The shells let you test for success right on the command line. This gives you a very efficient way to write quick and comprehensible shell scripts.
I'm referring to the || and && operators and in particular, the || operator. comm1
||
comm2 is typically explained as "execute the command
on the right if the command on the left failed." I prefer to explain it as an
"either-or" construct: "execute either comm1 or
comm2." While this isn't really precise, let's see what
it means in context:[3]
cat filea fileb > filec || exit
This means "either cat the files or exit." If you can't cat the files (if cat returns an exit status of 1), you exit (Section 24.4). If you can cat the files, you don't exit. You execute the left side or the right side.
I'm stretching
normal terminology a bit here, but I think it's necessary to clarify the purpose
of ||. By the way, we could give the poor
user an error message before flaming out (which, by the way, is a way to write
an "inverse if (Section
35.13)):
cat filea fileb > filec || {
echo sorry, no dice 1>&2
exit 1
}Similarly,
comm1
&&
comm2 means "execute comm1 AND
comm2," or execute comm2 if
comm1 succeeds. (But if you can't execute the first,
don't do any.) This might be helpful if you want to print a temporary file and
delete it
immediately.
lpr file && rm file
If lpr fails for some reason, you want to leave the file around. Again, I want to stress how to read this: print the file and delete it. (Implicitly: if you don't print it, don't delete it.)
— ML