system: Add -I, --list-installed option.

* guix/scripts/system.scm (display-system-generation): Add
 #:list-installed-regex and honor it.
(list-generations): Likewise.
(show-help, %options): Add "--list-installed".
(process-command): For 'describe' and 'list-generation', honor the
'list-installed option.
* doc/guix.texi (Invoking Guix System): Add information for
--list-installed flag.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index c348760dae..d8a3d2e90c 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -37781,6 +37781,13 @@ bootloader boot menu:
 Describe the running system generation: its file name, the kernel and
 bootloader used, etc., as well as provenance information when available.
 
+The @code{--list-installed} flag is available, with the same
+syntax that is used in @command{guix package --list-installed}
+(@pxref{Invoking guix package}).  When the flag is used,
+the description will include a list of packages that are currently
+installed in the system profile, with optional filtering based on a
+regular expression.
+
 @quotation Note
 The @emph{running} system generation---referred to by
 @file{/run/current-system}---is not necessarily the @emph{current}
@@ -37808,6 +37815,11 @@ generations that are up to 10 days old:
 $ guix system list-generations 10d
 @end example
 
+The @code{--list-installed} flag may also be specified, with the same
+syntax that is used in @command{guix package --list-installed}.  This
+may be helpful if trying to determine when a package was added to the
+system.
+
 @end table
 
 The @command{guix system} command has even more to offer!  The following