journal: beef up journal matches considerably

we now can take multiple matches, and they will apply as AND if they
apply to different fields and OR if they apply to the same fields. Also,
terms of this kind can be combined with an overreaching OR.

1 files changed, 50 insertions, 6 deletions

diff --git a/man/journalctl.xml b/man/journalctl.xml
index bb964b01e..f314fb6d2 100644
--- a/man/journalctl.xml
+++ b/man/journalctl.xml
@@ -49,7 +49,7 @@
 
         <refsynopsisdiv>
                 <cmdsynopsis>
-                        <command>journalctl <arg choice="opt" rep="repeat">OPTIONS</arg> <arg choice="opt">MATCH</arg></command>
+                        <command>journalctl <arg choice="opt" rep="repeat">OPTIONS</arg> <arg choice="opt" rep="repeat">MATCHES</arg></command>
                 </cmdsynopsis>
         </refsynopsisdiv>
 
@@ -66,12 +66,25 @@
                 contents of the journal, starting with the oldest
                 entry collected.</para>
 
-                <para>If a match argument is passed the output is
-                filtered accordingly. A match is in the format
-                <literal>FIELD=VALUE</literal>,
-                e.g. <literal>_SYSTEMD_UNIT=httpd.service</literal>. See
+                <para>If one or more match arguments are passed the
+                output is filtered accordingly. A match is in the
+                format <literal>FIELD=VALUE</literal>,
+                e.g. <literal>_SYSTEMD_UNIT=httpd.service</literal>,
+                referring to the components of a structured journal
+                entry. See
                 <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
-                for a list of well-known fields.</para>
+                for a list of well-known fields. If multiple matches
+                are specified matching different fields the log
+                entries are filtered by both, i.e. the resulting output
+                will show only entries matching all the specified
+                matches of this kind. If two matches apply to the same
+                field, then they are automatically matched as
+                alternatives, i.e. the resulting output will show
+                entries matching any of the specified matches for the
+                same field. Finally, if the character
+                "<literal>+</literal>" appears as separate word on the
+                command line all matches before and after are combined
+                in a disjunction (i.e. logical OR).</para>
 
                 <para>Output is interleaved from all accessible
                 journal files, whether they are rotated or currently
@@ -272,6 +285,37 @@
         </refsect1>
 
         <refsect1>
+                <title>Examples</title>
+
+                <para>Without arguments all collected logs are shown
+                unfiltered:</para>
+
+                <programlisting>journalctl</programlisting>
+
+                <para>With one match specified all entries with a field matching the expression are shown:</para>
+
+                <programlisting>journalctl _SYSTEMD_UNIT=avahi-daemon.service</programlisting>
+
+                <para>If two different fields are matched only entries matching both expressions at the same time are shown:</para>
+
+                <programlisting>journalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=28097</programlisting>
+
+                <para>If two matches refer to the same field all entries matching either expression are shown:</para>
+
+                <programlisting>journalctl _SYSTEMD_UNIT=avahi-daemon.service _SYSTEMD_UNIT=dbus.service</programlisting>
+
+                <para>If the separator "<literal>+</literal>" is used
+                two expression may be combined in a logical OR. The
+                following will show all messages from the Avahi
+                service process with the PID 28097 plus all messages
+                from the D-Bus service (from any of its
+                processes):</para>
+
+                <programlisting>journalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=28097 + _SYSTEMD_UNIT=dbus.service</programlisting>
+
+        </refsect1>
+
+        <refsect1>
                 <title>See Also</title>
                 <para>
                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,