Some useful sysadmin commands

Some useful sysadmin commands:

1. Show the disk usage per directory one level:

   FreeBSD: du -h -d 1
   Linux: du -h --max-depth=1

2. How to create a Symlink/Softlink?

   ln -s realpath fakepath

3. Emptying a file without deleting it:(i.e. quick way to empty system logs)

   cp /dev/null filename

4. This command will show exim processes including the path to the script being utilized to send mail. Very useful in locating a spamming script.

   ps -C exim -fH eww

5. To trace through the hops the email gets delivered. This is useful to determine if the message can reach its destination or not.

   exim -bt username@domain.com

6. Watching logs:

   tail -f /path/to/log-file