Capture log4J output with grep

Question

Hi,

I know that log4j by default outputs to stderror.

I have been capturing the out put of my application with the following command:

application_to_run 2> log ; cat log | grep FATAL

Is there a way to capture the output without the auxiliary file?

Answer 1

If you want both stdout and stderr, use:

( application_to_run 2>&1 ) | grep FATAL

If you want both stderr alone, you can use:

( application_to_run 2>&1 >/dev/null ) | grep FATAL

The first sends all output destined for file handle 2 (stderr) to file handle 1 (stdout), then pipes that through grep. The second does the same but also sends stdout to the bit bucket. This will work since redirection is a positional thing. First, stderr is redirected to the current stdout, then stdout is redirected to /dev/null.

Answer 2

If you are asking how to redirect stderr to stdout so you can use it in a pipe, there are two ways I know of:

$ command 2>&1 | ...

$ command |& ..