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SH(I)                                                                    SH(I)

       sh - shell (command interpreter)

       sh [ -t ] [ -c ] [ name [ arg1 ... [ arg9 ] ] ]

       Sh  is the standard command interpreter.  It is the program which reads
       and arranges the execution of the command lines typed  by  most  users.
       It  may  itself  be called as a command to interpret files of commands.
       Before discussing the arguments to the Shell used  as  a  command,  the
       structure of command lines themselves will be given.

       Commands.   Each  command  is a sequence of non-blank command arguments
       separated by blanks.  The first argument specifies the name of  a  com-
       mand  to  be  executed.   Except for certain types of special arguments
       discussed below, the arguments other than the command name  are  passed
       without interpretation to the invoked command.

       If the first argument is the name of an executable file, it is invoked;
       otherwise the string `/bin/' is prepended to the  argument.   (In  this
       way  most standard commands, which reside in `/bin', are found.)  If no
       such command is found, the string `/usr' is further prepended (to  give
       `/usr/bin/command')  and another attempt is made to execute the result-
       ing file.  (Certain lesser-used commands live in `/usr/bin'.)

       If a non-directory file has executable mode, but not  the  form  of  an
       executable  program  (does not begin with the proper magic number) then
       it is assumed to be an ASCII file of commands and a new Shell  is  cre-
       ated to execute it.  See ``Argument passing'' below.

       If the file cannot be found, a diagnostic is printed.

       Command lines.  One or more commands separated by `|' or `^' constitute
       a chain of filters.  The standard output of each command but  the  last
       is  taken  as  the standard input of the next command.  Each command is
       run as a separate process, connected by pipes  (see  pipe(II))  to  its
       neighbors.  A command line contained in parentheses `( )' may appear in
       place of a simple command as a filter.

       A command line consists of one or more pipelines separated, and perhaps
       terminated  by  `;' or `&'.  The semicolon designates sequential execu-
       tion.  The ampersand causes the preceding pipeline to be executed with-
       out  waiting  for  it  to finish.  The process id of such a pipeline is
       reported, so that it may be used if necessary for a subsequent kill.

       Termination Reporting.  If a command (not followed by  `&')  terminates
       abnormally,  a  message  is printed.  (All terminations other than exit
       and interrupt are considered abnormal.)  Termination reports  for  com-
       mands  followed by `&' are given upon receipt of the first command sub-
       sequent to the termination of the command, or when a wait is  executed.
       The following is a list of the abnormal termination messages:

            Bus error
            Trace/BPT trap
            Illegal instruction
            IOT trap
            EMT trap
            Bad system call
            Floating exception
            Memory violation
            Broken Pipe

       If  a  core  image  is  produced,  ` -- Core dumped' is appended to the
       appropriate message.

       Redirection of I/O.  There are three character sequences that cause the
       immediately following string to be interpreted as a special argument to
       the Shell itself.  Such an argument may appear anywhere among the argu-
       ments  of  a simple command, or before or after a parenthesized command
       list, and is associated with that command or command list.

       An argument of the form `<arg' causes the file `arg' to be used as  the
       standard input (file descriptor 0) of the associated command.

       An  argument  of  the  form  `>arg' causes file `arg' to be used as the
       standard output (file descriptor 1) for the associated command.   `Arg'
       is  created  if  it  did not exist, and in any case is truncated at the

       An argument of the form `>>arg' causes file `arg' to  be  used  as  the
       standard output for the associated command.  If `arg' did not exist, it
       is created; if it did exist, the command  output  is  appended  to  the

       For example, either of the command lines

            ls >junk; cat tail >>junk
            ( ls; cat tail ) >junk

       creates,  on  file `junk', a listing of the working directory, followed
       immediately by the contents of file `tail'.

       Either of the constructs `>arg' or `>>arg' associated with any but  the
       last  command of a pipeline is ineffectual, as is `<arg' in any but the

       In commands called by the Shell, file descriptor 2 refers to the  stan-
       dard  output  of  the  Shell  before any redirection.  Thus filters may
       write diagnostics to a location where they have a chance to be seen.

       Generation of argument lists.  If any  argument  contains  any  of  the
       characters  `?',  `*'  or `[', it is treated specially as follows.  The
       current directory is searched for files which match the given argument.

       The  character `*' in an argument matches any string of characters in a
       file name (including the null string).

       The character `?' matches any single character in a file name.

       Square brackets `[...]' specify a class of characters which matches any
       single  file-name  character  in  the class.  Within the brackets, each
       ordinary character is taken to be a member of the  class.   A  pair  of
       characters  separated  by  `-' places in the class each character lexi-
       cally greater than or equal to the first and less than or equal to  the
       second member of the pair.

       Other characters match only the same character in the file name.

       For  example, `*' matches all file names; `?' matches all one-character
       file names; `[ab]*.s' matches all file names beginning with `a' or  `b'
       and  ending  with  `.s'; `?[zi-m]' matches all two-character file names
       ending with `z' or the letters `i' through `m'.

       If the argument with `*' or `?' also contains a `/', a slightly differ-
       ent procedure is used:  instead of the current directory, the directory
       used is the one obtained by taking the argument  up  to  the  last  `/'
       before a `*' or `?'.  The matching process matches the remainder of the
       argument after this `/' against the files  in  the  derived  directory.
       For  example: `/usr/dmr/a*.s' matches all files in directory `/usr/dmr'
       which begin with `a' and end with `.s'.

       In any event, a list of names is obtained  which  match  the  argument.
       This list is sorted into alphabetical order, and the resulting sequence
       of arguments replaces the single argument containing the `*',  `[',  or
       `?'.   The same process is carried out for each argument (the resulting
       lists are not merged) and  finally  the  command  is  called  with  the
       resulting list of arguments.

       Quoting.   The character `\' causes the immediately following character
       to lose any special meaning it may have to the Shell;  in this way `<',
       `>', and other characters meaningful to the Shell may be passed as part
       of arguments.  A special case of this feature allows  the  continuation
       of  commands  onto  more  than one line:  a new-line preceded by `\' is
       translated into a blank.

       Sequences of characters enclosed in double (") or single (') quotes are
       also taken literally.  For example:

            ls  |  pr -h "My directory"

       causes a directory listing to be produced by ls, and passed on to pr to
       be printed with the heading `My directory'.  Quotes permit  the  inclu-
       sion of blanks in the heading, which is a single argument to pr.

       Argument passing.  When the Shell is invoked as a command, it has addi-
       tional string processing capabilities.  Recall that the form  in  which
       the Shell is invoked is

            sh [ name [ arg1 ... [ arg9 ] ] ]

       The  name  is the name of a file which is read and interpreted.  If not
       given, this subinstance of the Shell continues  to  read  the  standard
       input file.

       In  command  lines  in  the  file  (not  in  command  input), character
       sequences of the form `$n', where n is a digit, are replaced by the nth
       argument  to  the  invocation of the Shell (argn).  `$0' is replaced by

       The argument `-t', used alone, causes sh to read the standard input for
       a  single  line, execute it as a command, and then exit.  This facility
       replaces the older `mini-shell'.  It is useful for interactive programs
       which allow users to execute system commands.

       The  argument  `-c'  (used with one following argument) causes the next
       argument to be taken as a command line and executed.  No new-line  need
       be  present,  but  new-line characters are treated appropriately.  This
       facility is useful as an alternative  to  `-t'  where  the  caller  has
       already read some of the characters of the command to be executed.

       End of file.  An end-of-file in the Shell's input causes it to exit.  A
       side effect of this fact means that the way to log out from UNIX is  to
       type an EOT.

       Special  commands.  The following commands are treated specially by the

       chdir  is done without spawning a new process by executing chdir(II).

       login  is done by executing /bin/login without creating a new  process.

       wait   is done without spawning a new process by executing wait(II).

       shift  is done by manipulating the arguments to the Shell.

       :      is simply ignored.

       Command  file  errors;  interrupts.   Any  Shell-detected  error, or an
       interrupt signal, during the execution of a  command  file  causes  the
       Shell to cease execution of that file.

       Processes  that are created with `&' ignore interrupts.  Also if such a
       process has not redirected its input with a `<', its input is automati-
       cally redirected to the zero length file /dev/null.

       /etc/glob, which interprets `*', `?', and `['.
       /dev/null as a source of end-of-file.

       `The  UNIX Time-Sharing System', CACM, July, 1974, which gives the the-
       ory of operation of the Shell.
       chdir(I), login(I), shift(I), wait(I)

       There is no way to redirect the diagnostic output.

                                    5/15/74                              SH(I)