Cute "sed" Trick: rotate file

I wanted a script that would invoke a process, passing in a port number from a circular pool of ports.  When the process exits and restarts, I want the script to pass in the *next* port from the pool.

    PORT=`head -1 port_file.txt`

    sed -i -e '1h;1d;$G' port_file.txt

In the above "sed" command:

• The "-i" causes the file "port_file.txt" to be edited in-place.
• The "1h" sed command yanks the first line into the "hold" space.
• The "1d" sed command deletes the first line (prevents it from being output).
• The "$G" command appends the hold space after the last line of the file.

Thus, given that "port_file.txt" contains:

    12000

    12001

    12002

    12003

the above two commands will leave the file with:

    12001

    12002

    12003

    12000

and "PORT" set to 12000.