Simplify the meaning of %s

The rules governing %s where just too complicated. First of
all, looking at $SHELL is dangerous. For systemd --system,
it usually wouldn't be set. But it could be set if the admin
first started a debug shell, let's say /sbin/sash, and then
launched systemd from it. This shouldn't influence how daemons
are started later on, so is better ignored. Similar reasoning
holds for session mode. Some shells set $SHELL, while other
set it only when it wasn't set previously (e.g. zsh). This
results in fragility that is better avoided by ignoring $SHELL
totally.

With $SHELL out of the way, simplify things by saying that
%s==/bin/sh for root, and the configured shell otherwise.
get_shell() is the only caller, so it can be inlined.

Fixes one issue seen with 'make check'.

1 files changed, 8 insertions, 1 deletions

diff --git a/man/systemd.unit.xml.in b/man/systemd.unit.xml.in
index e99703f7b3..2196e73bb3 100644
--- a/man/systemd.unit.xml.in
+++ b/man/systemd.unit.xml.in
@@ -1270,7 +1270,14 @@
                       <row>
                         <entry><literal>%s</literal></entry>
                         <entry>User shell</entry>
-                        <entry>This is the shell of the configured user of the unit, or (if none is set) the user running the systemd instance.</entry>
+                        <entry>This is the shell of the configured
+                        user of the unit, or (if none is set) the user
+                        running the systemd instance.  If the user is
+                        <literal>root</literal> (UID equal to 0), the
+                        shell configured in account database is
+                        ignored and <filename>/bin/sh</filename> is
+                        always used.
+                        </entry>
                       </row>
                       <row>
                         <entry><literal>%m</literal></entry>