11/3/71                                                    SH (I)

NAME            sh  --  shell (command interpreter)

SYNOPSIS        sh [ name [ arg  ... [ arg  ] ] ]
                               1          9

DESCRIPTION     sh is the standard command interpreter.  It is

                the program which reads and arranges the execu-

                tion of the command lines typed by most users.

                It may itself be called as a command to interpret

                files of command lines.  Before discussing the

                arguments to the shell used as a command, the

                structure of command lines themselves will be


                Command lines are sequences of commands separated

                by command delimiters.  Each command is a se-

                quence of non-blank command arguments separated

                by blanks.  The first argument specifies the name

                of a command to be executed.  Except for certain

                types of special arguments discussed below, the

                arguments other than the command name are simply

                passed to the invoked command.

                If the first argument represents the path 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 this search too fails a

                diagnostic is printed.

                The remaining non-special arguments are simply

                passed to the command without further interpreta-

                tion by the shell.

                There are three command delimiters:  the new-

                line, ";", and "&".  The semicolon ";" specifies

                sequential execution of the commands so sepa-

                rated; that is,

                        coma; comb

                causes the execution first of command coma, then

                of comb.  The ampersand "&" causes simultaneous


                        coma & comb

                causes coma to be called, followed immediately by

                comb without waiting for coma to finish.  Thus

                coma and comb execute simultaneously.  As a spe-

                cial case,

                        coma &

                causes coma to be executed and the shell immedi-

                ately to request another command without waiting

                for coma.

                Two characters cause the immediately following

                string to be interpreted as a special argument to

                the shell itself, not passed to the command.  An

                argument of the form "<arg" causes the file arg

                to be used as the standard input file of the

                given command; an argument of the form ">arg"

                causes file "arg" to be used as the standard out-

                put file for the given command.

                If any argument contains either 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).  "?"

                matches any single character in a file name.

                Other argument characters match only the same

                character in the file name.  For example, "*"

                matches all file names; "?" matches all one-char-

                acter file names; "ab*.s" matches all file names

                beginning with "ab" and ending with ".s";

                If the argument with "*" or "?" also contains a

                "/", a slightly different procedure is used:  in-

                stead 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 ar-

                gument 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 al-

                phabetical order, and the resulting sequence of

                arguments replaces the single argument containing

                the "*" or "?".  The same process is carried out

                for each argument with a "*" or "?"  (the result-

                ing lists are not merged) and finally the command

                is called with the resulting list of arguments.

                For example: directory /usr/dmr contains the

                files a1.s, a2.s, ..., a9.s.  From any directory,

                the command

                     as /usr/dmr/a?.s

                calls as with arguments /usr/dmr/a1.s,

                /usr/dmr/a2.s, ...  /usr/dmr/a9.s in that order.

                The character "\" causes the immediately follow-

                ing 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.

                When the shell is invoked as a command, it has

                additional string processing capabilities.  Re-

                call that the form in which the shell is invoked


                     sh [ name [ arg  ... [ arg  ] ] ]
                                    1          9

                The name is the name of a file which will be read

                and interpreted.  If not given, this subinstance

                of the shell will continue to read the standard

                input file.

                In the file, character sequences of the form

                "$n", where n is a digit 0, ..., 9, are replaced

                by the nth argument to the invocation of the

                shell (arg ).  "$0" is replaced by name.

                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 end of


FILES           /etc/glob,

SEE ALSO        [reference], which gives the theory of operation

                of the shell.

DIAGNOSTICS     "?", in case of any difficulty.  The most common

                problem is inability to find the given command.

                Others:  input file ("<") cannot be found; no

                more processes can be created (this will allevi-

                ate itself with the passage of time).  Note that

                no diagnostic is given for inability to create an

                output (">") file; the standard output file has

                already been closed when the condition is discov-

                ered and there is no place to write the diagnos-


                If a "*" or "?" is used, the glob routine is in-

                voked; it types "No command" if it cannot find

                the given command, and "No match" if there were

                no files which matched an argument with "?" or


BUGS            Better diagnostics should be provided.  If a "*"

                or "?" is used, the command must be in /bin.

                (Not, for example, in the user's directory.)

                This is actually a glob bug.

OWNER           dmr, ken