compgen warning: -C option not working as I expected

What is the correct way to use the compgen -C option? I'm trying to learn about Bash programmable completion, and in particular the compgen builtin function. I'm experimenting with the different compgen command-line options, and I don't understand how the -C flag is supposed to work. From the [GNU Bash Reference Manual](http://www.gnu.org/software/bash/manual/html_node/Programmable-Completion-Builtins.html) : > -C command > >> command is executed in a subshell environment, and its output is used as the possible completions. Based on this, I expect something like the following to work: $ compgen -C 'echo "first_option second_option"' f first_option But instead, I get this: $ compgen -C 'echo "first_option second_option"' f -bash: compgen: warning: -C option may not work as you expect first_option second_option f I've tried this with Bash version 4.2.45 on OS X 10.7 and with Bash version 4.2.25 on Ubuntu 12.04, and in both cases I get the same error: -bash: compgen: warning: -C option may not work as you expect