How can I check if a program exists from a Bash script?

Question

How would I validate that a program exists, in a way that will either return an error and exit, or continue with the script?

It seems like it should be easy, but it\

Answer 1

Command -v works fine if the POSIX_BUILTINS option is set for the to test for, but it can fail if not. (It has worked for me for years, but I recently ran into one where it didn't work.)

I find the following to be more failproof:

test -x $(which )

Since it tests for three things: path, existence and execution permission.