path: root/bin/csh/USD.doc/csh.2

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.nr H1 1
.NH
Details on the shell for terminal users
.NH 2
Shell startup and termination
.PP
When you login, the shell is started by the system in your
.I home
directory and begins by reading commands from a file
.I \&.cshrc
in this directory.
All shells which you may start during your terminal session will
read from this file.
We will later see what kinds of commands are usefully placed there.
For now we need not have this file and the shell does not complain about
its absence.
.PP
A 
.I "login shell" ,
executed after you login to the system,
will, after it reads commands from
.I \&.cshrc,
read commands from a file
.I \&.login
also in your home directory.
This file contains commands which you wish to do each time you login
to the \s-2UNIX\s0 system.
My
.I \&.login
file looks something like:
.DS
set ignoreeof
set mail=(/usr/spool/mail/bill)
echo "${prompt}users" ; users
alias ts \e
	\'set noglob ; eval \`tset \-s \-m dialup:c100rv4pna \-m plugboard:?hp2621nl \!*\`\';
ts; stty intr ^C kill ^U crt
set time=15 history=10
msgs \-f
if (\-e $mail) then
	echo "${prompt}mail"
	mail
endif
.DE
.PP
This file contains several commands to be executed by \s-2UNIX\s0
each time I login.
The first is a
.I set
command which is interpreted directly by the shell.  It sets the shell
variable
.I ignoreeof
which causes the shell to not log me off if I hit ^D.  Rather,
I use the
.I logout
command to log off of the system.
By setting the
.I mail
variable, I ask the shell to watch for incoming mail to me.  Every 5 minutes
the shell looks for this file and tells me if more mail has arrived there.
An alternative to this is to put the command
.DS
biff y
.DE
in place of this
.I set;
this will cause me to be notified immediately when mail arrives, and to
be shown the first few lines of the new message.
.PP
Next I set the shell variable `time' to `15' causing the shell to automatically
print out statistics lines for commands which execute for at least 15 seconds
of \s-2CPU\s+2 time.  The variable `history' is set to 10 indicating that
I want the shell to remember the last 10 commands I type in its
.I "history list" ,
(described later).
.PP
I create an
.I alias
``ts'' which executes a
\fItset\fR\|(1) command setting up the modes of the terminal.
The parameters to
.I tset
indicate the kinds of terminal which I usually use when not on a hardwired
port.  I then execute ``ts'' and also use the
.I stty
command to change the interrupt character to ^C and the line kill
character to ^U.
.PP
I then run the `msgs' program, which provides me with any
system messages which I have not seen before; the `\-f' option here prevents
it from telling me anything if there are no new messages.
Finally, if my mailbox file exists, then I run the `mail' program to
process my mail.
.PP
When the `mail' and `msgs' programs finish, the shell will finish
processing my
.I \&.login
file and begin reading commands from the terminal, prompting for each with
`% '.
When I log off (by giving the 
.I logout
command) the shell
will print `logout' and execute commands from the file `.logout'
if it exists in my home directory.
After that the shell will terminate and \s-2UNIX\s0 will log
me off the system.
If the system is not going down, I will receive a new login message.
In any case, after the `logout' message the shell is committed to terminating
and will take no further input from my terminal.
.NH 2
Shell variables
.PP
The shell maintains a set of
.I variables.
We saw above the variables
.I history
and
.I time
which had values `10' and `15'.
In fact, each shell variable has as value an array of
zero or more
.I strings.
Shell variables may be assigned values by the set command.  It has
several forms, the most useful of which was given above and is
.DS
set name=value
.DE
.PP
Shell variables may be used to store values which are to
be used in commands later through a substitution mechanism.
The shell variables most commonly referenced are, however, those which the
shell itself refers to.
By changing the values of these variables one can directly affect the
behavior of the shell.
.PP
One of the most important variables is the variable
.I path.
This variable contains a sequence of directory names where the shell
searches for commands.
The
.I set
command with no arguments
shows the value of all variables currently defined (we usually say
.I set)
in the shell.
The default value for path will be shown by
.I set
to be
.DS
% set
.ta .75i
argv	()
cwd	/usr/bill
home	/usr/bill
path	(. /usr/ucb /bin /usr/bin)
prompt	%
shell	/bin/csh
status	0
term	c100rv4pna
user	bill
%
.so tabs
.DE
This output indicates that the variable path points to the current
directory `.' and then `/usr/ucb', `/bin' and `/usr/bin'.
Commands which you may write might be in `.' (usually one of
your directories).
Commands developed at Berkeley, live in `/usr/ucb'
while commands developed at Bell Laboratories live in `/bin' and `/usr/bin'.
.PP
A number of locally developed programs on the system live in the directory
`/usr/local'.
If we wish that all shells which we invoke to have
access to these new programs we can place the command
.DS
set path=(. /usr/ucb /bin /usr/bin /usr/local)
.DE
in our file
.I \&.cshrc
in our home directory.
Try doing this and then logging out and back in and do
.DS
set
.DE
again to see that the value assigned to
.I path
has changed.
.FS \(dg
Another directory that might interest you is /usr/new, which contains
many useful user-contributed programs provided with Berkeley Unix.
.FE
.PP
One thing you should be aware of is that the shell examines each directory
which you insert into your path and determines which commands are contained
there.  Except for the current directory `.', which the shell treats specially,
this means that if commands are added to a directory in your search path after
you have started the shell, they will not necessarily be found by the shell.
If you wish to use a command which has been added in this way, you should
give the command
.DS
rehash
.DE
to the shell, which will cause it to recompute its internal table of command
locations, so that it will find the newly added command.
Since the shell has to look in the current directory `.' on each command,
placing it at the end of the path specification usually works equivalently
and reduces overhead.
.PP
Other useful built in variables are the variable
.I home
which shows your home directory,
.I cwd
which contains your current working directory,
the variable
.I ignoreeof
which can be set in your
.I \&.login
file to tell the shell not to exit when it receives an end-of-file from
a terminal (as described above).
The variable `ignoreeof'
is one of several variables which the shell does not care about the
value of, only whether they are
.I set
or
.I unset.
Thus to set this variable you simply do
.DS
set ignoreeof
.DE
and to unset it do
.DS
unset ignoreeof
.DE
These give the variable `ignoreeof' no value, but none is desired or required.
.PP
Finally, some other built-in shell variables of use are the
variables
.I noclobber
and
.I mail.
The metasyntax
.DS
> filename
.DE
which redirects the standard output of a command
will overwrite and destroy the previous contents of the named file.
In this way you may accidentally overwrite a file which is valuable.
If you would prefer that the shell not overwrite files in this
way you can
.DS
set noclobber
.DE
in your
.I \&.login
file.
Then trying to do
.DS
date > now
.DE
would cause a diagnostic if `now' existed already.
You could type
.DS
date >!  now
.DE
if you really wanted to overwrite the contents of `now'.
The `>!' is a special metasyntax indicating that clobbering the
file is ok.\(dg
.FS
\(dgThe space between the `!' and the word `now' is critical here, as `!now'
would be an invocation of the
.I history
mechanism, and have a totally different effect.
.FE
.NH 2
The shell's history list
.PP
The shell can maintain a
.I "history list"
into which it places the words
of previous commands.
It is possible to use a notation to reuse commands or words
from commands in forming new commands.
This mechanism can be used to repeat previous commands or to
correct minor typing mistakes in commands.
.PP
The following figure gives a sample session involving typical usage of the
history mechanism of the shell.
.KF
.DS
% cat bug.c
main()

{
	printf("hello);
}
% cc !$
cc bug.c 
"bug.c", line 4: newline in string or char constant
"bug.c", line 5: syntax error
% ed !$
ed bug.c 
29
4s/);/"&/p
        printf("hello");
w
30
q
% !c
cc bug.c 
% a.out
hello% !e
ed bug.c 
30
4s/lo/lo\e\en/p
        printf("hello\en");
w
32
q
% !c \-o bug
cc bug.c \-o bug
% size a.out bug
a.out: 2784+364+1028 = 4176b = 0x1050b
bug: 2784+364+1028 = 4176b = 0x1050b
% ls \-l !*
ls \-l a.out bug 
\(mirwxr\(mixr\(mix 1 bill       3932 Dec 19 09:41 a.out
\(mirwxr\(mixr\(mix 1 bill       3932 Dec 19 09:42 bug
% bug
hello
% num bug.c | spp
spp: Command not found.
% ^spp^ssp
num bug.c | ssp 
    1	main()
    3	{
    4		printf("hello\en");
    5	}
% !! | lpr
num bug.c | ssp | lpr
% 
.DE
.KE
In this example we have a very simple C program which has a bug (or two)
in it in the file `bug.c', which we `cat' out on our terminal.  We then
try to run the C compiler on it, referring to the file again as `!$',
meaning the last argument to the previous command.  Here the `!' is the
history mechanism invocation metacharacter, and the `$' stands for the last
argument, by analogy to `$' in the editor which stands for the end of the line.
The shell echoed the command, as it would have been typed without use of
the history mechanism, and then executed it.
The compilation yielded error diagnostics so we now run the editor on the
file we were trying to compile, fix the bug, and run the C compiler again,
this time referring to this command simply as `!c', which repeats the last
command which started with the letter `c'.  If there were other
commands starting with `c' done recently we could have said `!cc' or even
`!cc:p' which would have printed the last command starting with `cc'
without executing it.
.PP
After this recompilation, we ran the resulting `a.out' file, and then
noting that there still was a bug, ran the editor again.  After fixing
the program we ran the C compiler again, but tacked onto the command
an extra `\-o bug' telling the compiler to place the resultant binary in
the file `bug' rather than `a.out'.  In general, the history mechanisms
may be used anywhere in the formation of new commands and other characters
may be placed before and after the substituted commands.
.PP
We then ran the `size' command to see how large the binary program images
we have created were, and then an `ls \-l' command with the same argument
list, denoting the argument list `\!*'.
Finally we ran the program `bug' to see that its output is indeed correct.
.PP
To make a numbered listing of the program we ran the `num' command on the file `bug.c'.
In order to compress out blank lines in the output of `num' we ran the
output through the filter `ssp', but misspelled it as spp.  To correct this
we used a shell substitute, placing the old text and new text between `^'
characters.  This is similar to the substitute command in the editor.
Finally, we repeated the same command with `!!', but sent its output to the
line printer.
.PP
There are other mechanisms available for repeating commands.  The
.I history
command prints out a number of previous commands with numbers by which
they can be referenced.  There is a way to refer to a previous command
by searching for a string which appeared in it, and there are other,
less useful, ways to select arguments to include in a new command.
A complete description of all these mechanisms
is given in the C shell manual pages in the \s-2UNIX\s0 Programmer's Manual.
.NH 2
Aliases
.PP
The shell has an
.I alias
mechanism which can be used to make transformations on input commands.
This mechanism can be used to simplify the commands you type,
to supply default arguments to commands,
or to perform transformations on commands and their arguments.
The alias facility is similar to a macro facility.
Some of the features obtained by aliasing can be obtained also
using shell command files, but these take place in another instance
of the shell and cannot directly affect the current shells environment
or involve commands such as
.I cd
which must be done in the current shell.
.PP
As an example, suppose that there is a new version of the mail program
on the system called `newmail'
you wish to use, rather than the standard mail program which is called
`mail'.
If you place the shell command
.DS
alias mail newmail
.DE
in your
.I \&.cshrc
file, the shell will transform an input line of the form
.DS
mail bill
.DE
into a call on `newmail'.
More generally, suppose we wish the command `ls' to always show
sizes of files, that is to always do `\-s'.
We can do
.DS
alias ls ls \-s
.DE
or even
.DS
alias dir ls \-s
.DE
creating a new command syntax `dir'
which does an `ls \-s'.
If we say
.DS
dir ~bill
.DE
then the shell will translate this to
.DS
ls \-s /mnt/bill
.DE
.PP
Thus the
.I alias
mechanism can be used to provide short names for commands,
to provide default arguments,
and to define new short commands in terms of other commands.
It is also possible to define aliases which contain multiple
commands or pipelines, showing where the arguments to the original
command are to be substituted using the facilities of the
history mechanism.
Thus the definition
.DS
alias cd \'cd \e!* ; ls \'
.DE
would do an
.I ls
command after each change directory
.I cd
command.
We enclosed the entire alias definition in `\'' characters to prevent
most substitutions from occurring and the character `;' from being
recognized as a metacharacter.
The `!' here is escaped with a `\e' to prevent it from being interpreted
when the alias command is typed in.
The `\e!*' here substitutes the entire argument list to the pre-aliasing
.I cd
command, without giving an error if there were no arguments.
The `;' separating commands is used here
to indicate that one command is to be done and then the next.
Similarly the definition
.DS
alias whois \'grep \e!^ /etc/passwd\'
.DE
defines a command which looks up its first argument in the password file.
.PP
.B Warning:
The shell currently reads the
.I \&.cshrc
file each time it starts up.  If you place a large number of commands
there, shells will tend to start slowly.  A mechanism for saving the shell
environment after reading the \fI\&.cshrc\fR file and quickly restoring it is
under development, but for now you should try to limit the number of
aliases you have to a reasonable number... 10 or 15 is reasonable,
50 or 60 will cause a noticeable delay in starting up shells, and make
the system seem sluggish when you execute commands from within the editor
and other programs.
.NH 2
More redirection; >> and >&
.PP
There are a few more notations useful to the terminal user
which have not been introduced yet.
.PP
In addition to the standard output, commands also have a
.I "diagnostic output"
which is normally directed to the terminal even when the standard output
is redirected to a file or a pipe.
It is occasionally desirable to direct the diagnostic output along with
the standard output.
For instance if you want to redirect the output of a long running command
into a file and wish to have a record of any error diagnostic it produces
you can do
.DS
command >& file
.DE
The `>&' here tells the shell to route both the diagnostic output and the
standard output into `file'.
Similarly you can give the command
.DS
command |\|& lpr
.DE
to route both standard and diagnostic output through the pipe
to the line printer daemon
.I lpr.\(dd
.FS
\(dd A command of the form
.br
.ti +5
command >&! file
.br
exists, and is used when
.I noclobber
is set and
.I file
already exists.
.FE
.PP
Finally, it is possible to use the form
.DS
command >> file
.DE
to place output at the end of an existing file.\(dg
.FS
\(dg If
.I noclobber
is set, then an error will result if
.I file
does not exist, otherwise the shell will create
.I file
if it doesn't exist.
A form
.br
.ti +5
command >>! file
.br
makes it not be an error for file to not exist when
.I noclobber
is set.
.FE
.NH 2
Jobs; Background, Foreground, or Suspended
.PP
When one or more commands
are typed together as a pipeline or as a sequence of commands separated by
semicolons, a single
.I job
is created by the shell consisting of these commands together as a unit.
Single commands without pipes or semicolons create the simplest jobs.
Usually, every line typed to the shell creates a job.
Some lines that create jobs (one per line) are
.DS
sort < data
ls \-s | sort \-n | head \-5
mail harold
.DE
.PP
If the metacharacter `&' is typed
at the end of the commands, then the job is started as a
.I background
job.  This means that the shell does not wait for it to complete but
immediately prompts and is ready for another command.  The job runs
.I "in the background"
at the same time that normal jobs, called
.I foreground
jobs, continue to be read and executed by the shell one at a time.
Thus
.DS
du > usage &
.DE
would run the
.I du
program, which reports on the disk usage of your working directory (as well as
any directories below it), put the output into the file `usage' and return
immediately with a prompt for the next command without out waiting for
.I du
to finish.  The
.I du
program would continue executing in the background
until it finished, even though you can type and execute more commands in the
mean time.
When a background
job terminates, a message is typed by the shell just before the next prompt
telling you that the job has completed.
In the following example the
.I du
job finishes sometime during the
execution of the
.I mail
command and its completion is reported just before
the prompt after the
.I mail
job is finished.
.DS
% du > usage &
[1] 503
% mail bill
How do you know when a background job is finished?
EOT
.ta 1.75i
[1] \- Done	du > usage
%
.so tabs
.DE
If the job did not terminate normally the `Done' message might say
something else like `Killed'.
If you want the 
terminations of background jobs to be reported at the time they occur
(possibly interrupting the output of other foreground jobs), you can set
the
.I notify
variable.  In the previous example this would mean that the
`Done' message might have come right in the middle of the message to
Bill.
Background jobs are unaffected by any signals from the keyboard like
the \s-2STOP\s0, \s-2INTERRUPT\s0, or \s-2QUIT\s0 signals mentioned earlier.
.PP
Jobs are recorded in a table inside the shell until they terminate.
In this table, the shell remembers the command names, arguments and the
.I "process numbers"
of all commands in the job as well as the working directory where the job was
started.
Each job in the table is either running
.I "in the foreground"
with the shell waiting for it to terminate, running
.I "in the background,"
or
.I suspended.
Only one job can be running in the foreground at one time, but several
jobs can be suspended or running in the background at once.  As each job
is started, it is assigned a small identifying
number called the
.I "job number"
which can be used later to refer to the job in the commands described below.
Job numbers remain
the same until the job terminates and then are re-used.
.PP
When a job is started in the backgound using `&', its number, as well
as the process numbers of all its (top level) commands, is typed by the shell
before prompting you for another command. For example,
.DS
% ls \-s | sort \-n > usage &
[2] 2034 2035
%
.DE
runs the `ls' program with the `\-s' options, pipes this output into
the `sort' program with the `\-n' option which puts its output into the
file `usage'.
Since the `&' was at the end of the line, these two programs were started
together as a background job.  After starting the job, the shell prints
the job number in brackets (2 in this case) followed by the process number
of each program started in the job.  Then the shell immediates prompts for
a new command, leaving the job running simultaneously.
.PP
As mentioned in section 1.8, foreground jobs become
.I suspended
by typing ^Z
which sends a \s-2STOP\s0 signal to the currently running
foreground job.  A background job can become suspended by using the
.I stop
command described below.  When jobs are suspended they merely stop
any further progress until started again, either in the foreground
or the backgound.  The shell notices when a job becomes stopped and
reports this fact, much like it reports the termination of background jobs.
For foreground jobs this looks like
.DS
% du > usage
^Z
Stopped
%
.DE
`Stopped' message is typed by the shell when it notices that the
.I du
program stopped.
For background jobs, using the
.I stop
command, it is
.DS
% sort usage &
[1] 2345
% stop %1
.ta 1.75i
[1] + Stopped (signal)	sort usage
%
.so tabs
.DE
Suspending foreground jobs can be very useful when you need to temporarily
change what you are doing (execute other commands) and then return to
the suspended job.  Also, foreground jobs can be suspended and then
continued as background jobs using the
.I bg
command, allowing you to continue other work and
stop waiting for the foreground job to finish.  Thus
.DS
% du > usage
^Z
Stopped
% bg
[1] du > usage &
%
.DE
starts `du' in the foreground, stops it before it finishes, then continues
it in the background allowing more foreground commands to be executed.
This is especially helpful
when a foreground job ends up taking longer than you expected and you
wish you had started it in the backgound in the beginning.
.PP
All
.I "job control"
commands can take an argument that identifies a particular
job.
All job name arguments begin with the character `%', since some of the
job control commands also accept process numbers (printed by the
.I ps
command.)
The default job (when no argument is given) is called the
.I current
job and is identified by a `+' in the output of the
.I jobs
command, which shows you which jobs you have.
When only one job is stopped or running in the background (the usual case)
it is always the current job thus no argument is needed.
If a job is stopped while running in the foreground it becomes the
.I current
job and the existing current job becomes the
.I previous
job \- identified by a `\-' in the output of
.I jobs.
When the current job terminates, the previous job becomes the current job.
When given, the argument is either `%\-' (indicating
the previous job); `%#', where # is the job number; 
`%pref' where pref is some unique prefix of the command name
and arguments of one of the jobs; or `%?' followed by some string found
in only one of the jobs.
.PP
The
.I jobs
command types the table of jobs, giving the job number,
commands and status (`Stopped' or `Running') of each backgound or
suspended job.  With the `\-l' option the process numbers are also
typed.
.DS
% du > usage &
[1] 3398
% ls \-s | sort \-n > myfile &
[2] 3405
% mail bill
^Z
Stopped
% jobs
.ta 1.75i
[1] \(mi Running	du > usage
[2]    Running	ls \-s | sort \-n > myfile
[3] \(pl Stopped	mail bill
% fg %ls
ls \-s | sort \-n > myfile
% more myfile
.so tabs
.DE
.PP
The
.I fg
command runs a suspended or background job in the foreground.  It is
used to restart a previously suspended job or change a background job
to run in the foreground (allowing signals or input from the terminal).
In the above example we used
.I fg
to change the `ls' job from the
background to the foreground since we wanted to wait for it to
finish before looking at its output file.
The
.I bg
command runs a suspended job in the background.  It is usually used
after stopping the currently running foreground job with the
\s-2STOP\s0 signal.  The combination of the \s-2STOP\s0 signal and the
.I bg
command changes a foreground job into a background job.
The
.I stop
command suspends a background job.
.PP
The
.I kill
command terminates a background or suspended job immediately.
In addition to jobs, it may be given process numbers as arguments,
as printed by
.I ps.
Thus, in the example above, the running
.I du
command could have been terminated by the command
.DS
% kill %1
.ta 1.75i
[1]  Terminated	du > usage
%
.so tabs
.DE
.PP
The
.I notify
command (not the variable mentioned earlier) indicates that the termination
of a specific job should be
reported at the time it finishes instead of waiting for the next prompt.
.PP
If a job running in the background tries to read input from the terminal
it is automatically stopped.  When such a job is then run in the
foreground, input can be given to the job.  If desired, the job can
be run in the background again until it requests input again.
This is illustrated in the following sequence where the `s' command in the
text editor might take a long time.
.ID
.nf
% ed bigfile
120000
1,$s/thisword/thatword/
^Z
Stopped
% bg
[1] ed bigfile &
% 
 . . .  some foreground commands
.ta 1.75i
[1] Stopped (tty input)	ed bigfile
% fg
ed bigfile
w
120000
q
% 
.so tabs
.DE
So after the `s' command was issued, the `ed' job was stopped with ^Z
and then put in the background using
.I bg.
Some time later when the `s' command was finished,
.I ed
tried to read another command and was stopped because jobs
in the backgound cannot read from the terminal.  The
.I fg
command returned the `ed' job to the foreground where it could once again
accept commands from the terminal.
.PP
The command
.DS
stty tostop
.DE
causes all background jobs run on your terminal to stop
when they are about to
write output to the terminal.  This prevents messages from background
jobs from interrupting foreground job output and allows you to run
a job in the background without losing terminal output.  It also
can be used for interactive programs that sometimes have long
periods without interaction.  Thus each time it outputs a prompt for more
input it will stop before the prompt.  It can then be run in the
foreground using
.I fg,
more input can be given and, if necessary stopped and returned to
the background.  This
.I stty
command might be a good thing to put in your
.I \&.login
file if you do not like output from background jobs interrupting
your work.  It also can reduce the need for redirecting the output
of background jobs if the output is not very big:
.DS
% stty tostop
% wc hugefile &
[1] 10387
% ed text
\&. . . some time later
q
.ta 1.75i
[1] Stopped (tty output)	wc hugefile
% fg wc
wc hugefile
   13371   30123   302577
% stty \-tostop
.so tabs
.DE
Thus after some time the `wc' command, which counts the lines, words
and characters in a file, had one line of output.  When it tried to
write this to the terminal it stopped.  By restarting it in the
foreground we allowed it to write on the terminal exactly when we were
ready to look at its output.
Programs which attempt to change the mode of the terminal will also
block, whether or not
.I tostop
is set, when they are not in the foreground, as
it would be very unpleasant to have a background job change the state
of the terminal.
.PP
Since the
.I jobs
command only prints jobs started in the currently executing shell,
it knows nothing about background jobs started in other login sessions
or within shell files.  The
.I ps
can be used in this case to find out about background jobs not started
in the current shell.
.NH 2
Working Directories
.PP
As mentioned in section 1.6, the shell is always in a particular
.I "working directory."
The `change directory' command
.I chdir
(its
short form
.I cd
may also be used)
changes the working directory of the shell,
that is, changes the directory you
are located in.
.PP
It is useful to make a directory for each project you wish to work on
and to place all files related to that project in that directory.
The `make directory' command,
.I mkdir,
creates a new directory.
The
.I pwd
(`print working directory') command
reports the absolute pathname of the working directory of the shell,
that is, the directory you are
located in.
Thus in the example below:
.DS
% pwd
/usr/bill
% mkdir newpaper
% chdir newpaper
% pwd
/usr/bill/newpaper
%
.DE
the user has created and moved to the
directory
.I newpaper.
where, for example, he might
place a group of related files.
.PP
No matter where you have moved to in a directory hierarchy,
you can return to your `home' login directory by doing just
.DS
cd
.DE
with no arguments.
The name `..' always means the directory above the current one in
the hierarchy, thus
.DS
cd ..
.DE
changes the shell's working directory to the one directly above the
current one.
The name `..' can be used in any
pathname, thus,
.DS
cd ../programs
.DE
means
change to the directory `programs' contained in the directory
above the current one.
If you have several directories for different
projects under, say, your home directory,
this shorthand notation
permits you to switch easily between them.
.PP
The shell always remembers the pathname of its current working directory in
the variable
.I cwd.
The shell can also be requested to remember the previous directory when
you change to a new working directory.  If the `push directory' command
.I pushd
is used in place of the
.I cd
command, the shell saves the name of the current working directory
on a
.I "directory stack"
before changing to the new one.
You can see this list at any time by typing the `directories'
command
.I dirs.
.ID
.nf
% pushd newpaper/references
~/newpaper/references  ~
% pushd /usr/lib/tmac
/usr/lib/tmac  ~/newpaper/references  ~
% dirs
/usr/lib/tmac  ~/newpaper/references  ~
% popd
~/newpaper/references  ~
% popd
~
%
.DE
The list is printed in a horizontal line, reading left to right,
with a tilde (~) as
shorthand for your home directory\(emin this case `/usr/bill'.
The directory stack is printed whenever there is more than one
entry on it and it changes.
It is also printed by a
.I dirs
command.
.I Dirs
is usually faster and more informative than
.I pwd
since it shows the current working directory as well as any
other directories remembered in the stack.
.PP
The
.I pushd
command with no argument
alternates the current directory with the first directory in the
list.
The `pop directory'
.I popd
command without an argument returns you to the directory you were in prior to
the current one, discarding the previous current directory from the
stack (forgetting it).
Typing
.I popd
several times in a series takes you backward through the directories
you had been in (changed to) by
.I pushd
command.
There are other options to
.I pushd
and
.I popd
to manipulate the contents of the directory stack and to change
to directories not at the top of the stack; see the
.I csh
manual page for details.
.PP
Since the shell remembers the working directory in which each job
was started, it warns you when you might be confused by restarting
a job in the foreground which has a different working directory than the
current working directory of the shell.  Thus if you start a background
job, then change the shell's working directory and then cause the
background job to run in the foreground, the shell warns you that the
working directory of the currently running foreground job is different
from that of the shell.
.DS
% dirs \-l
/mnt/bill
% cd myproject
% dirs
~/myproject
% ed prog.c
1143
^Z
Stopped
% cd ..
% ls
myproject
textfile
% fg
ed prog.c (wd: ~/myproject)
.DE
This way the shell warns you when there
is an implied change of working directory, even though no cd command was
issued.  In the above example the `ed' job was still in `/mnt/bill/project'
even though the shell had changed to `/mnt/bill'.
A similar warning is given when such a foreground job
terminates or is suspended (using the \s-2STOP\s0 signal) since
the return to the shell again implies a change of working directory.
.DS
% fg
ed prog.c (wd: ~/myproject)
 . . . after some editing
q
(wd now: ~)
%
.DE
These messages are sometimes confusing if you use programs that change
their own working directories, since the shell only remembers which
directory a job is started in, and assumes it stays there.
The `\-l' option of
.I jobs
will type the working directory
of suspended or background jobs when it is different
from the current working directory of the shell.
.NH 2
Useful built-in commands
.PP
We now give a few of the useful built-in commands of the shell describing
how they are used.
.PP
The
.I alias
command described above is used to assign new aliases and to show the
existing aliases.
With no arguments it prints the current aliases.
It may also be given only one argument such as
.DS
alias ls
.DE
to show the current alias for, e.g., `ls'.
.PP
The
.I echo
command prints its arguments.
It is often used in
.I "shell scripts"
or as an interactive command
to see what filename expansions will produce.
.PP
The
.I history
command will show the contents of the history list.
The numbers given with the history events can be used to reference
previous events which are difficult to reference using the
contextual mechanisms introduced above.
There is also a shell variable called
.I prompt.
By placing a `!' character in its value the shell will there substitute
the number of the current command in the history list.
You can use this number to refer to this command in a history substitution.
Thus you could
.DS
set prompt=\'\e! % \'
.DE
Note that the `!' character had to be
.I escaped
here even within `\'' characters.
.PP
The
.I limit
command is used to restrict use of resources.
With no arguments it prints the current limitations:
.DS
.ta 1i
cputime	unlimited
filesize	unlimited
datasize	5616 kbytes
stacksize	512 kbytes
coredumpsize	unlimited
.so tabs
.DE
Limits can be set, e.g.:
.DS
limit coredumpsize 128k
.DE
Most reasonable units abbreviations will work; see the
.I csh
manual page for more details.
.PP
The
.I logout
command can be used to terminate a login shell which has
.I ignoreeof
set.
.PP
The
.I rehash
command causes the shell to recompute a table of where commands are
located.  This is necessary if you add a command to a directory
in the current shell's search path and wish the shell to find it,
since otherwise the hashing algorithm may tell the shell that the
command wasn't in that directory when the hash table was computed.
.PP
The
.I repeat
command can be used to repeat a command several times.
Thus to make 5 copies of the file
.I one
in the file
.I five
you could do
.DS
repeat 5 cat one >> five
.DE
.PP
The
.I setenv
command can be used
to set variables in the environment.
Thus
.DS
setenv TERM adm3a
.DE
will set the value of the environment variable \s-2TERM\s0
to
`adm3a'.
A user program
.I printenv
exists which will print out the environment.
It might then show:
.DS
% printenv
HOME=/usr/bill
SHELL=/bin/csh
PATH=:/usr/ucb:/bin:/usr/bin:/usr/local
TERM=adm3a
USER=bill
%
.DE
.PP
The
.I source
command can be used to force the current shell to read commands from
a file.
Thus
.DS
source .cshrc
.DE
can be used after editing in a change to the
.I \&.cshrc
file which you wish to take effect right away.
.PP
The
.I time
command can be used to cause a command to be timed no matter how much
\s-2CPU\s0 time it takes.
Thus
.DS
% time cp /etc/rc /usr/bill/rc
0.0u 0.1s 0:01 8% 2+1k 3+2io 1pf+0w
% time wc /etc/rc /usr/bill/rc
     52    178   1347 /etc/rc
     52    178   1347 /usr/bill/rc
    104    356   2694 total
0.1u 0.1s 0:00 13% 3+3k 5+3io 7pf+0w
%
.DE
indicates that the
.I cp
command used a negligible amount of user time (u)
and about 1/10th of a system time (s); the elapsed time was 1 second (0:01),
there was an average memory usage of 2k bytes of program space and 1k
bytes of data space over the cpu time involved (2+1k); the program
did three disk reads and two disk writes (3+2io), and took one page fault
and was not swapped (1pf+0w).
The word count command
.I wc
on the other hand used 0.1 seconds of user time and 0.1 seconds of system
time in less than a second of elapsed time.
The percentage `13%' indicates that over the period when it was active
the command `wc' used an average of 13 percent of the available \s-2CPU\s0
cycles of the machine.
.PP
The
.I unalias
and
.I unset
commands can be used
to remove aliases and variable definitions from the shell, and
.I unsetenv
removes variables from the environment.
.NH 2
What else?
.PP
This concludes the basic discussion of the shell for terminal users.
There are more features of the shell to be discussed here, and all
features of the shell are discussed in its manual pages.
One useful feature which is discussed later is the
.I foreach
built-in command which can be used to run the same command
sequence with a number of different arguments.
.PP
If you intend to use \s-2UNIX\s0 a lot you should look through
the rest of this document and the csh manual pages (section1) to become familiar
with the other facilities which are available to you.
.bp