3/15/72 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 commands. Before discussing the argu-
ments to the shell used as a command, the struc-
ture of command lines themselves will be given.
Command lines
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 is the name of an exe-
cutable 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 the "/bin" file exists,
but is not executable, it is used by the shell as
a command file. That is to say it is executed as
though it were typed from the console. If all
attempts fail, a diagnostic is printed.
The remaining non-special arguments are simply
passed to the command without further interpreta-
tion by the shell.
Command delimiters
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.
Termination Reporting
If a command (not followed by "&") terminates ab-
normally, a message is printed. (All termina-
tions other than exit and interrupt are consid-
ered abnormal.) The following is a list of the
abnormal termination messages:
Bus error
Trace trap
Illegal instruction
IOT trap
Power fail trap
EMT trap
Bad system call
Quit
Error
If a core image is produced, " -- Core dumped" is
appended to the appropriate message.
Redirection of I/O
Three character sequences 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 output file for the
given command. "Arg" is created if it did not
exist, and in any case is truncated at the out-
set.
An argument of the form ">>arg" causes file "arg"
to be used as the standard output for the given
command. If "arg" did not exist, it is created;
if it did exist, the command output is appended
to the file.
Generation of argument lists
If any argument contains any of the characters
"?", "*" or '[', it is treated specially as fol-
lows. 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.
Each "[" must be paired with a matching "]". The
characters between "[" and "]" specify a class of
characters. It matches any single character in a
file name which is in the class. An ordinary
character in the brackets specifies that charac-
ter to be in the class. A pair of characters
separated by "-" specifies each character lexi-
cally greater than or equal to the first and less
than or equal to the second member of the pair is
to be included in the class. If the first member
of the pair lexically exceeds the second, the
second member is the sole character specified.
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 let-
ters "i" through "m".
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 car-
ried out for each argument (the resulting 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.
Quoting
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.
Argument passing
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
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 end of
file.
Special commands
Two commands are treated specially by the shell.
"Chdir" is done without spawning a new process by
executing the sys chdir primitive.
"Login" is done by executing /bin/login without
creating a new process.
These peculiarities are inexorably imposed upon
the shell by the basic structure of the UNIX pro-
cess control system. It is a rewarding exercise
to work out why.
Command file errors; interrupts
Any shell-detected error in a file of commands
causes the shell to cease execution of that file.
FILES /etc/glob, which interprets "*", "?", and "[".
SEE ALSO "The UNIX Time-sharing System", which gives the
theory of operation of the shell.
DIAGNOSTICS "Input not found", when a command file is speci-
fied which cannot be read;
"Arg count", if the number of arguments to the
chdir pseudo-command is not exactly 1, or if "*",
"?", or "[" is used inappropriately;
"Bad directory", if the directory given in
"chdir" cannot be switched to;
"Try again", if no new process can be created to
execute the specified command;
""' imbalance", if single or double quotes are
not matched;
"Input file", if an argument after "<" cannot be
read;
"Output file", if an argument after ">" or ">>"
cannot be written (or created);
"No command", if the specified command cannot be
executed.
"No match", if no arguments are generated for a
command which contains "*", "?", or "[".
Termination messages described above.
BUGS If any argument contains a quoted "*", "?", or
"[", then all instances of these characters must
be quoted. This is because sh calls the glob
routine whenever an unquoted "*", "?", or "[" is
noticed; the fact that other instances of these
characters occurred quoted is not noticed by
glob.
OWNER dmr, ken