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15.5.2.5 Invoking the envsubst program

     envsubst [option] [shell-format]

The envsubst program 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.

These substitutions are a subset of the substitutions that a shell performs on unquoted and double-quoted strings. Other kinds of substitutions done by a shell, such as ${variable-default} or $(command-list) or `command-list`, are not performed by the envsubst program, due to security reasons.

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.