NAME
sh - shell (command interpreter)
SYNOPSIS
sh [ -t ] [ -c ] [ name [ arg1 ... [ arg9 ] ] ]
DESCRIPTION
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 argu-
ments to the Shell used as a command, the structure of com-
mand 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 command to be executed. Except for certain
types of special arguments discussed below, the arguments
other than the command name are passed without interpreta-
tion 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 resulting 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 created 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 com-
mand 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 desig-
nates sequential execution. The ampersand causes the pre-
ceding pipeline to be executed without 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 wait or
kill.
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Termination Reporting. If a command (not followed by `&')
terminates abnormally, a message is printed. (All termina-
tions other than exit and interrupt are considered abnor-
mal.) Termination reports for commands followed by `&' are
given upon receipt of the first command subsequent 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
Quit
Floating exception
Memory violation
Killed
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 inter-
preted as a special argument to the Shell itself. Such an
argument may appear anywhere among the arguments 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 asso-
ciated command.
An argument of the form `>arg' causes file `arg' to be used
as the standard output (file descriptor 1) for the associ-
ated command. `Arg' is created if it did not exist, and in
any case is truncated at the outset.
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 file.
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
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`<arg' in any but the first.
In commands called by the Shell, file descriptor 2 refers to
the standard 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 char-
acters 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 mem-
ber of the class. A pair of characters separated by `-'
places in the class each character lexically greater than or
equal to the first and less than or equal to the second mem-
ber 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 different procedure is used: instead of the cur-
rent 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
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Shell; in this way `<', `>', and other characters meaning-
ful to the Shell may be passed as part of arguments. A spe-
cial case of this feature allows the continuation of com-
mands 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 inclusion of blanks in the heading, which
is a single argument to pr.
Argument passing. When the Shell is invoked as a command,
it has additional 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 inter-
preted. If not given, this subinstance of the Shell contin-
ues to read the standard input file.
In command lines in the file (not in command input), charac-
ter 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 name.
The argument `-t,' used alone, causes sh to read the stan-
dard 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 exe-
cuted. 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 spe-
cially by the Shell.
chdir is done without spawning a new process by executing
sys chdir (II).
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login is done by executing /bin/login without creating a new
process.
wait is done without spawning a new process by executing sys
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 automatically redirected to the zero length
file /dev/null.
FILES
/etc/glob, which interprets `*', `?', and `['.
/dev/null as a source of end-of-file.
SEE ALSO
`The UNIX Time-Sharing System', CACM, July, 1974, which
gives the theory of operation of the Shell.
chdir (I), login (I), wait (I), shift (I)
BUGS
There is no way to redirect the diagnostic output.
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