The program ps lets you see the running processes. There are (too) many option ps supports or requires.

To show processes including Process ID ps -A

To see all processes do a ps -e

To see also daemons ps -ax or simply ps ax

To see also the users ps -u

pstree is alternative that shows process tree.

top gives a live text view about the running processes and their CPU load.

nice -n<-20 to 19><command> starts a program with given runtime priority

renice -n<-20 to 19><PID> lets change the runtime priority of an already running program using their PID (Process id-> shown e.g. using ps). The values -20 mean low and 19 mean high priority.

Processes can be stopped by killing them. kill -9<Process ID> kills process.

To be successful use -9 the hard way

To find out that you are probably to nice use -150 the soft way.

KDE system guard is an application to show and kill processes

Linux was originally written for larger processors (Pentium and Athlon are considered very large processors) having a piece of hardware called memory managing unit. This allows that the individual user processes have their own memory space and can just do there damage. Therefore crashed processes can be killed without the need of doing a system shutdown. If X crashes or some keyboard/mouse processes then you might be logging via network (SSH, Telnet, tty).

A program can have more than one process. Of course there can be more than one process running (“quasi”) at the same time, Linux is a multi tasking system and manages the runtime for each process. Each process is registered in the kernel process table. Check out cat /proc/1/status that shows what the kernel knows about process number 1 (PID 1 is init). There are also threads. The difference between threads and processes are, that threads belong to a process and therefore all threads have the same memory space. But the threads run in parallel.

Linux got ported to smaller processors without physical memory managing unit. Special Linux versions got developed and are required for that as uclinux . The kernel 2.6 has also growing support for such devices.

A program can be started two times and run in parallel, however sometimes this is not desired, especially when there is just one resource (on CD burner, one Database, one Document). Programs that do not allow that often create a locking file when they start. When there is already a locking file present, the program decides that an other instance is already running and quits. After a crash locking files might be present and prevent the program to start. In such cases the locking files have to be deleted.

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