SH (I) 1/15/73 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 pro-
gram which reads and arranges the execution of the com-
mand lines typed by most users. It may itself be called
as a command to interpret files of commands. Before dis-
cussing the arguments to the shell used as a command, the
structure of command lines themselves will be given.
Command lines
Command lines are sequences of commands separated by com-
mand delimiters. Each command is a sequence of non-blank
command arguments separated by blanks. The first argu-
ment specifies the name of a command to be executed. Ex-
cept for certain types of special arguments discussed be-
low, 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 exe-
cute the resulting file. (Certain "overflow" commands
live in "/usr/bin".) If the "/usr/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 diagnos-
tic is printed.
The remaining non-special arguments are simply passed to
the command without further interpretation by the shell.
Command delimiters
There are three command delimiters: the new-line, ";",
and "&". The semicolon ";" specifies sequential execu-
tion of the commands so separated; 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 special case,
coma &
causes coma to be executed and the shell immediately to
request another command without waiting for coma.
Termination Reporting
If a command (not followed by "&") terminates abnormally,
a message is printed. (All terminations other than exit
and interrupt are considered abnormal.) The following is
a list of the abnormal termination messages:
Bus error
Trace/BPT trap
Illegal instruction
IOT trap
Power fail trap
EMT trap
Bad system call
Quit
PIR trap
Floating exception
Memory violation
Killed
User I/O
Error
If a core image is produced, " -- Core dumped" is ap-
pended 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 outset.
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.
Pipes and Filters
A pipe is a channel such that information can be written
into one end of the pipe by one program, and read at the
other end by another program. (See pipe (II)). A filter
is a program which reads the standard input file, per-
forms some transformation, and writes the result on the
standard output file. By extending the syntax used for
redirection of I/O, a command line can specify that the
output produced by a command be passed via a pipe through
another command which acts as a filter. For example:
command >filter>
More generally, special arguments of the form
>f >f >...>
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specify that output is to be passed successively through
the filters f , f , ..., and end up on the standard out-
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put stream. By saying instead
>f >f >...>file
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the output finally ends up in file. (The last ">" could
also have been a ">>" to specify concatenation onto the
end of file.)
In exactly analogous manner input filtering can be speci-
fied via one of
<f <f <...<
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<f <f <...<file
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Both input and output filtering can be specified in the
same command, though not in the same special argument.
For example:
ls >pr>
produces a listing of the current directory with page
headings, while
ls >pr>xx
puts the paginated listing into the file xx.
If any of the filters needs arguments, quotes can be used
to prevent the required blank characters from violating
the blankless syntax of filters. For example:
ls >"pr -h 'My directory'">
uses quotes twice, once to protect the entire pr command,
once to protect the heading argument of pr. (Quotes are
discussed fully below.)
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 ar-
gument.
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 different 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 sin-
gle 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.
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 following char-
acter to lose any special meaning it may have to the
shell; in this way "<", ">", and other characters mean-
ingful 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. Recall 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 in-
terpreted. If not given, this subinstance of the shell
will continue 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
0, ..., 9, are replaced by the nth argument to the invo-
cation 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 execut-
ing 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 process control sys-
tem. It is a rewarding exercise to work out why.
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.
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 specified 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);
"Command not found", 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.
When output is redirected, particularly through a
filter, diagnostics tend to be sent down the pipe
and are sometimes lost altogether.