List commands
A simple use of exit status is when using a list
command. A list command is a sequence of pipelines
separated by either || (pronounced 'or') or
&& (pronounded 'and'). In the case of an
or-list
$
pipeline1 ||
pipeline2
pipeline1 is run, and if a non-zero exit status is
returned pipeline2 is run. If pipeline1 returns
0, the exit status for this list command is 0, otherwise it is the
status of pipeline2. Thus the or-list succeeds if either
the first command or the second succeeds. In the case of an
and-list
$
pipeline1 &&
pipeline2
pipeline1 is run, and if a zero exit status is returned
pipeline2 is run. The exit status for this list command is
that of pipeline1, if non-zero, otherwise that of
pipeline2. An and-list succeeds if both its first and its
second component succeed. Both or-lists and and-lists can be strung
together, and the pipelines separated by || and
&& are evaluated from left to right.
Simple examples for || and &&
would be to check whether a command has in fact completed
successfully:
$ mycommand || echo Cannot run
mycommand
$ mycommand && echo mycommand ran
OK
In the first line above, if mycommand fails - that
is, returns exit status not zero - the subsequent echo
command is run and informs you that mycommand failed.
In the second, if mycommand succeeds the subsequent
echo command is run.
Worked example
Compare files named file1 and file2,
and if they are different mail yourself a message indicating the
differences.
Solution: Use diff to compare the two
files. We see from the manual page for diff that an
exit status of 0 is returned only when the two arguments to
diff are identical. You can therefore send the output
of diff to a file and then mail yourself the contents
of that file. || can be used so that the mail will
only be performed if the diff returned non-zero exit
status.
$ diff file1 file2 >diffout
||
> mailx -s "Output of diff" chris
<diffout
Using && we can sequence commands so that
subsequent commands will only run if earlier ones have been
completed successfully.
Worked example
Compare files named file1 and file2,
and if they are identical delete file2.
Solution: Since we do not require a list
of any differences it will be quicker to use cmp
which, like diff, returns 0 exit status if its
arguments have the same contents. Use && to
perform rm upon successful completion of
cmp.
$ cmp file1 file2 && rm
file2
Parentheses can also be used to group list
commands so that, for instance,
command1 || ( command2
&& command3 )
would cause command1 to be run, and if it failed the
and-list
command2 && command3
would then be run.
Two other commands it is appropriate to introduce here are
true and false. Both these commands do
nothing at all, but return exit status 0 and 1 respectively. We
shall use them later on in this chapter. There is also a command
: (colon), which has the same effect as
true.
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