So sometimes you have a command where you want to only grep stderr. For example I use Cronic to manage all my cronjobs. It’s really nice since it sends a nicely formatted email back to you if a command returns anything in stderr. Crontab alone will email me if anything in stdout/stderr is printed out from a command.

There is a problem with cronic though. There’s an app called s3cmd which uploads files to s3 and on large files this output can happen

So sometimes you have a command where you want to only grep stderr. For example I use Cronic to manage all my cronjobs. It’s really nice since it sends a nicely formatted email back to you if a command returns anything in stderr. Crontab alone will email me if anything in stdout/stderr is printed out from a command.

There is a problem with cronic though. There’s an app called s3cmd which uploads files to s3 and on large files this output can happen

This generally happens if the fule is very large. s3cmd will restart the transfer of the part and it will be uploaded just fine. So the main issue is s3cmd has no way to ignore warnings, at least as of 1.5.0-alpha2. If it does, I am overlooking it.

So in order to solve this I have decided I want to grep out the WARNING lines from stderr. Now the easy way to do this is re-direct stderr to stdout and pipe it to grep. Well this sucks cause you will lose valid error output. So the answer is something like this

cmd 2> >(grep -v "WARNING" >&2)

That will then allow you to grep just from stderr and the -v flag will set grep to ignore WARNING lines.

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