gettext: envsubst Invocation

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1 15.5.2.5 Invoking the ‘envsubst’ program
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1      envsubst [OPTION] [SHELL-FORMAT]
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1    The ‘envsubst’ program substitutes the values of environment
1 variables.
1 
1 *Operation mode*
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1 ‘-v’
1 ‘--variables’
1      Output the variables occurring in SHELL-FORMAT.
1 
1 *Informative output*
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1 ‘-h’
1 ‘--help’
1      Display this help and exit.
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1 ‘-V’
1 ‘--version’
1      Output version information and exit.
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1    In normal operation mode, standard input is copied to standard
1 output, with references to environment variables of the form ‘$VARIABLE’
1 or ‘${VARIABLE}’ being replaced with the corresponding values.  If a
1 SHELL-FORMAT is given, only those environment variables that are
1 referenced in SHELL-FORMAT are substituted; otherwise all environment
1 variables references occurring in standard input are substituted.
1 
1    These substitutions are a subset of the substitutions that a shell
1 performs on unquoted and double-quoted strings.  Other kinds of
1 substitutions done by a shell, such as ‘${VARIABLE-DEFAULT}’ or
1 ‘$(COMMAND-LIST)’ or ‘`COMMAND-LIST`’, are not performed by the
1 ‘envsubst’ program, due to security reasons.
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1    When ‘--variables’ is used, standard input is ignored, and the output
1 consists of the environment variables that are referenced in
1 SHELL-FORMAT, one per line.
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