Explanation
When starting a program with the nice command without any additional parameters, the nice level is set to 10 for the resulting process. This means that the process will have a lower priority than the default value of 0, and will be more willing to yield CPU time to other processes. The nice command can also take an optional argument -n followed by a number, which specifies the increment or decrement of the nice value from the default value of 0. For example, the command:
nice -n 5 /usr/bin/prog
will start the /usr/bin/prog process with a nice value of 5, which means a lower priority than the default.
Similarly, the command:
nice -n -5 /usr/bin/prog
will start the /usr/bin/prog process with a nice value of -5, which means a higher priority than the default. If the -n argument is omitted, the nice command will assume a default increment of 10. For example, the command:
nice /usr/bin/prog
will start the /usr/bin/prog process with a nice value of 10, which means a very low priority. Note that only the root user can start a process with a negative nice value, as this requires special privileges.
References:
* [LPI Exam 101 Detailed Objectives], Topic 103: GNU and Unix Commands, Objective 103.6: Modify process execution priorities, Weight: 2, Key Knowledge Areas: Know the default priority of a job that is created.
* How to Set Process Priorities With nice and renice on Linux, Topic: The nice Command.