I have a systemd service unit for sshd:

$ systemctl status sshd
* ssh.service - OpenBSD Secure Shell server
   Loaded: loaded (/lib/systemd/system/ssh.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2020-09-30 18:54:10 UTC; 6s ago
  Process: 13923 ExecReload=/bin/kill -HUP $MAINPID (code=exited, status=0/SUCCESS)
  Process: 13918 ExecReload=/usr/sbin/sshd -t (code=exited, status=0/SUCCESS)
  Process: 6287 ExecStartPre=/usr/sbin/sshd -t (code=exited, status=0/SUCCESS)
 Main PID: 6296 (sshd)
    Tasks: 1 (limit: 4915)
   CGroup: /system.slice/ssh.service
           `-6296 /usr/sbin/sshd -D

Sep 30 18:54:10 machine systemd[1]: Starting OpenBSD Secure Shell server...
Sep 30 18:54:10 machine sshd[6296]: Server listening on port 22.
Sep 30 18:54:10 machine sshd[6296]: Server listening on :: port 22.
Sep 30 18:54:10 machine systemd[1]: Started OpenBSD Secure Shell server.

I have some *.journal files that it wrote:

$ find /var/log/journal -type f -name 'system@*.journal' -exec grep -l 'OpenBSD' {} +

Given the former, how do I find the latter?


Journal files are not sorted by unit. The unit issuing the journal entry is simply metadata associated with the entry. Journal files are also not always in /var. They could also be found in /run if configured as non-persistent.

The correct way to extract journal entries for a specific unit is to use journalctl -u <unit_name> or to use the sd-journal API. In addition, the journal is a binary file so it's not exactly in a format that is parse-friendly. I'm not even sure that the internal format of the journal follows a stable specification. journalctl or the API is definitely the way to read these files.

If you are asking this question to back-up (or send for troubleshooting) a specific file, you'll need to treat all of these files as a single unit.

Since you asked specifically about the file-naming convention of these journal files, I spent some time investigating this. After a fresh boot, the only file in /run/log/journal was /run/log/journal/c13e4a2f54334a95891a6a471db3b7e0/system@edd0c14e44a240038601194807d5c28e-0000000000000001-0005b08ccb2c37b3.journal.

I broke that down into the following to try and understand the naming convention:

/run/                                  <-- run/var
  log/journal/                         <-- standard location 
    c13e4a2f54334a95891a6a471db3b7e0/  <-- _MACHINE_ID
    system@                            <-- system/user
    edd0c14e44a240038601194807d5c28e-  <-- Some bus/boot/session ID?
    0000000000000001-                  <-- Sequence number (also looks like offsets as 4-KB blocks in hex)
    0005b08ccb2c37b3                   <-- Timestamp (time since epoch in hex)
    .journal                           <-- Extension

When I look at a machine which has been running for a while, all system@*.journal files are 80M while all user@*.journal files are 8.0M. With such consistent sizing, it really suggests that the file splits are simply log rotations. I can also confirm that by looking at the modification dates (as we go forward in sequence, I see the dates increasing).

To figure some of these things out (like _MACHINE_ID) I had to look into the raw meta-data associated with a journal entry. There is more meta-data than what you can get with journalctl. I wrote a quick program with the sd-journal API and printed "Hello" from a service. I found it in the journal with the following data. It's amazing how much meta-data is associated with each printf.


If you're interested in the sd-journal program I wrote to investigate this, it's here:

// Compile with `gcc journalctl.c -lsystemd`
#include <systemd/sd-journal.h>
#include <stdio.h>
int main()
    // Open the log
    sd_journal* journal;

    if ( sd_journal_open(&journal, SD_JOURNAL_LOCAL_ONLY) < 0 )
        return 1; // Couldn't open journal

    // Subscribe to specific logs:
    // Messages from the service itself
    int r = sd_journal_add_match(journal, "_SYSTEMD_UNIT=counter.service", 0);
    r = r ? r : sd_journal_add_disjunction(journal);
    // Messages from PID 1 (systemd) about this service
    r = r ? r : sd_journal_add_match(journal, "_PID=1", 0);
    r = r ? r : sd_journal_add_match(journal, "UNIT=counter.service", 0);
    r = r ? r : sd_journal_add_disjunction(journal);

    if ( r < 0 )
        return 1; // Could not subscribe to all journal entries

    r = sd_journal_seek_head(journal);
    if ( r < 0 )
        return 1; // Problem finding the head of the log

    // Read the log
    int skip_seek=1;
    while( skip_seek || (sd_journal_next(journal) > 0 ) ) 
        const void* data = 0;
        size_t length;
        skip_seek = 0;


        SD_JOURNAL_FOREACH_DATA(journal, data, length)
            printf("%s\n", data);


    return 0;

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