envsubst

Get's a stream and replaces the ${variables} with the ones defined in the environment.

Example:

# not an environment variable yet (no export)
$ name='meleu'

$ echo 'Hello $name'
Hello $name

# $name is not exported...
$ echo 'Hello $name' | envsubst
Hello 

$ export name

# now envsubst works as expected
$ echo 'Hello $name' | envsubst
Hello meleu

Useful links:

• https://superuser.com/questions/1148950/what-is-shell-format-in-envsubst
• https://unix.stackexchange.com/questions/294378/replacing-only-specific-variables-with-envsubst