scripts: system: Add --expression option.

* guix/scripts/system.scm (show-help): Add expression option.
(%options): Ditto.
(guix-system): Allow commands taking a file as an argument to use an
expression instead.
(process-action): Read operating-system from expression or file.
* doc/guix.texi (Invoking guix system): Introduce the expression option.

1 files changed, 8 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 7625f30fbb..64f73b38a4 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -18744,6 +18744,14 @@ Build Options}).  In addition, @var{options} can contain one of the
 following:
 
 @table @option
+@item --expression=@var{expr}
+@itemx -e @var{expr}
+Consider the operating-system @var{expr} evaluates to.
+This is an alternative to specifying a file which evaluates to an
+operating system.
+This is used to generate the GuixSD installer @pxref{Building the
+Installation Image}).
+
 @item --system=@var{system}
 @itemx -s @var{system}
 Attempt to build for @var{system} instead of the host system type.