched explicitly.

          *    Matches any string, including the null string.
          ?    Matches any single character.
               Matches any one of the characters enclosed.  A pair of
               characters separated by - matches any character lexi-
               cally between the pair.

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

               ;   &   (   )   or   <   >   newline   space   tab

          A character may be quoted by preceding it with a \.
          \newline is ignored.  All characters enclosed between a pair
          of quote marks (''), except a single quote, are quoted.
          Inside double quotes ("") 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 newline is
          typed and further input is needed to complete a command then
          the secondary prompt ($PS2) is issued.

          Input output.
          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

     SH(1)                                                       SH(1)

          passed on to the invoked command.  Substitution occurs
          before word or digit is used.

               Use file word as standard input (file descriptor 0).

               Use file word as standard output (file descriptor 1).
               If the file does not exist then it is created; other-
               wise it is truncated to zero length.

               Use file word as standard output.  If the file exists
               then output is appended (by seeking to the end); other-
               wise the file is created.

               The shell input is read up to a line the same as word,
               or end of file.  The resulting document becomes the
               standard input.  If any character of word is quoted
               then no interpretation is placed upon the characters of
               the document; otherwise, parameter and command substi-
               tution occurs, \newline is ignored, and \ is used to
               quote the characters \ $ ` and the first character of

               The standard input is duplicated from file descriptor
               digit; see dup(2). Similarly for the standard output
               using >.

          <&-  The standard input is closed.  Similarly for the stan-
               dard output using >.

          If one of the above is preceded by a digit then the file
          descriptor created is that specified by the digit (instead
          of the default 0 or 1).  For example,

               ... 2>&1

          creates file descriptor 2 to be a duplicate of file descrip-
          tor 1.

          If a command is followed by & then the default standard
          input for the command is the empty file (/dev/null).  Other-
          wise, the environment for the execution of a command con-
          tains the file descriptors of the invoking shell as modified
          by input output specifications.

          The environment is a list of name-value pairs that is passed
          to an executed program in the same way as a normal argument

     SH(1)                                                       SH(1)

          list; see exec(2) and environ(5). 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.  Executed commands
          inherit the same environment.  If the user modifies the val-
          ues of these parameters or creates new ones, none of these
          affects the environment unless the export command is used to
          bind the shell's parameter to the environment.  The environ-
          ment seen by any executed command is thus composed of any
          unmodified name-value pairs originally inherited by the
          shell, 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 these two lines are equivalent

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

          If the -k flag is set, all keyword arguments are placed in
          the environment, even if the occur after the command name.
          The following prints `a=b c' and `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.
          (But see also trap.)

          Each time a command is executed the above substitutions are
          carried out.  Except for the `special commands' listed below
          a new process is created and an attempt is made to execute
          the command via an exec(2).

          The shell parameter $PATH defines the search path for the
          directory containing the command.  Each alternative direc-
          tory name is separated by a colon (:).  The default path is
          :/bin:/usr/bin.  If the command name contains a / then 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 subshell (i.e., a
          separate process) is spawned to read it.  A parenthesized
          command is also executed in a subshell.

          Special commands.
          The following commands are executed in the shell process and

     SH(1)                                                       SH(1)

          except where specified no input output redirection is per-
          mitted for such commands.

          :    No effect; the command does nothing.
          . file
               Read and execute commands from file and return.  The
               search path $PATH is used to find the directory con-
               taining file.
          break [n]
               Exit from the enclosing for or while loop, if any.  If
               n is specified then break n levels.
          continue [n]
               Resume the next iteration of the enclosing for or while
               loop.  If n is specified then resume at the n-th
               enclosing loop.
          cd [arg]
               Change the current directory to arg. The shell parame-
               ter $HOME is the default 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 argu-
               ments are given cause the shell input output to be mod-
          exit [n]
               Causes a non interactive shell to exit with the exit
               status specified by n. If n is omitted then the exit
               status is that of the last command executed.  (An end
               of file will also exit from the shell.)
          export [name ...]
               The given names are marked for automatic export to the
               environment of subsequently-executed commands.  If no
               arguments are given then a list of exportable names is
          login [arg ...]
               Equivalent to `exec login arg ...'.
          newgrp [arg ...]
               Equivalent to `exec newgrp arg ...'.
          read name ...
               One line is read from the standard input; successive
               words of the input are assigned to the variables name
               in order, with leftover words to the last variable.
               The return code is 0 unless the end-of-file is encoun-
          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 then a list of
               all readonly names is printed.

     SH(1)                                                       SH(1)

          set [-eknptuvx [arg ...]]
               -e If non interactive then exit immediately if a com-
                  mand fails.
               -k All keyword arguments are placed in the environment
                  for a command, not just those that precede the com-
                  mand 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 exe-
               -  Turn off the -x and -v options.

               These flags can also be used upon invocation of the
               shell.  The current set of flags may be found in $-.

               Remaining arguments are positional parameters and are
               assigned, in order, to $1, $2, etc.  If no arguments
               are given then the values of all names are printed.

               The positional parameters from $2...  are renamed $1...

               Print the accumulated user and system times for pro-
               cesses run from the shell.

          trap [arg] [n] ...
               Arg is a command 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.
               If arg is absent then all trap(s) n are reset to their
               original values.  If arg is the null string then this
               signal is ignored by the shell and by invoked commands.
               If n is 0 then the command arg is executed on exit from
               the shell, otherwise upon receipt of signal n as num-
               bered in signal(2). Trap with no arguments prints a
               list of commands associated with each signal number.

          umask [ nnn ]
               The user file creation mask is set to the octal value
               nnn (see umask(2)). If nnn is omitted, the current
               value of the mask is printed.

          wait [n]
               Wait for the specified process and report its termina-
               tion status.  If n is not given then all currently
               active child processes are waited for.  The return code
               from this command is that of the process waited for.

     SH(1)                                                       SH(1)

          If the first character of argument zero is -, commands are
          read from $HOME/.profile, if such a file exists.  Commands
          are then read as described below.  The following flags are
          interpreted by the shell when it is invoked.
          -c string  If the -c flag is present then commands are read
                     from string.
          -s         If the -s flag is present or if no arguments
                     remain then commands are read from the standard
                     input.  Shell output is written to file descrip-
                     tor 2.
          -i         If the -i flag is present or if the shell input
                     and output are attached to a terminal (as told by
                     gtty) then this shell is interactive. In this
                     case the terminate signal SIGTERM (see signal(2))
                     is ignored (so that `kill 0' does not kill an
                     interactive shell) and the interrupt signal SIG-
                     INT is caught and ignored (so that wait is inter-
                     ruptable).  In all cases SIGQUIT is ignored by
                     the shell.

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


          test(1), exec(2),

          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 then execution of the shell
          file is abandoned.  Otherwise, the shell returns the exit
          status of the last command executed (see also exit).

          If << is used to provide standard input to an asynchronous
          process invoked by &, the shell gets mixed up about naming
          the input document.  A garbage file /tmp/sh* is created, and
          the shell complains about not being able to find the file by
          another name.