In a Linux Foundation course we're told to use cat & at a terminal prompt to return what appears to be the current max (used) PID. I wanted to clarify what the & parameter means but both cat --help and man cat turned up nothing for the ampersand parameter.

Snapshot for clarity:

enter image description here

Can anyone please explain what the ampersand means in this context?

1 Answer 1


The & means that you send the command to the background (also called forking), and you are given "back" the prompt even though execution of the command continues (if any).

There is a very good discussion about this in this thread.

To know what the max_pid is, you can look in /proc:

cat /proc/sys/kernel/pid_max 

Or (as root):

sysctl kernel.pid_max 
kernel.pid_max = 32768
  • Cheers! So the ampersand is not not really a parameter specific to cat (explaining no mention in the docs). That helped a lot, the linked thread was useful too. Tx :)
    – Alxs
    Commented Dec 10, 2016 at 16:40
  • 1
    @Alxs No. You're right. The & can be used with any command. It is not a parameter.
    – anon
    Commented Dec 10, 2016 at 16:41
  • 1
    Sending command to background != Forking, please clear up your ideas first.
    – heemayl
    Commented Dec 11, 2016 at 3:44

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