Question

I’m trying to read shell script which I haven’t worked with before… What does this code do?

# Setup some command defaults (can be overriden by the config)
MYSQL=${MYSQL:-`which mysql`}
MYSQLDUMP=${MYSQLDUMP:-`which mysqldump`}
PHP=${PHP:-`which php`}

I have a feeling it determines the location of php, mysql and mysqldump if the variable is not already defined. Is that correct?

Answer 1

If variable is undefined or is the empty string, it replaces it with the result of the which command so that it serves as a default value.

Side note, you could make it more robust by using type -P mysql or hash mysql instead of which mysql since implementations of which may differ.

see http://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html and Check if a program exists from a Bash script