In a bash script, how can I tell if the script output is redirected to a file?

Question

I want to write a shell script that will use colored output when output is terminal, and normal output when redirected to a file. How can I do that?

Answer 1

Very simple:

if [ -t 1 ]; then
    echo "Hello, terminal."
else
    echo "Not a terminal."
fi

-t tests if the given file descriptor (here, 1 = stdout) is attached to a terminal.