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

Try using:

test -x filename

or

[ -x filename ]

From the Bash manpage under Conditional Expressions:

 -x file
          True if file exists and is executable.