Two streams: output and errors

A command prints normal output on stream 1 and errors on stream 2. > redirects only stream 1; 2> catches the errors; 2>&1 sends both to the same place.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

A script that logs its output but loses its errors is a script whose failure you find out about a week later.

How to think about it

Decide where each stream should go. Errors to a file with 2>, everything to one file with > file 2>&1, and nothing at all to /dev/null.

Worked example

ls missing.txt 2> errors.txt

The complaint goes into errors.txt, not the screen.

cat errors.txt

The error text, mentioning missing.txt.

ls missing.txt > all.txt 2>&1

Both streams into one file.

cat all.txt

The same complaint, captured through stream 1's destination.

Your turn

Keep only the errors from the build.

make  build-errors.txt

The trap

Writing 2>&1 > file. The order matters: that sends errors to the screen and output to the file, the opposite of what was meant.

Practise 2> (errors) on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.