Substitutes the values of environment variables.

-v, --variables

output the variables occurring in SHELL-FORMAT

-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.

info envsubst

should give you access to the complete manual.