Fill placeholders in file in single pass

Question

I have a skeleton text file with placeholder strings:

blah blah blah
blah $PLACEHOLDER_1$
blah
$PLACEHOLDER_2$

and so on. Specific \"form\" of

Answer 1

I just stumbled upon this question because I was just looking for the exact same, and I found envsubst(1).

You can use envsubst if you don't mind using environment variables:

PLACEHOLDER_1='string 1' PLACEHOLDER_2='multiline 
string 
2' envsubst < output.template 

If you have a lot of variables you can store them in a file and just source it (remember to use export at the end of the sourced file!)