11/3/71 SH (I)
NAME sh -- shell (command interpreter)
SYNOPSIS sh [ name [ arg ... [ arg ] ] ]
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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
given.
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
execution:
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
is
sh [ name [ arg ... [ arg ] ] ]
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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.
n
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
file.
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-
tic.
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