How to reliably clear a log file on Linux

Have you ever found your Linux server running out of disk space, only to discover a single log file taking up gigabytes of storage?

Well, I have ๐Ÿ˜…. Recently, in one of the servers I'm managing, I detected a 5 GB (and steadily increasing) /var/log/maillog file.

Turned out to be an issue with an incomplete removal of a hosting control panel tool that was causing the log file to be written to every second.

After fixing the issue that was causing this non-stop growth of the log, I began looking for a way to reliably get the disk space back.

I had a sneaky suspicion that simply rm'ing and recreating the file would not be such a great idea.

Turns out my hunch was right. My investigation led me to learn that when a process opens a file - either for read or write, it's actually holding a handle to that file's inode.

inode is a Linux filesystem data structure that's assigned to each created file. Its purpose is to store various metadata about the file.

If the file gets deleted by a different process, this may cause other programs that reference that inode to malfunction. Also, the disk space is not freed up until processes close their handle on it.

Moreover, file ownership and permissions will most likely get messed up, as the newly created file won't have the same inode as the deleted file.

File truncation

Ok, so deleting and recreating the log file is not a viable solution. What can we do instead to free up that disk space?

We need to truncate the contents of the file, while its inode remains the same. In other words, we need to clear the contents of the file without deleting it.

Manually editing the file is not clean solution a at all. What if the file gets written to by another process while we're editing it?

To truncate the log file we can rely on Linux's standard output redirect operator to redirect the output of nothing to the file:

> /var/log/maillog

Or, we can use command truncate, which truncates the file to a precise length in bytes:

truncate -s 0 /var/log/maillog

Truncating a file is also much faster than deleting the file, recreating it, and setting the correct ownership and permissions.

Log rotation

Before we wrap up, it's important to bring up the topic of log rotation.

In a more critical/production scenario, you may need to store the logs for archive. Log rotation helps us achieve this, by renaming and compressing old log files, making them ready to be backed up. logrotate is the Linux tool that achieves precisely this.

After you've backed up a rotated log file, it's fine to just remove it and gain that disk space back.