Unix systems (and thus Linux) are multi-tasking and multi-user systems. Indeed, several processes can run in parallel, and be owned by different users: the kernel mediates access to the resources between the different processes. As a part of this task, it has a concept of priority, which allows it to favor certain processes over others, as needed. When you know that a process can run in low priority, you can indicate so by running it with nice program. The program will then have a smaller share of the CPU, and will have a smaller impact on other running processes. Of course, if no other process needs to run, the program will not be artificially held back.

nice 添加 “niceness” 優先序：正數層級 (1 至 19) 逐漸減少優先序，負數層級 (-1 至 -20) 則逐漸增加 — 但祗有超級用戶才能使用負數層級。除非特別指明 (見 nice(1) 手冊頁面)，nice 把現在的執行層級增加 10。 nice 会给优先级添加“niceness”：正值优先级（从１到１９）逐渐降低优先级，而负值（从－１到－２０）增加优先级－但是仅有超级用户可以使用负值优先级。除非特别指明（参看 nice(1) 手册）， nice 将当前优先级加１０。

若發現已執行的工作應以 nice 啟動，修正它並不算太遲；renice 命令可改變執行中程序的優先序，增加或減少均可 (但減少 “niceness” 是超級使用者的權限)。