journalctl: add output mode where time is shown in seconds since 1st Jan 1970 UTC

aka "UNIX time".

Fixes: #2120

man/journalctl.xml

@@ -270,6 +270,16 @@
               </listitem>
             </varlistentry>
 
+            <varlistentry>
+              <term>
+                <option>short-unix</option>
+              </term>
+              <listitem>
+                <para>is very similar, but shows seconds passed since January 1st 1970 UTC instead of wallclock
+                timestamps ("UNIX time"). The time is shown with microsecond accuracy.</para>
+              </listitem>
+            </varlistentry>
+
             <varlistentry>
               <term>
                 <option>verbose</option>

src/shared/logs-show.c

@@ -344,16 +344,22 @@ static int output_short(
 
                 t = (time_t) (x / USEC_PER_SEC);
 
-                switch(mode) {
+                switch (mode) {
+
+                case OUTPUT_SHORT_UNIX:
+                        r = snprintf(buf, sizeof(buf), "%10llu.%06llu", (unsigned long long) t, (unsigned long long) (x % USEC_PER_SEC));
+                        break;
+
                 case OUTPUT_SHORT_ISO:
                         r = strftime(buf, sizeof(buf), "%Y-%m-%dT%H:%M:%S%z", gettime_r(&t, &tm));
                         break;
+
                 case OUTPUT_SHORT_PRECISE:
                         r = strftime(buf, sizeof(buf), "%b %d %H:%M:%S", gettime_r(&t, &tm));
                         if (r > 0)
-                                snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf),
-                                         ".%06llu", (unsigned long long) (x % USEC_PER_SEC));
+                                snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), ".%06llu", (unsigned long long) (x % USEC_PER_SEC));
                         break;
+
                 default:
                         r = strftime(buf, sizeof(buf), "%b %d %H:%M:%S", gettime_r(&t, &tm));
                 }
@@ -894,6 +900,7 @@ static int (*output_funcs[_OUTPUT_MODE_MAX])(
         [OUTPUT_SHORT_ISO] = output_short,
         [OUTPUT_SHORT_PRECISE] = output_short,
         [OUTPUT_SHORT_MONOTONIC] = output_short,
+        [OUTPUT_SHORT_UNIX] = output_short,
         [OUTPUT_VERBOSE] = output_verbose,
         [OUTPUT_EXPORT] = output_export,
         [OUTPUT_JSON] = output_json,

src/shared/output-mode.c

@@ -25,6 +25,7 @@ static const char *const output_mode_table[_OUTPUT_MODE_MAX] = {
         [OUTPUT_SHORT_ISO] = "short-iso",
         [OUTPUT_SHORT_PRECISE] = "short-precise",
         [OUTPUT_SHORT_MONOTONIC] = "short-monotonic",
+        [OUTPUT_SHORT_UNIX] = "short-unix",
         [OUTPUT_VERBOSE] = "verbose",
         [OUTPUT_EXPORT] = "export",
         [OUTPUT_JSON] = "json",

src/shared/output-mode.h

@@ -26,6 +26,7 @@ typedef enum OutputMode {
         OUTPUT_SHORT_ISO,
         OUTPUT_SHORT_PRECISE,
         OUTPUT_SHORT_MONOTONIC,
+        OUTPUT_SHORT_UNIX,
         OUTPUT_VERBOSE,
         OUTPUT_EXPORT,
         OUTPUT_JSON,