How does this bash statemnet test ".$TOMCAT_USER" = . && TOMCAT_USER=tomcat work?

Question

I am trying to understand the tomcat daemon.sh script which contains a bunch of lines along the lines test ".$TOMCAT_USER" = . && TOMCAT_USER=tomcat

I figured out that test ".$TOMCAT_USER" = . && TOMCAT_USER=tomcat checks if an environment variable TOMCAT_USER is already defined if so it uses it and if not it uses the default value of tomcat

What I understand is the how the statement is evaluated by bash.

Answer 1

&& is a logical "and"; essentially, it lets you do something based on whether the previous command completed successfully (although this might not be true in more complex cases, for example, consider true || false && echo true, here's an article on on boolean logic rules). Another way of writing the statement in your question would be:

if test ".$TOMCAT_USER" = .; then
    TOMCAT_USER=tomcat
fi

This is a strange way of testing for an unset/null variable, though. POSIX already has multiple ways to do that:

: "${TOMCAT_USER:=tomcat}"  # Assign as part of the expansion,
                            # `:' is a noop to avoid running the expansion.
TOMCAT_USER="${TOMCAT_USER:-tomcat}"  # Expand, and then assign

The difference between the :=/:- and :-less versions are that the : versions also test if the variable is null in addition to testing if it is unset (which is closer to your original test).