Why doesn't “cvs --help | less” work?

Question

I don't know why, but piping cvs --help to less doesn't work. Probably it's because cvs --help outputs not to standard output(1) or error output(2), but somewhere else. Could you tell me where does it output?

Answer 1

The reason is because there's two types of output.

• Normal output (STDOUT)
• Error output (STDERR)

cvs --help prints the help info to STDERR. The pipe (|) is expecting data that was written to STDOUT so that it can be read STDIN.

In order to get the help text from STDERR to STDOUT (so that it's readable by the pipe) you need to redirect it.

cvs --help 2>&1 | less

Further explanation here.