How do I detect whether stderr is connected to a terminal from within a script?

Question

I want to experiment with terminals and colored output, but at the same time I want to make stdout/stderr redirection to file possible without having to deal with these escape sequences.

Is there any way to check whether stderr is connected to a terminal from within a shell script?

Answer 1

The file descriptor for standard error, stderr, is 2. The test or [ command (which is often a shell built-in) has an option, -t , to test whether a file descriptor is connected to a terminal:

if [ -t 2 ]
then : stderr is a terminal
else : stderr is not a terminal
fi