# 2.11BSD#431 - /usr/man/cat1/sh.0

    SH(1)		    UNIX Programmer's Manual		    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 argu-
         ment 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
         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 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 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
    	  omitted, in "$@" is assumed.	Execution ends when there
    	  are no more words in the list.

         case word in [pattern [ | pattern ] ... ) list ;;] ... esac

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    	  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-
    	  tion.

         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
         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 parame-
         ters.  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 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 parame-
    	  ter is a digit, it is a positional parameter.  If

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    	  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 substituted.  Positional parame-
    	  ters 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 omit-
    	  ted, a standard message is printed.

         ${parameter+word}
    	  If parameter is set, substitute word; otherwise substi-
    	  tute 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.

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         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
         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 pat-
         tern.  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 lexi-
    	  cally 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 \.  \new-
         line 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

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         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 stan-
    	  dard 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 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.

         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

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         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 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 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 followed 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 command.	Each alternative direc-
         tory 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 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

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         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 con-
    	  taining 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 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
    	  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 encoun-
    	  tered.
         readonly [name ...]
    	  The given names are marked readonly and the values of
    	  the these names may not be changed by subsequent

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    	  assignment.  If no arguments 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 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-
    	     cuted.
    	  -  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 ori-
    	  ginal 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 termina-
    	  tion status.	If n is not given, all currently active
    	  child processes are waited for.  The return code from

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    	  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 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 sigvec(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-
    		ruptible).  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 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.

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