How To Manually Add Messages To Linux System Log Files

A log file is a file that contains a set of records (or list of events) which have been logged by the system itself. Using the log files, the sysadmin can track what has happened on a particular day or time. Mostly log files are used by the admins for troubleshooting purpose. The log files are generated automatically and saved under a common directory - /var/log/. We can also manually add messages to Linux system log files. For instance, after setting up a Log server, you might want to check if the log server is properly functioning or not. To do so, we can manually add some entries in the log files to test the log server. This is where are logger command comes in handy.

Add Messages to Linux System Log Files using Logger Command

The logger command is part of the util-linux package, so don't bother installing it. Let me show you some logger command examples.

Adding entries manually to the system log files is absolutely easy! The typical usage of logger command is:

logger <message-to-add>

Example:

$ logger Welcome to OSTechNix

The above command will add the entry 'Welcome to OSTechNix' to the system log files.

Let us verify if the message has been added or not using tail command:

$ tail -l /var/log/syslog

Sample Output:

Jan 31 07:19:23 ubuntuserver systemd[1705]: Listening on REST API socket for snapd user session agent.
Jan 31 07:19:23 ubuntuserver systemd[1705]: Listening on GnuPG network certificate management daemon.
Jan 31 07:19:23 ubuntuserver systemd[1705]: Listening on GnuPG cryptographic agent and passphrase cache.
Jan 31 07:19:23 ubuntuserver systemd[1705]: Reached target Sockets.
Jan 31 07:19:23 ubuntuserver systemd[1705]: Reached target Timers.
Jan 31 07:19:23 ubuntuserver systemd[1705]: Reached target Basic System.
Jan 31 07:19:23 ubuntuserver systemd[1]: Started User Manager for UID 1000.
Jan 31 07:19:23 ubuntuserver systemd[1705]: Reached target Default.
Jan 31 07:19:23 ubuntuserver systemd[1705]: Startup finished in 298ms.
Jan 31 07:33:14 ubuntuserver sk: Welcome to OSTechNix

As you see in the output, the given message has been added to the syslog file.

You can also add the messages by enclosing them in single or double quotes.

$ logger "Welcome to OSTechNix blog"

Or,

$ logger 'Howdy, Welcome to OSTechNix blog'

Check if the entries have been added to the syslog file using tail command:

$ tail -l /var/log/syslog 
[...]
Jan 31 07:40:02 ubuntuserver sk: Welcome to OSTechNix blog
Jan 31 07:40:31 ubuntuserver sk: Howdy, Welcome to OSTechNix blog

Add logger PID to system logs

We can even log the PID of the logger process with each line using -i flag like below.

$ logger -i 'Howdy, Welcome to OSTechNix blog'

Verify the log messages using tail command:

$ tail -l /var/log/syslog 
[...]
Jan 31 07:49:22 ubuntuserver sk[1879]: Howdy, Welcome to OSTechNix blog

Here, 1879 is the process ID of logger.

Add tags to messages

As you may noticed in the above outputs, the newly added entries are tagged with the currently logged-in username i.e. sk. The default tag is the name of the user logged in on the terminal. We can, however, mark every line to be logged with a specific tag using -t flag.

$ logger -i -t ostechnix 'Howdy, Welcome to OSTechNix blog'

Check the log entries:

$ tail -l /var/log/syslog 
[...]
Jan 31 07:54:02 ubuntuserver ostechnix[1881]: Howdy, Welcome to OSTechNix blog

See? the last entry is tagged with ostechnix and its process ID is 1881.

Add messages from a file to the log files

It is also possible to add entries from a file to our system log files.

Let us create a sample text file.

$ echo "This is test file" > file.txt

$ cat file.txt 
This is test file

Now add the contents of the file.txt to system log files using command:

$ logger -f file.txt

Check log files:

$ tail -l /var/log/syslog 
[...]
Jan 31 08:43:06 ubuntuserver sk: This is test file

If he the input file has any empty lines, you can exclude them from being added to the log files using -e flag like below.

$ logger -e -f file.txt

Log messages with priority

It is possible to log messages with a given priority.

$ logger "Welcome To OSTechNix" --priority user.warning

Default priority is 'user.notice'. See logger man page to know all available priority options.

Take input from 'stdin' and messages to system logs

We can pipe the input from stand input and push them to the syslog using command:

$ echo "Welcome to OSTechNix" | logger

Send messages to remote log server

To send the messages to a remote syslog server running at a specific port, run:

$ logger "Welcome to OSTechNix" --server <hostname-or-ip> --port <port-no>

Or,

$ logger "Welcome to OSTechNix" -n <hostname-or-ip> -p <port-no>

The default port no is 514.

Limit the size of the messages

We can set the maximum permitted message size using --size flag.

$ logger --size 5 abcdefghijklmnopqrswxyz

Check the log message size:

$ tail -l /var/log/syslog 
[...]
Jan 31 09:09:02 ubuntuserver sk: abcde

The default is 1KiB characters.

For more details check man pages.

$ man logger

Conclusion

In this brief tutorial, we discussed what is logger command, how you can use logger command to manually add messages to Linux system log files to test if a log server is properly working or not.