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

It depends on whether you want to know whether it exists in one of the directories in the $PATH variable or whether you know the absolute location of it. If you want to know if it is in the $PATH variable, use

if which programname >/dev/null; then
    echo exists
else
    echo does not exist
fi

otherwise use

if [ -x /path/to/programname ]; then
    echo exists
else
    echo does not exist
fi

The redirection to /dev/null/ in the first example suppresses the output of the which program.