csh.2

早期freebsd实现

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 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