Evaluate systemd logs using Journalctl

Finding the Time

If necessary, you can filter logs to show events at exact times. This is very helpful for computers that are not restarted often. Use the options --since and --until to narrow down the time window to the minute or second as required. You can also combine these options.

You can see an example of using one of the options on its own in the first line of Listing 5. The second line contains an example of using both. If you are specifying the time, it must be in the format YYYY-MM-DD HH:MM:SS. You can also use more general options such as --since yesterday, as shown in Listing 5.

Listing 5

Filtering Logs by Time

$ sudo journalctl --since "2017-05-10 12:00:23"
$ sudo journalctl --since "2017-05-10 12:00:23" --until "2017-05-10 19:00:23"
$ sudo journalctl --since yesterday
$ sudo journalctl --since 12:00 --until "now"

Filter by component

There are other filters you can use to search for specific events. This means you can detect messages relating to individual components. For example, if you are experiencing problems with your web server, use journalctl -u apache2.service to display only related events (Figure 3).

Figure 3: Journalctl can query the logs for individual services.

If you have an idea of where the error may be occurring within the web server, refine the filter further, as in the example query (Listing 6, line 1) of a suspect PHP module. To investigate specific processes, you can also filter by process, user or group ID.

First, look for the corresponding process ID. You can do this using the commands pidof or ps (Listing 6, lines 2 and 3). Next, query the corresponding process using the option _PID= (line 4). In Debian, you can find the user for the web server using the command id -u www-data (line 5), for instance to display all events since midnight (line 6). To display all possible filters, use man systemd.journal-fields.

Listing 6

Filtering by Component

01 $ sudo journalctl -u apache2.service -u php-fpm.service --since yesterday
02 $ pidof apache2
03 $ ps aux | grep apache2
04 $ sudo journalctl _PID=PID
05 $ id -u www-data
06 $ sudo journalctl _UID=UID --since today

You can also search for notifications from misbehaving applications. To do this, simply enter the relevant path. For applications installed via your package manager, these should be located in /usr/bin, for instance journalctl /usr/bin/amarok. For other apps, use which <name> to determine the exact path.

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