tldr/pages/linux/envsubst.md

envsubst

Substitutes shell format strings with environment variables in text. Strings to be replaced should be in either ${var} or $var format.

• Replace environment variables in stdin and output to stdout:

echo '{{$HOME}}' | envsubst

• Replace environment variables in an input file and output to stdout:

envsubst < {{path/to/input}}

• Replace environment variables in an input file and output to a file:

envsubst < {{path/to/input}} > {{path/to/output}}

• Replace environment variables in input from a space-separated list:

envsubst {{variables}} < {{path/to/input}}