journalctl: allow --root argument for journal watching

It is useful to look at a (possibly inactive) container or other os tree
with --root=/path/to/container. This is similar to specifying
--directory=/path/to/container/var/log/journal --directory=/path/to/container/run/systemd/journal
(if using --directory multiple times was allowed), but doesn't require
as much typing.

2 files changed, 10 insertions, 6 deletions

diff --git a/man/journalctl.xml b/man/journalctl.xml
index c448a29a51..63b4a267b8 100644
--- a/man/journalctl.xml
+++ b/man/journalctl.xml
@@ -659,10 +659,12 @@
         <term><option>--root=<replaceable>ROOT</replaceable></option></term>
 
         <listitem><para>Takes a directory path as an argument. If
-        specified, journalctl will operate on catalog file hierarchy
+        specified, journalctl will operate on journal directories and catalog file hierarchy
         underneath the specified directory instead of the root
         directory (e.g. <option>--update-catalog</option> will create
-        <filename><replaceable>ROOT</replaceable>/var/lib/systemd/catalog/database</filename>).
+        <filename><replaceable>ROOT</replaceable>/var/lib/systemd/catalog/database</filename>,
+        and journal files under <filename><replaceable>ROOT</replaceable>/run/journal</filename>
+        or <filename><replaceable>ROOT</replaceable>/var/log/journal</filename> will be displayed).
         </para></listitem>
       </varlistentry>
 
diff --git a/src/journal/journalctl.c b/src/journal/journalctl.c
index 6f841efb69..381e219390 100644
--- a/src/journal/journalctl.c
+++ b/src/journal/journalctl.c
@@ -310,7 +310,7 @@ static void help(void) {
                "  -m --merge               Show entries from all available journals\n"
                "  -D --directory=PATH      Show journal files from directory\n"
                "     --file=PATH           Show journal file\n"
-               "     --root=ROOT           Operate on catalog files below a root directory\n"
+               "     --root=ROOT           Operate on files below a root directory\n"
 #ifdef HAVE_GCRYPT
                "     --interval=TIME       Time interval for changing the FSS sealing key\n"
                "     --verify-key=KEY      Specify FSS verification key\n"
@@ -848,8 +848,8 @@ static int parse_argv(int argc, char *argv[]) {
         if (arg_follow && !arg_no_tail && !arg_since && arg_lines == ARG_LINES_DEFAULT)
                 arg_lines = 10;
 
-        if (!!arg_directory + !!arg_file + !!arg_machine > 1) {
-                log_error("Please specify either -D/--directory= or --file= or -M/--machine=, not more than one.");
+        if (!!arg_directory + !!arg_file + !!arg_machine + !!arg_root > 1) {
+                log_error("Please specify at most one of -D/--directory=, --file=, -M/--machine=, --root.");
                 return -EINVAL;
         }
 
@@ -1267,7 +1267,7 @@ static int add_boot(sd_journal *j) {
          * We can do this only when we logs are coming from the current machine,
          * so take the slow path if log location is specified. */
         if (arg_boot_offset == 0 && sd_id128_is_null(arg_boot_id) &&
-            !arg_directory && !arg_file)
+            !arg_directory && !arg_file && !arg_root)
 
                 return add_match_this_boot(j, arg_machine);
 
@@ -2161,6 +2161,8 @@ int main(int argc, char *argv[]) {
 
         if (arg_directory)
                 r = sd_journal_open_directory(&j, arg_directory, arg_journal_type);
+        else if (arg_root)
+                r = sd_journal_open_directory(&j, arg_root, arg_journal_type | SD_JOURNAL_OS_ROOT);
         else if (arg_file_stdin) {
                 int ifd = STDIN_FILENO;
                 r = sd_journal_open_files_fd(&j, &ifd, 1, 0);