What does colon (':') in bash variable resolution syntax mean? [duplicate]

Question

I need to decode this assignment:

jvm_xmx=${jvm_xmx:-1024}

Answer 1

man page for bash:

${parameter:-word}
          Use Default Values.  If parameter is unset or null, the expansion of
          word is substituted.  Otherwise, the value of parameter is substituted.

So if jvm_xmx is already set to something, it is left unchanged.
If it is not already set to something, it is set to 1024.

Example:

$ echo $jvm_xmx

$ jvm_xmx=${jvm_xmx:-1024}
$ echo $jvm_xmx
1024
$ jvm_xmx=2048
$ jvm_xmx=${jvm_xmx:-1024}
$ echo $jvm_xmx
2048
$