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

If you check for program existence, you are probably going to run it later anyway. Why not try to run it in the first place?

if foo --version >/dev/null 2>&1; then
    echo Found
else
    echo Not found
fi

It's a more trustworthy check that the program runs than merely looking at PATH directories and file permissions.

Plus you can get some useful result from your program, such as its version.

Of course the drawbacks are that some programs can be heavy to start and some don't have a --version option to immediately (and successfully) exit.