guix system: 'describe' shows the running system, not the current one.

* guix/profiles.scm (generation-number): Add optional 'base-profile'
parameter and use it.
* guix/scripts/system.scm (process-command): Add "/run/current-system"
as first argument to 'generation-number'.
* doc/guix.texi (Invoking guix system): Clarify that 'guix system
describe' shows the running system.

1 files changed, 13 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index b10f0d085b..ceec0d0cf5 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -35811,9 +35811,21 @@ bootloader boot menu:
 @table @code
 
 @item describe
-Describe the current system generation: its file name, the kernel and
+Describe the running system generation: its file name, the kernel and
 bootloader used, etc., as well as provenance information when available.
 
+@quotation Note
+The @emph{running} system generation---referred to by
+@file{/run/current-system}---is not necessarily the @emph{current}
+system generation---referred to by @file{/var/guix/profiles/system}: it
+differs when, for instance, you chose from the bootloader menu to boot
+an older generation.
+
+It can also differ from the @emph{booted} system generation---referred
+to by @file{/run/booted-system}---for instance because you reconfigured
+the system in the meantime.
+@end quotation
+
 @item list-generations
 List a summary of each generation of the operating system available on
 disk, in a human-readable way.  This is similar to the