cat /dev/null > messages
You typically cat /dev/null > [something] when you want to wipe file contents while ensuring there is absolutely zero risk of interruption to the actual file state. The contents of the file will clearly be wiped by the cat /dev/null yet the file itself—as it exists and is known to the file system it resides on—will still be there with the same inode number, ownership and permissions.
In the case of a log file, it could be that the log file itself is marked “in use” by another process. So doing—for example—an rm /var/log/messages && touch /var/log/messages would be disruptive to other processes and might cause running processes to choke. Meaning a process that somehow is locked to a specific inode number connected to the file /var/log/messages could suddenly panic and say, “Hey! What happened to /var/log/messages!” even if the file is still there. Not to mention potential issues with ownership and permissions being incorrectly recreated.
Because of this uncertainty in usage/state of a file the use of cat /dev/null > [something] is preferred by system admins who want to clear out a log but do not want to potentially interfere with the operation of already existing processes.
