csh.2

早期freebsd实现

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.\"	@(#)csh.2	8.1 (Berkeley) 6/8/93
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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.