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

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’s been stumping me. …

Answer:

Example use:

if ! command -v <the_command> &> /dev/null
then
    echo "<the_command> could not be found"
    exit
fi

Explanation

Avoid which. Not only is it an external process you’re launching for doing very little (meaning builtins like hash, type or command are way cheaper), you can also rely on the builtins to actually do what you want, while the effects of external commands can easily vary from system to system.

Why care?

Many operating systems have a which that doesn’t even set an exit status, meaning the if which foo won’t even work there and will always report that foo exists, even if it doesn’t (note that some POSIX shells appear to do this for hash too).
Many operating systems make which do custom and evil stuff like change the output or even hook into the package manager.