oam/knowledge base/redirect outputs.md

Redirect outputs

1. TL;DR
2. Further readings

TL;DR

Pipes are set up before the I/O redirections are interpreted.
The sequence of I/O redirections is interpreted left-to-right.

command 2>&1 >'/dev/null' | grep 'something'
command 2>'/dev/stdout' 1>'/dev/null' | grep 'something'

# Swap the standard error and standard output over, then close the spare descriptor used for the swap.
command 3>&1 1>&2 2>&3 3>&-

Further readings

• How can I pipe stderr, and not stdout?
• Pipe only STDERR through a filter
• File Descriptors in Bourne shell