csh.3

早期freebsd实现

end
.DE
.PP
This script makes use of the
.I foreach
command, which causes the shell to execute the commands between the
.I foreach
and the matching
.I end
for each of the values given between `(' and `)' with the named
variable, in this case `i' set to successive values in the list.
Within this loop we may use the command
.I break
to stop executing the loop 
and
.I continue
to prematurely terminate one iteration
and begin the next.
After the
.I foreach
loop the iteration variable
(\fIi\fR in this case)
has the value at the last iteration.
.PP
We set the variable
.I noglob
here to prevent filename expansion of the members of
.I argv.
This is a good idea, in general, if the arguments to a shell script
are filenames which have already been expanded or if the arguments
may contain filename expansion metacharacters.
It is also possible to quote each use of a `$' variable expansion,
but this is harder and less reliable.
.PP
The other control construct used here is a statement of the form
.DS
\fBif\fR ( expression ) \fBthen\fR
	command
	...
\fBendif\fR
.DE
The placement of the keywords here is
.B not
flexible due to the current implementation of the shell.\(dg
.FS
\(dgThe following two formats are not currently acceptable to the shell:
.sp
.in +5
.nf
\fBif\fR ( expression )		# \fBWon't work!\fR
\fBthen\fR
	command
	...
\fBendif\fR
.fi
.in -5
.sp
and
.sp
.in +5
.nf
\fBif\fR ( expression ) \fBthen\fR command \fBendif\fR		# \fBWon't work\fR
.in -5
.fi
.FE
.PP
The shell does have another form of the if statement of the form
.DS
\fBif\fR ( expression ) \fBcommand\fR
.DE
which can be written
.DS
\fBif\fR ( expression ) \e
	command
.DE
Here we have escaped the newline for the sake of appearance.
The command must not involve `\||\|', `&' or `;'
and must not be another control command.
The second form requires the final `\e' to
.B immediately
precede the end-of-line.
.PP
The more general
.I if
statements above also admit a sequence of
.I else\-if
pairs followed by a single
.I else
and an
.I endif,
e.g.:
.DS
\fBif\fR ( expression ) \fBthen\fR
	commands
\fBelse\fR \fBif\fR (expression ) \fBthen\fR
	commands
\&...

\fBelse\fR
	commands
\fBendif\fR
.DE
.PP
Another important mechanism used in shell scripts is the `:' modifier.
We can use the modifier `:r' here to extract a root of a filename or
`:e' to extract the
.I extension.
Thus if the variable
.I i
has the value
`/mnt/foo.bar'
then
.sp
.in +5
.nf
% echo $i $i:r $i:e
/mnt/foo.bar /mnt/foo bar
%
.sp
.in -5
.fi
shows how the `:r' modifier strips off the trailing `.bar' and the
the `:e' modifier leaves only the `bar'.
Other modifiers will take off the last component of a pathname leaving
the head `:h' or all but the last component of a pathname leaving the
tail `:t'.
These modifiers are fully described in the
.I csh
manual pages in the User's Reference Manual.
It is also possible to use the
.I "command substitution"
mechanism described in the next major section to perform modifications
on strings to then reenter the shell's environment.
Since each usage of this mechanism involves the creation of a new process,
it is much more expensive to use than the `:' modification mechanism.\(dd
.FS
\(dd It is also important to note that
the current implementation of the shell limits the number of `:' modifiers
on a `$' substitution to 1.
Thus
.sp
.nf
.in +5
% echo $i $i:h:t
/a/b/c /a/b:t
%
.in -5
.fi
.sp
does not do what one would expect.
.FE
Finally, we note that the character `#' lexically introduces a shell
comment in shell scripts (but not from the terminal).
All subsequent characters on the input line after a `#' are discarded
by the shell.
This character can be quoted using `\'' or `\e' to place it in
an argument word.
.NH 2
Other control structures
.PP
The shell also has control structures
.I while
and
.I switch
similar to those of C.
These take the forms
.DS
\fBwhile\fR ( expression )
	commands
\fBend\fR
.DE
and
.DS
\fBswitch\fR ( word )

\fBcase\fR str1:
	commands
	\fBbreaksw\fR

\& ...

\fBcase\fR strn:
	commands
	\fBbreaksw\fR

\fBdefault:\fR
	commands
	\fBbreaksw\fR

\fBendsw\fR
.DE
For details see the manual section for
.I csh.
C programmers should note that we use
.I breaksw
to exit from a
.I switch
while
.I break
exits a
.I while
or
.I foreach
loop.
A common mistake to make in
.I csh
scripts is to use
.I break
rather than
.I breaksw
in switches.
.PP
Finally,
.I csh
allows a
.I goto
statement, with labels looking like they do in C, i.e.:
.DS
loop:
	commands
	\fBgoto\fR loop
.DE
.NH 2
Supplying input to commands
.PP
Commands run from shell scripts receive by default the standard
input of the shell which is running the script.
This is different from previous shells running
under \s-2UNIX\s0.  It allows shell scripts to fully participate
in pipelines, but mandates extra notation for commands which are to take
inline data.
.PP
Thus we need a metanotation for supplying inline data to commands in
shell scripts.
As an example, consider this script which runs the editor to
delete leading blanks from the lines in each argument file:
.DS
% cat deblank
# deblank \-\- remove leading blanks
foreach i ($argv)
ed \- $i << \'EOF\'
1,$s/^[ ]*//
w
q
\&\'EOF\'
end
%
.DE
The notation `<< \'EOF\''
means that the standard input for the
.I ed
command is to come from the text in the shell script file
up to the next line consisting of exactly `\'EOF\''.
The fact that the `EOF' is enclosed in `\'' characters, i.e. quoted,
causes the shell to not perform variable substitution on the
intervening lines.
In general, if any part of the word following the `<<' which the
shell uses to terminate the text to be given to the command is quoted
then these substitutions will not be performed.
In this case since we used the form `1,$' in our editor script
we needed to insure that this `$' was not variable substituted.
We could also have insured this by preceding the `$' here with a `\e',
i.e.:
.DS
1,\e$s/^[ ]*//
.DE
but quoting the `EOF' terminator is a more reliable way of achieving the
same thing.
.NH 2
Catching interrupts
.PP
If our shell script creates temporary files, we may wish to catch
interruptions of the shell script so that we can clean up
these files.
We can then do
.DS
onintr label
.DE
where
.I label
is a label in our program.
If an interrupt is received the shell will do a
`goto label'
and we can remove the temporary files and then do an
.I exit
command (which is built in to the shell)
to exit from the shell script.
If we wish to exit with a non-zero status we can do
.DS
exit(1)
.DE
e.g. to exit with status `1'.
.NH 2
What else?
.PP
There are other features of the shell useful to writers of shell
procedures.
The
.I verbose
and
.I echo
options and the related
.I \-v
and
.I \-x
command line options can be used to help trace the actions of the shell.
The
.I \-n
option causes the shell only to read commands and not to execute
them and may sometimes be of use.
.PP
One other thing to note is that
.I csh
will not execute shell scripts which do not begin with the
character `#', that is shell scripts that do not begin with a comment.
Similarly, the `/bin/sh' on your system may well defer to `csh'
to interpret shell scripts which begin with `#'.
This allows shell scripts for both shells to live in harmony.
.PP
There is also another quotation mechanism using `"' which allows
only some of the expansion mechanisms we have so far discussed to occur
on the quoted string and serves to make this string into a single word
as `\'' does.
.bp