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Chapter 2. Troubleshooting problems by using log files

You can use information from log files for troubleshooting and monitoring system functions. Log files contain messages about the system, including the kernel and the services and applications running on it. The logging system in Red Hat Enterprise Linux is based on the built-in syslog protocol. Various programs then use syslog to record events and organize them into log files.

2.1. Services that handle syslog messages
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The following services handle syslog messages:

The systemd-journald daemon

Collects messages from the following sources and forwards them to Rsyslog for further processing:

  • Kernel
  • Early stages of the boot process
  • Standard and error output of daemons as they start up and run
  • syslog
The Rsyslog service
Sorts syslog messages by type and priority and writes them to the files in the /var/log directory. The /var/log directory persistently stores the log messages.

2.2. Subdirectories that store syslog messages
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The following subdirectories under the /var/log directory store syslog messages:

/var/log/messages
all syslog messages except the following
/var/log/secure
security and authentication-related messages and errors
/var/log/maillog
mail server-related messages and errors
/var/log/cron
log files related to periodically executed tasks
/var/log/boot.log
log files related to system startup

2.3. Commands for viewing logs
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You can view and manage log files by using the Journal, which is a component of systemd. It addresses problems connected with traditional logging, is closely integrated with the rest of the system, and supports various logging technologies and access management for the log files.

You can use the journalctl command to view messages in the system journal, for example: 

$ journalctl -b | grep kvm
May 15 11:31:41 localhost.localdomain kernel: kvm-clock: Using msrs 4b564d01 and 4b564d00
May 15 11:31:41 localhost.localdomain kernel: kvm-clock: cpu 0, msr 76401001, primary cpu clock
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Viewing system information

journalctl
Shows all collected journal entries.
journalctl FILEPATH
Shows logs related to a specific file. For example, the journalctl /dev/sda command displays logs related to the /dev/sda file system.
journalctl -b
Shows logs for the current boot.
journalctl -k -b -1
Shows kernel logs for the current boot.

Viewing information about specific services

journalctl -b _SYSTEMD_UNIT=<name.service>
Filters log to show entries matching the systemd service.
journalctl -b _SYSTEMD_UNIT=<name.service> _PID=<number>
Combines matches. For example, this command shows logs for systemd-units that match <name.service> and the PID <number>.
journalctl -b _SYSTEMD_UNIT=<name.service> _PID=<number> + _SYSTEMD_UNIT=<name2.service>
The plus sign (+) separator combines two expressions in a logical OR. For example, this command shows all messages from the <name.service> service process with the PID plus all messages from the <name2.service> service (from any of its processes).
journalctl -b _SYSTEMD_UNIT=<name.service> _SYSTEMD_UNIT=<name2.service>
This command shows all entries matching either expression, referring to the same field. Here, this command shows logs matching a systemd-unit <name.service> or a systemd-unit <name2.service>.

Viewing logs related to specific boots

journalctl --list-boots
Shows a tabular list of boot numbers, their IDs, and the timestamps of the first and last message pertaining to the boot. You can use the ID in the next command to view detailed information.
journalctl --boot=ID _SYSTEMD_UNIT=<name.service>
Shows information about the specified boot ID.
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