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How to Read Linux System Logs using Journalctl – Guide
Journalctl is a utility for querying and viewing logs from journald, the systemd log service. Because journald stores log data in binary format rather than plain text, journalctl is the standard method for reading log messages processed by journald. In the following sections, we’ll show you several ways you can use journalctl to get, format, and parse your logs. These methods can be used alone or in combination with other commands to refine your search. For a complete list of journalctl options, see the Journalctl man page.
Some of the most attractive benefits of systemd are process and system logging. Typically, when using other tools, logs are distributed across the system, processed by multiple daemons and processes, and can be quite difficult to interpret if they span multiple applications. systemd tries to solve these problems by providing a centralized management solution to log all kernel and user processes. The system that collects and manages these logs is called a journal. The journal is implemented with the journal daemon, which processes all messages generated by the kernel, the initrd, services, and so on. In that guide, let’s discuss using the journalctl utility to access and manipulate the data kept in the journal.
Using journalctl for the first time
Default log format and ordering
journalctl will display your logs in a format similar to the traditional syslog format. Each line starts with the date (in the server’s local time), followed by the server’s hostname, the process name, and the message for the log.
Paging your records
journalctl pipes its output to the less command, which displays your logs one page at a time in your terminal. If a log line exceeds the horizontal width of your terminal window, you can use the left and right arrow keys to scroll horizontally and see the rest of the line:
Also, your logs can be browsed and searched using all the same key commands available in minus:
View journalctl without pagination
To send your logs to standard output and avoid paging them, use the –no-pager option:
It is not recommended that you do this without first filtering the number of logs shown.
Monitor new log messages
The minus key commands are not available in this mode. Type Control-C on the keyboard to return to the command prompt this way.
Filter output journalctl
In addition to searching your logs with less key commands, you can invoke journalctl with options that filter your log messages before they are displayed. These filters can be used with the normal paged view and with the –no-pager and -f options. Filters of different types can also be combined to further restrict the output.
Show records within a time range
Use the –since option to show logs after a specified date and time:
Dates and times must be specified in the format YYYY-MM-DD HH:MM:SS. If the time is omitted (that is, only the YYYY-MM-DD date is specified), the time is assumed to be 00:00:00.
journalctl can also accept some alternative terms when specifying dates:
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