Please explain these symbols I found in a shell script

Question

I see the following line in a shell script.

CATALINA_BASE=${CATALINA_BASE:-${APP_HOME}/tomcat}

Is the :- like an if statement? That is, if the environment variable $CATALINA_BASE exists, use its value for the variable CATALINA_BASE?

I also see this line:

APP_USER=${APP_USER:?}

What does the ? mean? In this case, there is no -.

...also, see http://mywiki.wooledge.org/BashSheet#Parameter_Operations — Charles Duffy, Apr 29 '14 at 18:41
...also, see http://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html — Charles Duffy, Apr 29 '14 at 18:41
possible duplicate of [Usage of :- in bash](http://stackoverflow.com/questions/10390406/usage-of-in-bash) -- note that accepted answer also covers the `:` case. — Charles Duffy, Apr 30 '14 at 02:21

score 3 · Accepted Answer · answered Apr 29 '14 at 18:43

${foo:-bar} and ${foo:bar} are both parameter expansions with defaults. They vary in terms of how they expand an explicitly-set empty string (as opposed to a null, unset string).

See http://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html, http://mywiki.wooledge.org/BashFAQ/073, or http://mywiki.wooledge.org/BashSheet#Parameter_Operations for explanations of these and more.

Please explain these symbols I found in a shell script

1 Answers1