(gettext.info)envsubst Invocation

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15.5.2.5 Invoking the `envsubst' program
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     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.