csh.1

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beginning are treated specially.
Neither `*' or `?' or 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 `/usr/bill'
if the home directory for `bill' was `/usr/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 your
.I home
directory, i.e. into `/usr/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 /usr/bill
.DE
since my home directory is /usr/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 an 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, `/', `.' or `\-' 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
\s-2INTERRUPT\s0
.I signal
to the
.I cat
command by typing ^C on your terminal.*
.FS
*On some older Unix systems the \s-2DEL\s0 or \s-2RUBOUT\s0 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
\s-2INTERRUPT\s0
will cause it to terminate.
The shell notices that
.I cat
has terminated and prompts you again with `% '.
If you hit \s-2INTERRUPT\s0 again, the shell will just
repeat its prompt since it handles \s-2INTERRUPT\s0 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';
\s-2UNIX\s0 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 \s-2INTERRUPT\s0.
.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 \s-2STOP\s0 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
`Stopped' and then prompts for a new command.
The previously executing command has been suspended, but otherwise
unaffected by the \s-2STOP\s0 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
Stopped
% ls
funnyfile
prog.c
prog.o
% jobs
.ta 1.75i
[1]  + Stopped	mail harold
% fg
mail harold
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 `Stopped' 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 \s-2INTERRUPT\s0, and \s-2QUIT\s0
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 \s-2QUIT\s0
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 \s-2QUIT\s0 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 \s-2INTERRUPT\s0 and \s-2QUIT\s0 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 screenful and types `\-\-More\-\-'
at which point you can hit a space to get another screenful, a return
to get another line, a `?' 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 \s-2UNIX\s0 and type the following
command to the system:
.DS
chsh myname /bin/csh
.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 bill /bin/csh'.
.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