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

I'd say there isn't any portable and 100% reliable way due to dangling aliases. For example:

alias john='ls --color'
alias paul='george -F'
alias george='ls -h'
alias ringo=/

Of course, only the last one is problematic (no offence to Ringo!). But all of them are valid aliases from the point of view of command -v.

In order to reject dangling ones like ringo, we have to parse the output of the shell built-in alias command and recurse into them (command -v isn't a superior to alias here.) There isn't any portable solution for it, and even a Bash-specific solution is rather tedious.

Note that a solution like this will unconditionally reject alias ls='ls -F':

test() { command -v $1 | grep -qv alias }