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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 terminates 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 com-
       mand.  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 precedence.  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  pattern  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 exe-
	      cutes 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 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.  Positional 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  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 parame-
	      ter  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 parameter is then substi-
	      tuted.  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 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 set.
	      ?      The value returned by the last executed command in decimal.
	      $      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 newline.  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  argu-
       ments  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 characters  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 nota-
       tion 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.	Substitu-
       tion 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 seek-
	      ing 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 result-
	      ing  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 invoking shell as modified by input output specifications.

       Environment.
       The environment is a list of name-value pairs that is passed to an executed 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.  Executed com-
       mands 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  exe-
       cuted  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  com-
       mands.

       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

       Signals.
       The  INTERRUPT  and QUIT signals for an invoked command are ignored if the command is fol-
       lowed by &; otherwise signals have the values inherited 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 com-
       mand.  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.  Other-
       wise, each directory in the path is searched for an executable file.  If the file has exe-
       cute permission but is not an a.out file, it is assumed to be a file containing shell com-
       mands.  A subshell (i.e., a separate process) is spawned to read it.  A parenthesized com-
       mand 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 commands.

       #      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 lev-
	      els.
       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 arguments may appear and if  no  other  argu-
	      ments 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 environment 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 arguments are given, a list of  all  read-
	      only 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 executed 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 commands.  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  status.   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 interactively then execution of the shell
       file is abandoned.  Otherwise, the shell returns the exit status of the last command  exe-
       cuted (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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