guix system: Add '--graph-backend'.

* guix/scripts/system.scm (lookup-backend): New procedure.
(export-extension-graph, export-shepherd-graph): Add #:backend parameter
and honor it.
(show-help, %options): Add "--graph-backend".
(%default-options): Add 'graph-backend'.
(process-action): Pass #:backend to 'export-extension-graph' and
'export-shepherd-graph'.
* doc/guix.texi (Invoking guix system): Document '--graph-backend'.

1 files changed, 9 insertions, 4 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 1081ed26a3..0f6e95a65a 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -21,7 +21,7 @@
 @set SUBSTITUTE-URL https://@value{SUBSTITUTE-SERVER}
 
 @copying
-Copyright @copyright{} 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès@*
+Copyright @copyright{} 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès@*
 Copyright @copyright{} 2013, 2014, 2016 Andreas Enge@*
 Copyright @copyright{} 2013 Nikita Karetnikov@*
 Copyright @copyright{} 2014, 2015, 2016 Alex Kost@*
@@ -31548,10 +31548,12 @@ each other:
 @table @code
 
 @item extension-graph
-Emit in Dot/Graphviz format to standard output the @dfn{service
+Emit to standard output the @dfn{service
 extension graph} of the operating system defined in @var{file}
 (@pxref{Service Composition}, for more information on service
-extensions).
+extensions).  By default the output is in Dot/Graphviz format, but you
+can choose a different format with @option{--graph-backend}, as with
+@command{guix graph} (@pxref{Invoking guix graph, @option{--backend}}):
 
 The command:
 
@@ -31563,11 +31565,14 @@ shows the extension relations among services.
 
 @anchor{system-shepherd-graph}
 @item shepherd-graph
-Emit in Dot/Graphviz format to standard output the @dfn{dependency
+Emit to standard output the @dfn{dependency
 graph} of shepherd services of the operating system defined in
 @var{file}.  @xref{Shepherd Services}, for more information and for an
 example graph.
 
+Again, the default output format is Dot/Graphviz, but you can pass
+@option{--graph-backend} to select a different one.
+
 @end table
 
 @node Invoking guix deploy