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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.
 

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