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 wanted the same question answered but to run within a Makefile.

install:
    @if [[ ! -x "$(shell command -v ghead)" ]]; then \
        echo 'ghead does not exist. Please install it.'; \
        exit -1; \
    fi