Substitute environment variables in files

envsubst [OPTION] [SHELL-FORMAT]

In normal operation mode, standard input is copied to standard output, with references to environment variables of the form $VARIABLE or ${VARIABLE} being replaced with the corresponding values.

If a SHELL-FORMAT is given, only those environment variables that are referenced in SHELL-FORMAT are substituted; otherwise all environment variables references occurring in standard input are substituted.

When --variables is used, standard input is ignored, and the output consists of the environment variables that are referenced in SHELL-FORMAT, one per line.