Bash Templating: How to build configuration files from templates with Bash?

Try envsubst

$ cat envsubst-template.txt
Variable FOO is (${FOO}).
Variable BAR is (${BAR}).

$ FOO=myfoo

$ BAR=mybar

$ export FOO BAR

$ cat envsubst-template.txt | envsubst
Variable FOO is (myfoo).
Variable BAR is (mybar).