BSD licenced version of envsubst. Same tool, different license.

Usage: envsubstbsd [OPTION] [SHELL-FORMAT]

Substitutes the values of environment variables.

Operation mode: -v, --variables output the variables occurring in SHELL-FORMAT

Informative output: -h, --help display this help and exit -V, --version output version information and exit

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.