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.EH 'USD:4-%''An Introduction to the C shell'
.OH 'An Introduction to the C shell''USD:4-%'
.\".RP
.TL
An Introduction to the C shell
.AU
William Joy
(revised for 4.3BSD by Mark Seiden)
.AI
Computer Science Division
.br
Department of Electrical Engineering and Computer Science
.br
University of California, Berkeley
.br
Berkeley, California 94720
.AB
.I Csh
is a new command language interpreter for
.UX
systems.
It incorporates good features of other shells and a
.I history
mechanism similar to the
.I redo
of INTERLISP.
While incorporating many features of other shells which make
writing shell programs (shell scripts) easier,
most of the features unique to
.I csh
are designed more for the interactive UNIX user.
.PP
UNIX
users who have read a general introduction to the system
will find a valuable basic explanation of the shell here.
Simple terminal interaction with
.I csh
is possible after reading just the first section of this document.
The second section describes the shell's capabilities which you can
explore after you have begun to become acquainted with the shell.
Later sections introduce features which are useful, but not necessary
for all users of the shell.
.PP
Additional information includes an appendix listing special characters of the shell
and a glossary of terms and commands introduced in this manual.
.AE
.SH
.if n .ND
Introduction
.PP
A
.I shell
is a command language interpreter.
.I Csh
is the name of one particular command interpreter on
UNIX.
The primary purpose of
.I csh
is to translate command lines typed at a terminal into
system actions, such as invocation of other programs.
.I Csh
is a user program just like any you might write.
Hopefully,
.I csh
will be a very useful program for you
in interacting with the UNIX system.
.PP
In addition to this document, you will want to refer to a copy
of the UNIX User Reference Manual.
The
.I csh
documentation in section 1 of the manual provides a full description of all
features of the shell and is the definitive reference for questions
about the shell.
.PP
Many words in this document are shown in
.I italics .
These are important words;
names of commands, and words which have special meaning in discussing
the shell and UNIX.
Many of the words are defined in a glossary at the end of this document.
If you don't know what is meant by a word, you should look
for it in the glossary.
.SH
Acknowledgements
.PP
Numerous people have provided good input about previous versions
of
.I csh
and aided in its debugging and in the debugging of its documentation.
I would especially like to thank Michael Ubell
who made the crucial observation that history commands could be
done well over the word structure of input text, and implemented
a prototype history mechanism in an older version of the shell.
Eric Allman has also provided a large number of useful comments on the
shell, helping to unify those concepts which are present and to identify
and eliminate useless and marginally useful features.
Mike O'Brien suggested the pathname hashing
mechanism which speeds command execution.
Jim Kulp added the job control and directory stack primitives and
added their documentation to this introduction.
.br
.bp
.NH
Terminal usage of the shell
.NH 2
The basic notion of commands
.PP
A
.I shell
in
UNIX
acts mostly as a medium through which other
.I programs
are invoked.
While it has a set of
.I built-in
functions which it performs directly,
most commands cause execution of programs that are, in fact,
external to the shell.
The shell is thus distinguished from the command interpreters of other
systems both by the fact that it is just a user program, and by the fact
that it is used almost exclusively as a mechanism for invoking other programs.
.PP
.I Commands
in the UNIX system consist of a list of strings or
.I words
interpreted as a
.I "command name"
followed by
.I arguments .
Thus the command
.DS
mail bill
.DE
consists of two words.
The first word
.I mail
names the command to be executed, in this case the
mail program which sends messages to other users.
The shell uses the name of the command in attempting to execute it for you.
It will look in a number of
.I directories
for a file with the name
.I mail
which is expected to contain the mail program.
.PP
The rest of the words of the command are given as
.I arguments
to the command itself when it is executed.
In this case we specified also the argument
.I bill
which is interpreted by the
.I mail
program to be the name of a user to whom mail is to be sent.
In normal terminal usage we might use the
.I mail
command as follows.
.DS
% mail bill
I have a question about the csh documentation.
My document seems to be missing page 5.
Does a page five exist?
	Bill
EOT
%
.DE
.PP
Here we typed a message to send to
.I bill
and ended this message with a ^D which sent an end-of-file to
the mail program.
(Here and throughout this document, the notation ``^\fIx\fR''
is to be read ``control-\fIx\fR'' and represents the striking of the \fIx\fR
key while the control key is held down.)
The mail program
then echoed the characters `EOT' and transmitted our message.
The characters `% ' were printed before and after the mail command
by the shell to indicate that input was needed.
.PP
After typing the `% ' prompt, the shell was reading command input from
our terminal.
We typed a complete command `mail bill'.
The shell then executed the
.I mail
program with argument
.I bill
and went dormant waiting for it to complete.
The mail program then read input from our terminal until we signaled
an end-of-file via typing a ^D after which the shell noticed
that mail had completed
and signaled us that it was ready to read from the terminal again by
printing another `% ' prompt.
.PP
This is the essential pattern of all interaction with UNIX
through the shell.
A complete command is typed at the terminal, the shell executes
the command and when this execution completes, it prompts for a new command.
If you run the editor for an hour, the shell will patiently wait for
you to finish editing and obediently prompt you again whenever you finish
editing.
.PP
An example of a useful command you can execute now is the
.I tset
command, which sets the default
.I erase
and
.I kill
characters on your terminal \- the erase character erases the last
character you typed and the kill character erases the entire line you
have entered so far.
By default, the erase character is the delete key (equivalent to `^?')
and the kill character is `^U'.  Some people prefer to make the erase character
the backspace key (equivalent to `^H').
You can make this be true by typing
.DS
tset \-e
.DE
which tells the program
.I tset
to set the erase character to tset's default setting for this character
(a backspace).
.NH 2
Flag arguments
.PP
A useful notion in UNIX is that of a
.I flag
argument.
While many arguments to commands specify file names or user names,
some arguments rather specify an optional capability of the command
which you wish to invoke.
By convention, such arguments begin with the character `\-' (hyphen).
Thus the command
.DS
ls
.DE
will produce a list of the files in the current
.I "working directory" .
The option
.I \-s
is the size option, and
.DS
ls \-s
.DE
causes
.I ls
to also give, for each file the size of the file in blocks of 512
characters.
The manual section for each command in the UNIX reference manual
gives the available options for each command.
The
.I ls
command has a large number of useful and interesting options.
Most other commands have either no options or only one or two options.
It is hard to remember options of commands which are not used very
frequently, so most UNIX utilities perform only one or two functions
rather than having a large number of hard to remember options.
.NH 2
Output to files
.PP
Commands that normally read input or write output on the terminal
can also be executed with this input and/or output done to
a file.
.PP
Thus suppose we wish to save the current date in a file called `now'.
The command
.DS
date
.DE
will print the current date on our terminal.
This is because our terminal is the default
.I "standard output"
for the date command and the date command prints the date on its
standard output.
The shell lets us
.I redirect
the
.I "standard output"
of a command through a
notation using the
.I metacharacter
`>' and the name of the file where output is to be placed.
Thus the command
.DS
date > now
.DE
runs the
.I date
command such that its standard output is
the file `now' rather than the terminal.
Thus this command places the current date and time into the file `now'.
It is important to know that the
.I date
command was unaware that its output was going to a file rather than
to the terminal.
The shell performed this
.I redirection
before the command began executing.
.PP
One other thing to note here is that the file `now'
need not have existed before the
.I date
command was executed; the shell would have created the file if it did
not exist.
And if the file did exist?
If it had existed previously these previous contents would have been discarded!
A shell option
.I noclobber
exists to prevent this from happening accidentally;
it is discussed in section 2.2.
.PP
The system normally keeps files which you create with `>' and all other files.
Thus the default is for files to be permanent.  If you wish to create a file
which will be removed automatically, you can begin its name with a `#'
character, this `scratch' character denotes the fact that the file will
be a scratch file.*
.FS
*Note that if your erase character is a `#', you will have to precede the
`#' with a `\e'.  The fact that the `#' character is the old (pre-CRT)
standard erase character means that it seldom appears in a file name, and
allows this convention to be used for scratch files.  If you are using a
CRT, your erase character should be a ^H, as we demonstrated
in section 1.1 how this could be set up.
.FE
The system will remove such files after a couple of days,
or sooner if file space becomes very tight.
Thus, in running the
.I date
command above, we don't really want to save the output forever, so we
would more likely do
.DS
date > #now
.DE
.NH 2
Metacharacters in the shell
.PP
The shell has a large number of
special characters (like `>')
which indicate special functions.
We say that these notations have
.I syntactic
and
.I semantic
meaning to the shell.
In general, most characters which are neither letters nor digits
have special meaning to the shell.
We shall shortly learn a means of
.I quotation
which allows us to use
.I metacharacters
without the shell treating them in any special way.
.PP
Metacharacters normally have effect only when the shell is reading
our input.
We need not worry about placing shell metacharacters in a letter
we are sending via
.I mail ,
or when we are typing in text or data to some other program.
Note that the shell is only reading input when it has prompted with
`% ' (although we can type our input even before it prompts).
.NH 2
Input from files; pipelines
.PP
We learned above how to
.I redirect
the
.I "standard output"
of a command
to a file.
It is also possible to redirect the
.I "standard input"
of a command from a file.
This is not often necessary since most commands will read from
a file whose name is given as an argument.
We can give the command
.DS
sort < data
.DE
to run the
.I sort
command with standard input, where the command normally
reads its input, from the file
`data'.
We would more likely say
.DS
sort data
.DE
letting the
.I sort
command open the file
`data'
for input itself since this is less to type.
.PP
We should note that if we just typed
.DS
sort
.DE
then the sort program would sort lines from its
.I "standard input" .
Since we did not
.I redirect
the standard input, it would sort lines as we typed them on the terminal
until we typed a ^D to indicate an end-of-file.
.PP
A most useful capability is the ability to combine the standard output
of one command with the standard input of another, i.e. to run the
commands in a sequence known as a
.I pipeline .
For instance the command
.DS
ls \-s
.DE
normally produces a list of the files in our directory with the size
of each in blocks of 512 characters.
If we are interested in learning which of our files is largest we
may wish to have this sorted by size rather than by name, which is
the default way in which
.I ls
sorts.
We can look at the many options of
.I ls
to see if there was an option to do this,
or instead we can use a couple of simple options of the
.I sort
command, combining it with
.I ls
to get what we want.
.PP
The
.I \-n
option of sort specifies a numeric sort rather than an alphabetic sort.
Thus
.DS
ls \-s | sort \-n
.DE
specifies that the output of the
.I ls
command run with the option
.I \-s
is to be
.I piped
to the command
.I sort
run with the numeric sort option.
This would give us a sorted list of our files by size, but with the
smallest first.
We could then use the
.I \-r
reverse sort option and the
.I head
command in combination with the previous command doing
.DS
ls \-s | sort \-n \-r | head \-5
.DE
Here we have taken a list of our files sorted alphabetically,
each with the size in blocks.
We have run this to the standard input of the
.I sort
command asking it to sort numerically in reverse order (largest first).
This output has then been run into the command
.I head
which gives us the first few lines.
In this case we have asked
.I head
for the first 5 lines.
Thus this command gives us the names and sizes of our 5 largest files.
.PP
The notation introduced above is called the
.I pipe
mechanism.
Commands separated by `\||\|' characters are connected together by the
shell, and the standard output of each is run into the standard input of the
next.
The leftmost command in a pipeline will normally take its standard
input from the terminal and the rightmost will place its standard
output on the terminal.
Other examples of pipelines will be given later when we discuss the
history mechanism;
one important use of pipes which is illustrated there is in the
routing of information to the line printer.
.NH 2
Filenames
.PP
Many commands to be executed will need the names of files as arguments.
UNIX
.I pathnames
consist of a number of
.I components
separated by `/'.
Each component except the last names a directory in which the next
component resides, in effect specifying the
.I path
of directories to follow to reach the file.
Thus the pathname
.DS
/etc/motd
.DE
specifies a file in the directory
`etc'
which is a subdirectory of the
.I root
directory `/'.
Within this directory the file named is `motd' which stands
for `message of the day'.
A
.I pathname
that begins with a slash is said to be an
.I absolute
pathname since it is specified from the absolute top of the entire
directory hierarchy of the system (the
.I root ).
.I Pathnames
which do not begin with `/' are interpreted as starting in the current
.I "working directory" ,
which is, by default, your
.I home
directory and can be changed dynamically by the
.I cd
change directory command.
Such pathnames are said to be
.I relative
to the working directory since they are found by starting
in the working directory and descending to lower levels of directories
for each
.I component
of the pathname.  If the pathname contains no slashes at all then the
file is contained in the working directory itself and the pathname is merely
the name of the file in this directory.
Absolute pathnames have no relation
to the working directory.
.PP
Most filenames consist of a number of alphanumeric characters and
`.'s (periods).
In fact, all printing characters except `/' (slash) may appear in filenames.
It is inconvenient to have most non-alphabetic characters in filenames
because many of these have special meaning to the shell.
The character `.' (period) is not a shell-metacharacter and is often used
to separate the
.I extension
of a file name from the base of the name.
Thus
.DS
prog.c prog.o prog.errs prog.output
.DE
are four related files.
They share a
.I base
portion of a name
(a base portion being that part of the name that is left when a trailing
`.' and following characters which are not `.' are stripped off).
The file
`prog.c'
might be the source for a C program,
the file `prog.o' the corresponding object file,
the file
`prog.errs' the errors resulting from a compilation of the program
and the file
`prog.output' the output of a run of the program.
.PP
If we wished to refer to all four of these files in a command, we could
use the notation
.DS
prog.*
.DE
This expression is expanded by the shell, before the command to which it is
an argument is executed, into a list of names which begin with `prog.'.
The character `*' here matches any sequence (including the empty sequence)
of characters in a file name.
The names which match are alphabetically sorted and placed in the
.I "argument list"
of the command.
Thus the command
.DS
echo prog.*
.DE
will echo the names
.DS
prog.c prog.errs prog.o prog.output
.DE
Note that the names are in sorted order here, and a different
order than we listed them above.
The
.I echo
command receives four words as arguments, even though we only typed
one word as an argument directly.
The four words were generated by
.I "filename expansion"
of the one input word.
.PP
Other notations for
.I "filename expansion"
are also available.
The character `?' matches any single character in a filename.
Thus
.DS
echo ? \|?? \|???
.DE
will echo a line of filenames; first those with one character names,
then those with two character names, and finally those with three
character names.
The names of each length will be independently sorted.
.PP
Another mechanism consists of a sequence of characters between `[' and `]'.
This metasequence matches any single character from the enclosed set.
Thus
.DS
prog.[co]
.DE
will match
.DS
prog.c prog.o
.DE
in the example above.
We can also place two characters around a `\-' in this notation to denote
a range.
Thus
.DS
chap.[1\-5]
.DE
might match files
.DS
chap.1 chap.2 chap.3 chap.4 chap.5
.DE
if they existed.
This is shorthand for
.DS
chap.[12345]
.DE
and otherwise equivalent.
.PP
An important point to note is that if a list of argument words to
a command (an
.I "argument list)"
contains filename expansion syntax, and if this filename expansion syntax
fails to match any existing file names, then the shell considers this
to be an error and prints a diagnostic
.DS
No match.
.DE
and does not execute the command.
.PP
Another very important point is that files with the character `.' at the
beginning are treated specially.
Neither `*' nor `?' nor the `[' `]' mechanism will match it.
This prevents accidental matching of the filenames `.' and `..'
in the working directory which have special meaning to the system,
as well as other files such as
.I \&.cshrc
which are not normally
visible.
We will discuss the special role of the file
.I \&.cshrc
later.
.PP
Another filename expansion mechanism gives access to the pathname of
the
.I home
directory of other users.
This notation consists of the character `~' (tilde) followed by another user's
login name.
For instance the word `~bill' would map to the pathname `/home/bill'
if the home directory for `bill' was `/home/bill'.
Since, on large systems, users may have login directories scattered over
many different disk volumes with different prefix directory names,
this notation provides a convenient way of accessing the files
of other users.
.PP
A special case of this notation consists of a `~' alone, e.g. `~/mbox'.
This notation is expanded by the shell into the file `mbox' in
.I your
home directory, i.e. into `/home/bill/mbox' for me on Ernie Co-vax, the UCB
Computer Science Department VAX machine, where this document was prepared.
This can be very useful if you have used
.I cd
to change to another directory and have found a file you wish to
copy using
.I cp .
If I give the command
.DS
cp thatfile ~
.DE
the shell will expand this command to
.DS
cp thatfile /home/bill
.DE
since my home directory is /home/bill.
.PP
There also exists a mechanism using the characters `{' and `}' for
abbreviating a set of words which have common parts but cannot
be abbreviated by the above mechanisms because they are not files,
are the names of files which do not yet exist,
are not thus conveniently described.
This mechanism will be described much later,
in section 4.2,
as it is used less frequently.
.NH 2
Quotation
.PP
We have already seen a number of metacharacters used by the shell.
These metacharacters pose a problem in that we cannot use them directly
as parts of words.
Thus the command
.DS
echo *
.DE
will not echo the character `*'.
It will either echo a sorted list of filenames in the
current
.I "working directory" ,
or print the message `No match' if there are
no files in the working directory.
.PP
The recommended mechanism for placing characters which are neither numbers,
digits, `/', `.', nor `\-' in an argument word to a command, is to enclose
it with single quotation characters `\'', i.e.
.DS
echo \'*\'
.DE
There is one special character `!' which is used by the
.I history
mechanism of the shell and which cannot be
.I escaped
by placing it within `\'' characters.
It and the character `\'' itself can be preceded by a single `\e'
to prevent their special meaning.
Thus
.DS
echo \e\'\e!
.DE
prints
.DS
\'!
.DE
These two mechanisms suffice to place any printing character into a word
which is an argument to a shell command.  They can be combined, as in
.DS
echo \e\'\'*\'
.DE
which prints
.DS
\'*
.DE
since the first `\e' escaped the first `\'' and the `*' was enclosed
between `\'' characters.
.NH 2
Terminating commands
.PP
When you are executing a command and the shell is
waiting for it to complete, there are several ways
to force it to stop.
For instance if you type the command
.DS
cat /etc/passwd
.DE
the system will print a copy of a list of all users of the system
on your terminal.
This is likely to continue for several minutes unless you stop it.
You can send an
INTERRUPT
.I signal
to the
.I cat
command by typing ^C on your terminal.*
.FS
*On some older Unix systems the DEL or RUBOUT key
has the same effect. "stty all" will tell you the `intr' key value.
.FE
Since
.I cat
does not take any precautions to avoid or otherwise handle this signal,
the
INTERRUPT
will cause it to terminate.
The shell notices that
.I cat
has terminated and prompts you again with `% '.
If you hit INTERRUPT again, the shell will just
repeat its prompt since it handles INTERRUPT signals
and chooses to continue to execute commands rather than terminating
like
.I cat
did, which would have the effect of logging you out.
.PP
Another way in which many programs terminate is when they get an end-of-file
from their standard input.
Thus the
.I mail
program in the first example above was terminated when we typed a ^D
which generates an end-of-file from the standard input.
The shell also terminates when it gets an end-of-file, printing `logout';
UNIX then logs you off the system.
Since this means that typing too many ^D's can accidentally log us off,
the shell has a mechanism for preventing this.
This
.I ignoreeof
option will be discussed in section 2.2.
.PP
If a command has its standard input redirected from a file, then it will
normally terminate when it reaches the end of this file.
Thus if we execute
.DS
mail bill < prepared.text
.DE
the mail command will terminate without our typing a ^D.
This is because it read to the end-of-file of our file
`prepared.text' in which we placed a message for `bill' with an editor program.
We could also have done
.DS
cat prepared.text \||\| mail bill
.DE
since the
.I cat
command would then have written the text through the pipe to the
standard input of the mail command.
When the
.I cat
command completed it would have terminated,
closing down the pipeline
and the
.I mail
command would have received an end-of-file from it and terminated.
Using a pipe here is more complicated than redirecting input
so we would more likely use the first form.
These commands could also have been stopped by sending an INTERRUPT.
.PP
Another possibility for stopping a command is to suspend its execution
temporarily, with the possibility of continuing execution later.  This is
done by sending a STOP signal via typing a ^Z.
This signal causes all commands running on the terminal
(usually one, but more if a pipeline is executing) to become suspended.
The shell notices that the command(s) have been suspended, types
`Suspended', and then prompts for a new command.
The previously executing command has been suspended, but otherwise
unaffected by the STOP signal.  Any other commands can be executed
while the original command remains suspended.  The suspended command can
be continued using the
.I fg
command with no arguments.  The shell will then retype the command
to remind you which command is being continued, and cause the command
to resume execution.  Unless any input files in use by the suspended
command have been changed in the meantime, the suspension has no effect
whatsoever on the execution of the command.  This feature can be very useful
during editing, when you need to look at another file before continuing. An
example of command suspension follows.
.DS
% mail harold
Someone just copied a big file into my directory and its name is
^Z
Suspended
% ls
funnyfile
prog.c
prog.o
% jobs
.ta 1.75i
[1]  + Suspended	mail harold
% fg
mail harold
(continue)
funnyfile. Do you know who did it?
EOT
%
.so tabs
.DE
In this example someone was sending a message to Harold and forgot the
name of the file he wanted to mention.  The mail command was suspended
by typing ^Z.  When the shell noticed that the mail program was
suspended, it typed `Suspended' and prompted for a new command.  Then the
.I ls
command was typed to find out the name of the file.  The
.I jobs
command was run to find out which command was suspended. At this time the
.I fg
command was typed to continue execution of the mail program.  Input
to the mail program was then continued and ended with a ^D
which indicated the end of the message at which time the mail
program typed EOT.  The
.I jobs
command will show which commands are suspended.
The ^Z should only be typed at the beginning of a line since
everything typed on the current line is discarded when a signal is sent
from the keyboard.  This also happens on INTERRUPT, and QUIT
signals.  More information on
suspending jobs and controlling them is given in
section 2.6.
.PP
If you write or run programs which are not fully debugged, then it may
be necessary to stop them somewhat ungracefully.
This can be done by sending them a QUIT
signal, sent by typing a ^\e.
This will usually provoke the shell to produce a message like:
.DS
Quit (core dumped)
.DE
indicating that a file
`core' has been created containing information about the running program's
state when it terminated due to the QUIT signal.
You can examine this file yourself, or forward information to the
maintainer of the program telling him/her where the
.I "core file"
is.
.PP
If you run background commands (as explained in section 2.6) then these
commands will ignore INTERRUPT and QUIT signals at the
terminal.  To stop them you must use the
.I kill
command.  See section 2.6 for an example.
.PP
If you want to examine the output of a command without having it move
off the screen as the output of the
.DS
cat /etc/passwd
.DE
command will, you can use the command
.DS
more /etc/passwd
.DE
The
.I more
program pauses after each complete screen-full and types `[filename] %',
at which point you can hit a space to get another screen full, a return
to get another line, an `h' to get some help on other commands, or a `q' to end the
.I more
program.  You can also use more as a filter, i.e.
.DS
cat /etc/passwd | more
.DE
works just like the more simple more command above.
.PP
For stopping output of commands not involving
.I more ,
you can use the
^S key to stop the typeout.  The typeout will resume when you
hit ^Q or any other key, but ^Q is normally used because
it only restarts the output and does not become input to the program
which is running.  This works well on low-speed terminals, but at 9600
baud it is hard to type ^S and ^Q fast enough to paginate
the output nicely, and a program like
.I more
is usually used.
.PP
An additional possibility is to use the ^O flush output
character; when this character is typed, all output from the current
command is thrown away (quickly) until the next input read occurs
or until the next shell prompt.  This can be used to allow a command
to complete without having to suffer through the output on a slow
terminal; ^O is a toggle, so flushing can be turned off by
typing ^O again while output is being flushed.
.NH 2
What now?
.PP
We have so far seen a number of mechanisms of the shell and learned a lot
about the way in which it operates.
The remaining sections will go yet further into the internals of the
shell, but you will surely want to try using the
shell before you go any further.
To try it you can log in to UNIX and type the following
command to the system:
.DS
chsh -s /bin/csh myname
.DE
Here `myname' should be replaced by the name you typed to
the system prompt of `login:' to get onto the system.
Thus I would use `chsh -s /bin/csh bill'.
.B
You only have to do this once; it takes effect at next login.
.R
You are now ready to try using
.I csh .
.PP
Before you do the `chsh' command, the shell you are using when
you log into the system is `/bin/sh'.
In fact, much of the above discussion is applicable to `/bin/sh'.
The next section will introduce many features particular to
.I csh
so you should change your shell to
.I csh
before you begin reading it.
.bp