confusion about renice command

Hi,

Does the renice +number command increase the niceness to "number"
unit or set the niceness to that "number" ?

Set.

For instance, I can't use the command renice +5 process if that process
have a nice value set to 10, unless I use sudo. Because this command is
going set the nice value to 5 and thus decrease the old value rather than
increasing it!

I didn't understand the details of your test, but if you are increasing the priority of a process, you need to be root or use sudo. You can see that in man renice(1):

NOTES
Users other than the superuser may only alter the priority of processes they own. Furthermore, an unprivi‐
leged user can only increase the nice value'' (i.e., choose a lower priority) and such changes are irre‐ versible unless (since Linux 2.6.12) the user has a suitablenice'' resource limit (see ulimit(1) and getr‐
limit(2)).

   The superuser may alter the priority of any process and set the priority to any value in the range -20 to  19.
   Useful  priorities  are: 19 (the affected processes will run only when nothing else in the system wants to), 0
   (the ``base'' scheduling priority), anything negative (to make things go very fast).

On my machine renice +5 process and renice 5 process do the same.
Is that normal ? what's the difference between the two commands ?

That happens because the niceness of a process (started by you) starts on 0. So, if you add 5 or set to 5, the result will be the same. To help understand how it works, you can use the following:

1.- Start a process, say gnome-clocks.
2.- Print basic information of the processes and look at gnome-clocks (or whatever you are testing with):
ps -eo pid,tid,ni,pri,stat,comm

3.- See that "NI" and "PRI" indicates for the process.
4.- Change the niceness of the process.
5.- Print the information again, and compare. Ask yourself: what happened with the niceness and priority value when adding 7, for example?

Regards,
Luis.

you are correct. The argument to "renice" is the nice value to set to, not the increment or decrement. The text is somewhat ambiguous as it explains properly in once place and screws up in the earlier place. this will be fixed in the next update due in a month or two at the most. Thanks!

BTW, your example has to be run as root, at least on my workstation, because only root can set the niceness to -7

Nice to see someone pay attention to details like /etc/security/limits.conf. you would be surprised how many things can slip by. I suspect the error in the renice description has been there literally for years and noone has tagged us on it before as best I can remember, as we fix these things as we notice them!

as explained in the posts, please read more carefully. renice sets it to the value given, not as an increment. That's exactly what this thread was about so you are getting the correct behaviour (if I am understanding correctly!)

Hello,

Just to remind that the question 3.2 still waits the correction:
Question 3.2
Which command will increase the niceness by 5 units of the process with PID = 444?

Incorrect Answer

A. renice 5 444Correct Answer
B. renice +5 444Your Answer
C. nice -5 444
D. nice +5 444Next Question

BR
C