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.pa 1
.he 'SH (I)'1/15/73'SH (I)'
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NAME		sh  --  shell (command interpreter)
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SYNOPSIS	sh__ [ name [ arg\d1\u ... [ arg\d9\u ] ] ]
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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 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 command
Each command is a sequence of non-blank command arguments
separated by blanks.
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 interpretation to the invoked
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
(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 diagnostic 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 execution 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
	PIR trap
	Floating exception
	Memory violation
	User I/O
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 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, performs 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


specify that output is to be passed successively
through the filters f\d1\u, f\d2\u, ...,
and end up on the standard output stream.
By saying instead


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 specified via one of


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 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.

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 lexically 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
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

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, ...
in that order.

The character "\\" causes the immediately following 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.
Recall that the form in which the shell is invoked is

     sh [ name [ arg\d1\u ... [ arg\d9\u ] ] ]

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 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 n_th argument to the invocation
of the shell (arg\dn\u).
"$0" is replaced by name____.

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_____

"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 system.
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.

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FILES		/etc/glob,
which interprets "*", "?", and "[".
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SEE ALSO	"The UNIX Time-sharing System",
which gives the theory of operation of the
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"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.
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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.