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@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.
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