What does this shell script fragment do?

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?

http://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion — Lukáš Lalinský, Feb 25 '12 at 14:59

score 2 · Accepted Answer · edited May 23 '17 at 09:59

2

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

edited May 23 '17 at 09:59

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answered Feb 25 '12 at 14:58

Gregory Pakosz

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The first sentence should begin "If the variable is undefined or is the empty string" since the operator is ':-' rather than '-'. – William Pursell Feb 25 '12 at 17:34

What does this shell script fragment do?

1 Answers1