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SH(1)                       General Commands Manual                      SH(1)



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

SYNOPSIS
       sh [ -ceiknrstuvx ] [ arg ] ...

DESCRIPTION
       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.

       Commands.
       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 argument 0 (see exec(2)).  The value of a simple-command
       is its exit status if it terminates normally or 200+status if it termi-
       nates 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.

       A list is a sequence of one or more pipelines separated by ;, &&, &&&&  or
       ||  and optionally terminated by ; or &&.  ; and && have equal precedence
       which is lower than that of &&&& and ||, &&&& and || also have equal prece-
       dence.   A  semicolon  causes sequential execution; an ampersand causes
       the preceding pipeline to be executed without waiting for it to finish.
       The symbol &&&& (||) 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 semicolons, to delimit commands.

       A  command  is  either  a  simple-command or one of the following.  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 omitted then in "$@"  is
              assumed.   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  pat-
              tern that matches word.  The form of the patterns is the same as
              that used for file name generation.

       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  termi-
              nates.   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  parameters.   Posi-
       tional  parameters may be assigned values by set.  Variables may be set
       by writing

              name=value [ name=value ] ...

       ${parameter}
              A parameter is a sequence of letters, digits or  underscores  (a
              name),  a  digit,  or  any of the characters * @ # ? - $ !.  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 a digit then it is a positional parameter.  If
              parameter is * or @ then all the positional parameters, starting
              with  $1,  are  substituted separated by spaces.  $0 is set from
              argument zero when the shell is invoked.

       ${parameter-word}
              If parameter is set then substitute its value; otherwise substi-
              tute word.

       ${parameter=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.

       ${parameter?word}
              If  parameter is set then substitute its value; otherwise, print
              word and exit from the shell.  If word is omitted then  a  stan-
              dard message is printed.

       ${parameter+word}
              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  sub-
       stituted string.  (So that, for example, echo ${d-`pwd`} will only exe-
       cute 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 set.
              ?      The value returned by the last executed command in  deci-
                     mal.
              $      The process number of this shell.
              !      The   process  number  of  the  last  background  command
                     invoked.

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

              HOME   The default argument (home directory) for the cd command.
              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  new-
                     line.

       Blank interpretation.
       After  parameter  and command substitution, any 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 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 pattern then the word is left unchanged.  The  character  .
       at the start of a file name or immediately following a /, and the char-
       acter /, 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  charac-
              ters  separated by - matches any character lexically between the
              pair.

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

            ;   &&amp;   (   )   |   <&lt;   >&gt;   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" ... .

       Prompting.
       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  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 then it is created; otherwise it is trun-
              cated to zero length.

       >>word Use file word as standard output.  If the file exists then  out-
              put  is  appended (by seeking to the end); otherwise the file is
              created.

       <<word 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 substitution occurs, \newline is ignored, and \ is  used
              to quote the characters \ $ ` and the first character of word.

       <&digit
              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 standard 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 descriptor 1.

       If  a  command is followed by &&amp; then 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 invok-
       ing shell as modified by input output specifications.

       Environment.
       The environment is a list of name-value pairs that is passed to an exe-
       cuted  program  in  the same way as a normal argument 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.  Exe-
       cuted  commands inherit the same environment.  If the user modifies the
       values 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 environment  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 environ-
       ment, 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

       Signals.
       The  INTERRUPT  and  QUIT signals for an invoked command are ignored if
       the command is followed by &&amp;; otherwise signals have the values  inher-
       ited by the shell from its parent.  (But see also trap.)

       Execution.
       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 directory 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 except
       where specified no input output redirection is permitted for such  com-
       mands.

       :      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 containing 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 parameter  $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 argu-
              ments may appear and if no other arguments are given  cause  the
              shell input output to be modified.
       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  environ-
              ment  of  subsequently-executed  commands.   If no arguments are
              given then a list of exportable names is printed.
       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 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  argu-
              ments are given then a list of all readonly names is printed.
       set [-eknptuvx [arg ...]]
              -e If non interactive then exit immediately if a command fails.
              -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.
              -  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.

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

       times  Print  the  accumulated  user and system times for processes 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  exe-
              cuted  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 termination sta-
              tus.  If n is not given then all  currently  active  child  pro-
              cesses  are  waited  for.   The return code from this command is
              that of the process waited for.

       Invocation.
       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 descriptor 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  SIGINT  is
                  caught  and ignored (so that wait is interruptable).  In all
                  cases SIGQUIT is ignored by the shell.

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

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

SEE ALSO
       test(1), exec(2),

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

BUGS
       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.



                                                                         SH(1)