bash syntax question

pan64 · 05-29-2015, 04:41 AM

I found the following syntax and do not really understand how does it work.

So I have a command, which can be more or less anything and a counter, which is usually a grep -c pattern or wc -l or similar.
Here is a construct:

Code:

COUNT=$(<command> | <counter> || :)

The strange thing: COUNT is always set even if the <command> itself is incorrect and cannot be executed. For example:

Code:

COUNT=$(lasd-l/tmp | wc -l || :)
# COUNT is now 0

So in such cases what's happened? Is this syntax ok? Does it work using any bash versions?

kmhuntly · 05-29-2015, 06:53 AM

I'm not sure what's after your || , LQ made it a smiley face. however, generally || means "do this of the command fails", so that's probably what's making it zero

Also if you're typesetting as an integer it'll always be zero unless your command makes it different

sgrlscz · 05-29-2015, 06:56 AM

With

Code:

COUNT=$(<command> | <counter> || :)

you get a 0 in because wc counts the lines of output from lasd-l/tmp, which results in output to stderr, but nothing to stdout, so no lines written to the pipe, so 'wc -l' is 0, and the || never gets used.

In

Code:

$(<command> | <counter> || :)

if <command> | <counter> fails, then the || part of the expression is executed, and that is just a :, which is a special builtin meaning true.

pan64 · 06-02-2015, 03:11 AM

so the correct answer looks like: regardless of <command> (even if it was not executable, broken pipe, set -e) <counter> will be run and return something.
|| : has no any effect on it. Looks like the return code is 0 anyway. set -o pipefail will force to set exitcode, that can be escaped by || :

But if you have any comments on it....

sgrlscz · 06-02-2015, 10:05 AM

One purpose of ||: is to ensure that you always get a successful return status.

The construct is useful when you're using 'set -o errexit'. It ensures that specific pieces of code won't generate an error.

For example, if your script uses 'set -o errexit', then without the ||:, your script would exit if either <counter> failed for some reason or if <command> failed and you have pipefail set. With the ||:, setting COUNT will always have a successful return code, so it won't stop your script.