@pindex envsubst

@cindex @code{envsubst} program, usage

@example

envsubst [@var{option}] [@var{shell-format}]

@end example

@cindex shell format string

@cindex dollar substitution

@cindex environment variables

The @code{envsubst} program substitutes the values of environment variables.

@noindent @strong{Operation mode}

@table @samp

@item -v

@itemx --variables

@opindex -v@r{, @code{envsubst} option}

@opindex --variables@r{, @code{envsubst} option}

Output the variables occurring in @var{shell-format}.

@end table

@noindent @strong{Informative output}

@table @samp

@item -h

@itemx --help

@opindex -h@r{, @code{envsubst} option}

@opindex --help@r{, @code{envsubst} option}

Display this help and exit.

@item -V

@itemx --version

@opindex -V@r{, @code{envsubst} option}

@opindex --version@r{, @code{envsubst} option}

Output version information and exit.

@end table

In normal operation mode, standard input is copied to standard output,

with references to environment variables of the form @code{$VARIABLE} or

@code{$@{VARIABLE@}} being replaced with the corresponding values.  If a

@var{shell-format} is given, only those environment variables that are

referenced in @var{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 @code{$@{@var{variable}-@var{default}@}} or

@code{$(@var{command-list})} or @code{`@var{command-list}`}, are not performed

by the @code{envsubst} program, due to security reasons.

When @code{--variables} is used, standard input is ignored, and the output

consists of the environment variables that are referenced in

@var{shell-format}, one per line.