tldr/pages/common/envsubst.md

envsubst

Substitutes environment variables with their value in shell format strings. Variables 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_file}}

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

envsubst < {{path/to/input_file}} > {{path/to/output_file}}

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

envsubst '{{$USER $SHELL $HOME}}' < {{path/to/input_file}}