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Manual of the Bourne Shell on System III

released under the license of Caldera

original sh.1 massaged, nroffed and htmlized.

SH(1)                                                                    SH(1)

       sh - shell, the standard command programming language

       sh [ -ceiknrstuvx ] [ args ]

       Sh is a command programming language that executes commands read from a
       terminal or a file.  See Invocation below 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 argument 0 (see exec(2)).  The value of a simple-command
       is its exit status if it terminates normally, or (octal) 200+status  if
       it terminates 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 &.  Of these four symbols, ; and
       & have equal precedence, which is lower than that of &&  and  ||.   The
       symbols  &&  and || also have equal precedence.  A semicolon (;) causes
       sequential execution of the preceding pipeline; an ampersand (&) causes
       asynchronous  execution of the preceding pipeline (i.e., the shell does
       not wait for that pipeline to finish).  The symbol && (||)  causes  the
       list following it to be executed only if the preceding pipeline returns
       a zero (non-zero) exit status.  An arbitrary number  of  new-lines  may
       appear in a list, instead of semicolons, to delimit commands.

       A  command  is either a simple-command or one of the following.  Unless
       otherwise stated, 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 taken from the in word list.  If in word ...   is  omitted,
              then  the  for  command executes the do list once for each posi-
              tional parameter that is set (see Parameter Substitution below).
              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 (see File Name Generation
       if list then list [ elif list then list ] ... [ else list ] fi
              The list following if is executed and, if it returns a zero exit
              status,  the  list following the first then is executed.  Other-
              wise, the list following elif is executed and, if its  value  is
              zero,  the  list  following  the next then is executed.  Failing
              that, the else list is executed.  If no else list or  then  list
              is executed, then the if command returns a zero exit status.
       while list do list done
              A  while  command repeatedly executes the while list and, if the
              exit status of the last command in the list  is  zero,  executes
              the  do  list; otherwise the loop terminates.  If no commands in
              the do list are executed, then the while command returns a  zero
              exit  status;  until may be used in place of while to negate the
              loop termination test.
              Execute list in a sub-shell.
              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 esac for while until do done { }

       A  word beginning with # causes that word and all the following charac-
       ters up to a new-line to be ignored.

   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 new-lines are

   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 ] ...

       Pattern-matching is not performed on value.

              A parameter is a sequence of letters, digits, or underscores  (a
              name),  a  digit, or any of the characters *, @, #, ?, -, $, and
              !.  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.   A  name must begin with a letter or underscore.  If
              parameter is a digit then it  is  a  positional  parameter.   If
              parameter  is * or @, then all the positional parameters, start-
              ing with $1, are substituted (separated by  spaces).   Parameter
              $0 is set from argument zero when the shell is invoked.
              If  parameter  is set and is non-null then substitute its value;
              otherwise substitute word.
              If parameter is not set or is null then  set  it  to  word;  the
              value  of the parameter is then substituted.  Positional parame-
              ters may not be assigned to in this way.
              If parameter is set and is non-null then substitute  its  value;
              otherwise, print word and exit from the shell.  If word is omit-
              ted, then the message ``parameter null or not set'' is  printed.
              If parameter is set and is non-null then substitute word; other-
              wise substitute nothing.

       In the above, word is not evaluated unless it is to be used as the sub-
       stituted  string,  so  that,  in the following example, pwd is executed
       only if d is not set or is null:

              echo ${d:-`pwd`}

       If the colon (:) is omitted from the above expressions, then the  shell
       only checks whether parameter is set or not.

       The following parameters are automatically set by the shell:
              #      The number of positional parameters in decimal.
              -      Flags  supplied  to the shell on invocation or by the set
              ?      The decimal value returned by the last synchronously exe-
                     cuted command.
              $      The process number of this shell.
              !      The   process  number  of  the  last  background  command

       The following parameters are used by the shell:
              HOME   The default argument (home directory) for the cd command.
              PATH   The search path for commands (see Execution below).
              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-

       The shell gives default values to PATH, PS1, PS2, and IFS,  while  HOME
       and  MAIL  are  not  set  at  all by the shell (although HOME is set by

   Blank Interpretation.
       After parameter and command substitution, the 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 argu-
       ments (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 pattern, then the word is left unchanged.  The character .
       at the start of a file name or immediately following a /,  as  well  as
       the character / itself, must be matched explicitly.

              *      Matches any string, including the null string.
              ?      Matches any single character.
              [...]  Matches  any  one  of the enclosed characters.  A pair of
                     characters separated by - matches any character lexically
                     between the pair, inclusive.

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

              ;  &  (  )  |  <  >  new-line  space  tab

       A character may be quoted (i.e., made to stand for itself) by preceding
       it  with  a \.  The pair \new-line is ignored.  All characters enclosed
       between a pair of single quote marks (''), except a single  quote,  are
       quoted.   Inside double quote marks (""), parameter and command substi-
       tution 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 new-line is typed and further
       input is needed to complete a command, then the secondary prompt (i.e.,
       the value of PS2) is issued.

       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 truncated to zero length.
       >>word        Use  file  word  as  standard output.  If the file exists
                     then output is appended to it (by first  seeking  to  the
                     end-of-file); otherwise, the file is created.
       <<[-]word     The  shell input is read up to a line that is the same as
                     word, or  to  an  end-of-file.   The  resulting  document
                     becomes  the standard input.  If any character of word is
                     quoted, then no interpretation is placed upon the charac-
                     ters  of  the  document; otherwise, parameter and command
                     substitution occurs, (unescaped)  \new-line  is  ignored,
                     and  \  must be used to quote the characters \, $, `, and
                     the first character of word.  If -  is  appended  to  <<,
                     then all leading tabs are stripped from word and from the
       <&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 that is 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.

       The environment (see environ(7)) is a list of name-value pairs that  is
       passed  to  an  executed  program  in the same way as a normal argument
       list.  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 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 com-
       posed  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:

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

       are equivalent (as far as the above execution of cmd is concerned).

       If the -k flag is set, all keyword arguments are placed in the environ-
       ment,  even  if they occur after the command name.  The following first
       prints a=b c and then 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 inher-
       ited by the shell from its parent, with the exception of signal 11 (but
       see also the trap command below).

       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 exec(2).

       The shell parameter PATH defines the search path for the directory con-
       taining the command.  Alternative directory names are  separated  by  a
       colon  (:).  The default path is :/bin:/usr/bin (specifying the current
       directory, /bin, and /usr/bin, in that order).  Note that  the  current
       directory  is  specified  by a null path name, which can appear immedi-
       ately after the equal sign or between  the  colon  delimiters  anywhere
       else  in  the  path  list.   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  sub-shell  (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 as
       specified, no input/output redirection is permitted for such commands:

       :      No  effect;  the  command  does  nothing.   A  zero exit code is
       . file Read and execute commands from file and return.  The search path
              specified by 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 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 cause the shell to exit.)
       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 all names that are exported in this shell
              is printed.
       newgrp [ arg ... ]
              Equivalent to exec newgrp arg ....
       read [ name ... ]
              One line is read from the standard input and the first  word  is
              assigned  to the first name, the second word to the second name,
              etc., with leftover words assigned to the last name.  The return
              code is 0 unless an 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 [ -ekntuvx [ arg ... ] ]
              -e     If  the shell is non-interactive then exit immediately if
                     a command exits with a non-zero exit status.
              -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.
              --     Do not change any of the flags; useful in setting  $1  to
              Using  +  rather  than  -  causes  these flags to be turned off.
              These flags can also be used upon invocation of the shell.   The
              current  set  of  flags may be found in $-.  The remaining argu-
              ments are positional parameters and are assigned, in  order,  to
              $1,  $2,  ....  If no arguments are given then the values of all
              names are printed.
              The positional parameters from $2 ...  are renamed $1 ....
              Evaluate conditional expressions.  See  test(1)  for  usage  and
              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.  Any attempt to set a trap on a
              signal that was ignored on entry to the current shell  is  inef-
              fective.   An  attempt  to trap on signal 11 (memory fault) pro-
              duces an error.  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 the commands  it  invokes.
              If  n  is  0  then  the command arg is executed on exit from the
              shell.  The trap command with no arguments prints a list of com-
              mands associated with each signal number.
       umask [ nnn ]
              The  user  file-creation  mask is set to nnn (see umask(2)).  If
              nnn is omitted, the current value of the mask is printed.
       wait   Wait for all child processes to terminate report the termination
              status.   If n is not given then all currently active child pro-
              cesses are waited for.  The return code  from  this  command  is
              always zero.

       If  the  shell  is  invoked  through exec(2) and the first character of
       argument zero is -, commands are initially read from  /etc/profile  and
       then  from  $HOME/.profile,  if such files exist.  Thereafter, commands
       are read as described below, which is also the case when the  shell  is
       invoked  as  /bin/sh.   The flags below are interpreted by the shell on
       invocation only; Note that unless the -c or -s flag is  specified,  the
       first argument is assumed to be the name of a file containing commands,
       and the remaining arguments are passed as positional parameters to that
       command file:

       -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 com-
                 mands  are read from the standard input.  Any remaining argu-
                 ments specify the positional  parameters.   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, then this shell  is  interactive.
                 In  this  case  TERMINATE is ignored (so that kill 0 does not
                 kill an  interactive  shell)  and  INTERRUPT  is  caught  and
                 ignored  (so that wait is interruptible).  In all cases, QUIT
                 is ignored by the shell.
       -r        If the -r flag is present the shell  is  a  restricted  shell
                 (see rsh(1)).

       The  remaining  flags and arguments are described under the set command

       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
       the exit command above).


       cd(1), env(1), login(1), newgrp(1), rsh(1), test(1), umask(1),  dup(2),
       exec(2), fork(2), pipe(2), signal(2), umask(2), wait(2), a.out(5), pro-
       file(5), environ(7).

       The command readonly (without arguments) produces the  same  output  as
       the command export.
       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 that file by another name.