.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved. The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\" @(#)csh.2 6.1 (Berkeley) 5/23/86
Details on the shell for terminal users
Shell startup and termination
When you login, the shell is started by the system in your
directory and begins by reading commands from a file
All shells which you may start during your terminal session will
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
executed after you login to the system,
will, after it reads commands from
read commands from a file
also in your home directory.
This file contains commands which you wish to do each time you login
to the \s-2UNIX\s0 system.
file looks something like:
set mail=(/usr/spool/mail/bill)
echo "${prompt}users" ; users
\'set noglob ; eval \`tset \-s \-m dialup:c100rv4pna \-m plugboard:?hp2621nl \!*\`\';
ts; stty intr ^C kill ^U crt
This file contains several commands to be executed by \s-2UNIX\s0
command which is interpreted directly by the shell. It sets the shell
which causes the shell to not log me off if I hit ^D. Rather,
command to log off of the system.
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
this will cause me to be notified immediately when mail arrives, and to
be shown the first few lines of the new message.
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
\fItset\fR\|(1) command setting up the modes of the terminal.
indicate the kinds of terminal which I usually use when not on a hardwired
port. I then execute ``ts'' and also use the
command to change the interrupt character to ^C and the line kill
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
When the `mail' and `msgs' programs finish, the shell will finish
file and begin reading commands from the terminal, prompting for each with
When I log off (by giving the
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
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.
The shell maintains a set of
We saw above the variables
which had values `10' and `15'.
In fact, each shell variable has as value an array of
Shell variables may be assigned values by the set command. It has
several forms, the most useful of which was given above and is
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
By changing the values of these variables one can directly affect the
One of the most important variables is the variable
This variable contains a sequence of directory names where the shell
command with no arguments
shows the value of all variables currently defined (we usually say
The default value for path will be shown by
path (. /usr/ucb /bin /usr/bin)
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
Commands developed at Berkeley, live in `/usr/ucb'
while commands developed at Bell Laboratories live in `/bin' and `/usr/bin'.
A number of locally developed programs on the system live in the directory
If we wish that all shells which we invoke to have
access to these new programs we can place the command
set path=(. /usr/ucb /bin /usr/bin /usr/local)
Try doing this and then logging out and back in and do
again to see that the value assigned to
Another directory that might interest you is /usr/new, which contains
many useful user-contributed programs provided with Berkeley Unix.
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
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
Other useful built in variables are the variable
which shows your home directory,
which contains your current working directory,
file to tell the shell not to exit when it receives an end-of-file from
a terminal (as described above).
is one of several variables which the shell does not care about the
value of, only whether they are
Thus to set this variable you simply do
These give the variable `ignoreeof' no value, but none is desired or required.
Finally, some other built-in shell variables of use are the
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
would cause a diagnostic if `now' existed already.
if you really wanted to overwrite the contents of `now'.
The `>!' is a special metasyntax indicating that clobbering the
\(dgThe space between the `!' and the word `now' is critical here, as `!now'
would be an invocation of the
mechanism, and have a totally different effect.
into which it places the words
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.
The following figure gives a sample session involving typical usage of the
history mechanism of the shell.
"bug.c", line 4: newline in string or char constant
"bug.c", line 5: syntax error
a.out: 2784+364+1028 = 4176b = 0x1050b
bug: 2784+364+1028 = 4176b = 0x1050b
\(mirwxr\(mixr\(mix 1 bill 3932 Dec 19 09:41 a.out
\(mirwxr\(mixr\(mix 1 bill 3932 Dec 19 09:42 bug
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'
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.
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.
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
There are other mechanisms available for repeating commands. The
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.
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
which must be done in the current shell.
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
If you place the shell command
file, the shell will transform an input line of the form
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'.
creating a new command syntax `dir'
then the shell will translate this to
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
alias cd \'cd \e!* ; ls \'
command after each change directory
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
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.
alias whois \'grep \e!^ /etc/passwd\'
defines a command which looks up its first argument in the password file.
The shell currently reads the
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
More redirection; >> and >&
There are a few more notations useful to the terminal user
which have not been introduced yet.
In addition to the standard output, commands also have a
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
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
The `>&' here tells the shell to route both the diagnostic output and the
standard output into `file'.
Similarly you can give the command
to route both standard and diagnostic output through the pipe
to the line printer daemon
\(dd A command of the form
Finally, it is possible to use the form
to place output at the end of an existing file.\(dg
is set, then an error will result if
does not exist, otherwise the shell will create
makes it not be an error for file to not exist when
Jobs; Background, Foreground, or Suspended
When one or more commands
are typed together as a pipeline or as a sequence of commands separated by
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
ls \-s | sort \-n | head \-5
If the metacharacter `&' is typed
at the end of the commands, then the job is started as a
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
at the same time that normal jobs, called
jobs, continue to be read and executed by the shell one at a time.
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
program would continue executing in the background
until it finished, even though you can type and execute more commands in the
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
job finishes sometime during the
command and its completion is reported just before
How do you know when a background job is finished?
If the job did not terminate normally the `Done' message might say
something else like `Killed'.
terminations of background jobs to be reported at the time they occur
(possibly interrupting the output of other foreground jobs), you can set
variable. In the previous example this would mean that the
`Done' message might have come right in the middle of the message to
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.
Jobs are recorded in a table inside the shell until they terminate.
In this table, the shell remembers the command names, arguments and the
of all commands in the job as well as the working directory where the job was
Each job in the table is either running
with the shell waiting for it to terminate, running
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
which can be used later to refer to the job in the commands described below.
the same until the job terminates and then are re-used.
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,
% ls \-s | sort \-n > usage &
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
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.
As mentioned in section 1.8, foreground jobs become
which sends a \s-2STOP\s0 signal to the currently running
foreground job. A background job can become suspended by using the
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
`Stopped' message is typed by the shell when it notices that the
For background jobs, using the
[1] + Stopped (signal) sort usage
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
command, allowing you to continue other work and
stop waiting for the foreground job to finish. Thus
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.
commands can take an argument that identifies a particular
All job name arguments begin with the character `%', since some of the
job control commands also accept process numbers (printed by the
The default job (when no argument is given) is called the
job and is identified by a `+' in the output of the
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
job and the existing current job becomes the
job \- identified by a `\-' in the output of
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
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
% ls \-s | sort \-n > myfile &
[1] \(mi Running du > usage
[2] Running ls \-s | sort \-n > myfile
[3] \(pl Stopped mail bill
ls \-s | sort \-n > myfile
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
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.
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
command changes a foreground job into a background job.
command suspends a background job.
command terminates a background or suspended job immediately.
In addition to jobs, it may be given process numbers as arguments,
Thus, in the example above, the running
command could have been terminated by the command
[1] Terminated du > usage
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.
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.
. . . some foreground commands
[1] Stopped (tty input) ed bigfile
So after the `s' command was issued, the `ed' job was stopped with ^Z
and then put in the background using
Some time later when the `s' command was finished,
tried to read another command and was stopped because jobs
in the backgound cannot read from the terminal. The
command returned the `ed' job to the foreground where it could once again
accept commands from the terminal.
causes all background jobs run on your terminal to stop
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
more input can be given and, if necessary stopped and returned to
command might be a good thing to put in your
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:
[1] Stopped (tty output) wc hugefile
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
is set, when they are not in the foreground, as
it would be very unpleasant to have a background job change the state
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
can be used in this case to find out about background jobs not started
As mentioned in section 1.6, the shell is always in a particular
The `change directory' command
changes the working directory of the shell,
that is, changes the directory you
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,
(`print working directory') command
reports the absolute pathname of the working directory of the shell,
that is, the directory you are
Thus in the example below:
the user has created and moved to the
where, for example, he might
place a group of related files.
No matter where you have moved to in a directory hierarchy,
you can return to your `home' login directory by doing just
The name `..' always means the directory above the current one in
changes the shell's working directory to the one directly above the
The name `..' can be used in any
change to the directory `programs' contained in the directory
If you have several directories for different
projects under, say, your home directory,
permits you to switch easily between them.
The shell always remembers the pathname of its current working directory in
The shell can also be requested to remember the previous directory when
you change to a new working directory. If the `push directory' command
command, the shell saves the name of the current working directory
before changing to the new one.
You can see this list at any time by typing the `directories'
% pushd newpaper/references
/usr/lib/tmac ~/newpaper/references ~
/usr/lib/tmac ~/newpaper/references ~
The list is printed in a horizontal line, reading left to right,
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.
is usually faster and more informative than
since it shows the current working directory as well as any
other directories remembered in the stack.
alternates the current directory with the first directory in the
command without an argument returns you to the directory you were in prior to
the current one, discarding the previous current directory from the
several times in a series takes you backward through the directories
you had been in (changed to) by
There are other options to
to manipulate the contents of the directory stack and to change
to directories not at the top of the stack; see the
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
ed prog.c (wd: ~/myproject)
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.
ed prog.c (wd: ~/myproject)
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.
will type the working directory
of suspended or background jobs when it is different
from the current working directory of the shell.
We now give a few of the useful built-in commands of the shell describing
command described above is used to assign new aliases and to show the
With no arguments it prints the current aliases.
It may also be given only one argument such as
to show the current alias for, e.g., `ls'.
command prints its arguments.
or as an interactive command
to see what filename expansions will produce.
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
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.
Note that the `!' character had to be
here even within `\'' characters.
command is used to restrict use of resources.
With no arguments it prints the current limitations:
Most reasonable units abbreviations will work; see the
manual page for more details.
command can be used to terminate a login shell which has
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.
command can be used to repeat a command several times.
Thus to make 5 copies of the file
to set variables in the environment.
will set the value of the environment variable \s-2TERM\s0
exists which will print out the environment.
PATH=:/usr/ucb:/bin:/usr/bin:/usr/local
command can be used to force the current shell to read commands from
can be used after editing in a change to the
file which you wish to take effect right away.
command can be used to cause a command to be timed no matter how much
\s-2CPU\s0 time it takes.
% 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
0.1u 0.1s 0:00 13% 3+3k 5+3io 7pf+0w
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).
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
to remove aliases and variable definitions from the shell, and
removes variables from the environment.
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
built-in command which can be used to run the same command
sequence with a number of different arguments.
If you intend to use \s-2UNIX\s0 a lot you 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.