Meaning of `cat /dev/null > file`

Question

In a document created by a former coworker there is this command:

cat /dev/null > /var/spool/mail/root   

It says next to it that it will clean out mailbox.

Can someone please explain how/why these commands do that. I need to know what will happen before I run the command.

We are trying to clean up space on var, as of right now it's at 91%.

Answer 1

The command will output the data from device /dev/null to the given file (mailbox of the root account). Since /dev/null responds just with end-of-file when reading from it nothing will be written to the file, but with the redirection > the shell will have cleared the file already. Actually this is equivalent to writing just

> /var/spool/mail/root

(i.e., the same without cat or /dev/null).

Answer 2

cat /dev/null > /var/spool/mail/root truncates /var/spool/mail/root

Alternative is > /var/spool/mail/root