SH(1)                                                                    SH(1)


NAME
       sh,  for, case, if, while, :, ., break, continue, cd, eval, exec, exit,
       export, login, 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 execve(2)).  The value of a simple-command
       is its exit status if it terminates normally or 200+status if it termi‐
       nates abnormally (see sigvec(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,  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  back  quotes
       (``)  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,  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, substitute its value; otherwise  substitute
              word.

       ${parameter=word}
              If parameter is not set, set it to word; the value of the param‐
              eter is then substituted.   Positional  parameters  may  not  be
              assigned to in this way.

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

       ${parameter+word}
              If parameter is set, substitute word; otherwise substitute noth‐
              ing.

       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, 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.  IFS is ignored if sh is running as root or if  the
                     effective user id differs from the real user id.

       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, 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 /, 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  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.

            ;   &   (   )   |   <   >   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, 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, it is created; otherwise it is truncated to
              zero length.

       >>word Use file word as standard output.  If the file exists, output 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,  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, 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 & 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 execve(2)
       and environ(7).  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 &; 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 execve(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 /, the search path is not used.  Otherwise, each direc‐
       tory 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 sub‐
       shell.

       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.

       #      For  non-interactive  shells,  everything  following  the  #  is
              treated as a comment, i.e. the rest of the line is ignored.  For
              interactive shells, the # has no special effect.

       :      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, 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.
       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, 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, a list of exportable names is printed.
       login [arg ...]
              Equivalent to ’exec login 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, a list of all readonly names is printed.
       set [-eknptuvx [arg ...]]
              -e If non interactive, 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, 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, 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 invoked  com‐
              mands.   If n is 0, the command arg is executed on exit from the
              shell, otherwise  upon  receipt  of  signal  n  as  numbered  in
              sigvec(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, all currently active child processes
              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, 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  sigvec(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 interruptible).  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
       csh(1), test(1), execve(2), environ(7)

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.


7th Edition                       May 5, 1986                            SH(1)
 
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