Why can't I view view log files with cat/grep/etc? (clog)

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pfSense uses a Circular Log format known as clog to maintain a constant log size.

There are multiple benefits to this method. The primary advantage is that the log files cannot grow and fill up filesystems. The downside is that they are no longer plain text logs, and cannot be directly used by programs like cat or grep. If they are viewed directly, "random" binary data may be seen inside the log and entries may appear out of order.

Viewing Log Contents

The clog command must be used to view the contents of log files from the shell, like so:

clog /var/log/filter.log

Then the output may be piped to tools like grep if needed.

In order to follow the log files in a manner like tail -f would allow, use:

clog -f /var/log/filter.log

The contents of the log will be printed to the console, and then new entries will appear as they are written.

Log Sizes

The log files created for use by pfSense with clog are a fixed size that holds a certain amount of data total, not log entries. As such, the number of log entries may vary widely depending on the length of the lines and message content. Log files could contain anywhere between 2000 and 4000 entries, or even more/less than that. The GUI only shows 50 lines per log by default but the files contain many more entries. See Log Settings for more information on that setting.

On pfSense 2.2 and later, the sizes of these log files may be adjusted. Navigate to Status > System Logs on the Settings tab. There, a new Log File Size (Bytes) may be entered.

The default is 500 Kilobytes per log file, and there are around 20 log files. When increasing log sizes, keep disk space in mind. There is a disk space indicator for the filesystem containing the logs under the Log File Size (Bytes) text description.