The priority of a process is determined by the policies of its scheduling class and by its nice number. Each timesharing process has a global priority. The global priority is calculated by adding the user-supplied priority, which can be influenced by the nice or priocntl commands, and the system-calculated priority.
The execution priority number of a process is assigned by the operating system. The priority number is determined by several factors, including the process's scheduling class, how much CPU time it has used, and in the case of a timesharing process, its nice number.
Each timesharing process starts with a default nice number, which it inherits from its parent process. The nice number is shown in the NI column of the ps report.
A user can lower the priority of a process by increasing its user-supplied priority. However, only superuser can lower a nice number to increase the priority of a process. This restriction prevents users from increasing the priorities of their own processes, thereby monopolizing a greater share of the CPU.
The nice numbers range from 0 to +39, with 0 representing the highest priority. The default nice value for each timesharing process is 20. Two versions of the command are available: the standard version, /usr/bin/nice, and the C shell built-in command.