Redirecting shell output via grep

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I have a node.js script and I want both stdout and stderr to go to a log file, but I want to pass the stdout and stderr through grep first.

This works:

node a.js > a.log 2>&1

But this doesn't:

node a.js | grep "error" > a.log 2>&1

It doesn't output anything to the file, but worse it starts outputting stdout/stderr to the terminal again, which is just wrong.

What I have tried:

(See original question description.)

Posted 23-Nov-16 18:12pm

Alexander Mills

Updated 23-Nov-16 21:39pm

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Jochen Arndt · Answer 1 · 2016-11-23T21:39:00

Just redirect stderr to stdout before piping to grep:

node a.js 2>&1 | grep "error" > a.log

When using bash, there is a shorthand for the above:

node a.js |& grep "error" > a.log

With |&, stdout and stderr of the first command are piped to the second. See the Bash Reference Manual[^].