Template Variable Substitution¶

When doing a stacker build, the behavior of stacker is specified by the yaml directives below. In addition to these, stacker allows variable substitions of several forms. For example, a line like:

$ONE ${{TWO}} ${{THREE:3}}

When run with stacker build --substitute ONE=1 --substitute TWO=2 is processed in stacker as:

1 2 3

That is, variables of the form $FOO or ${FOO} are supported, and variables with ${FOO:default} a default value will evaluate to their default if not specified on the command line. It is an error to specify a ${FOO} style without a default; to make the default an empty string, use ${FOO:}.