Definition : A process is a program that is executed (or currently running on the computer). It reside in the memory (RAM) of the computer. Every process is unique and is identified by a Process Number called PID. The PID is a random number starting from 0 to 65534. Also, a process has various period of execution. (Eg, sleeping)

Every command you run will start at least one new process, and there are a number of system processes that run all the time, keeping the system functional.

When you run a specific command, usually that command is in the foreground and it's using the computer until it is finished. For example, if you want to run other commands rather than wait for the program to finish, you can type CTRL+Z to put it in the background. To stop a running application (from the foreground) you usually have to type CTRL+C.

Once an application is put into the background, it can be brought to the foreground using the fg command.It is possible to launch a program directly into the background by using an amperstand (&).To do this you should look at the example below :

root@linuxedu:~# updatedb &
[1] 5467

This means that the program has been started, the PID of the program is 5467 and it was launched in the background.

There are two useful commands to see the processes on a system, ps(1) and top(1). The ps command shows a static list of the running processes on the system.It shows you the PID, the amount of memory a process is using, the command line they were started with and so on.The top command displays all the running processes, and updates the display every few second so that you can easily observe what your system is doing.

By default, ps outputs only the processes that you own. For example:

tress@linuxedu:~$ ps
PID TTY TIME CMD
5632 pts/1 00:00:00 bash
5782 pts/1 00:00:00 ps

The output of ps, as you can see from the example, is organised into a number of columns.:

• PID is the process ID discussed earlier.
• TT shows the tty the program is running on.
• STAT shows the status of the process.
• TIME shows the amount of time the program has been running on the CPU.
• CMD is the command line used to start the program.

ps(1) has a number of different options to change the information that is displayed. One of the most useful combinations is auxww :

• a displays all the running programs, not just yours.
• u displays the memory usage and the username of the process owner.
• x displays information about daemon processes
• ww causes ps(1) to display the full command line used to start a specific program.

The output of the top command is quite similar.

tress@linxuedu:~$ top
 
 
 18:25:39 up 128 days,  2:17,  2 users,  load average: 1.75, 1.29, 1.11
71 processes: 68 sleeping, 2 running, 0 zombie, 0 stopped
CPU states:  98.0% user,   2.0% system,   0.0% nice,   0.0% idle
Mem:    125508K total,   121836K used,     3672K free,     4736K buffers
Swap:   240932K total,        0K used,   240932K free,    47728K cached
 
  PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME COMMAND
  543 nobody     9   0 19596  19M  1304 S     1.1 15.6  60:38 squid
    1 root       8   0   512  512   456 S     0.0  0.4   0:09 init
    2 root       9   0     0    0     0 SW    0.0  0.0   0:00 keventd
    3 root      19  19     0    0     0 SWN   0.0  0.0   0:00 ksoftirqd_CPU0
    4 root       9   0     0    0     0 SW    0.0  0.0   0:03 kswapd
    5 root       9   0     0    0     0 SW    0.0  0.0   0:00 bdflush
    6 root       9   0     0    0     0 SW    0.0  0.0   0:00 kupdated
  143 daemon     9   0   540  540   464 S     0.0  0.4   0:00 portmap
  540 root       9   0  1084 1084   916 S     0.0  0.8   0:00 squid
  554 nobody     9   0   312  308   260 S     0.0  0.2   0:13 unlinkd
  599 root       9   0   612  612   516 S     0.0  0.4   0:06 syslogd
  651 root       9   0  1320 1320   448 S     0.0  1.0   0:01 klogd
  688 root       9   0  2840 2836  1864 S     0.0  2.2   0:00 named
 1018 amavis     9   0   556  552   460 S     0.0  0.4   0:00 amavis-milter
 1021 root       9   0   700  700   616 S     0.0  0.5   0:00 rpc.statd
 1029 root       9   0   460  460   404 S     0.0  0.3   0:00 courierlogger
  705 root       8   0  2260 2256  1672 S     0.0  1.7   0:00 lwresd
  713 root       9   0  2260 2256  1672 S     0.0  1.7   0:00 lwresd

As you can see, the output of the top command is split into two sections. The header (first 5 lines) shows the PID of the last process to run, the system load averages (shows you the measure of how busy is the system), the system uptime (time since last reboot) and the current time. The other figures in the header relate to how many processes are running (71 in our case), how much memory and swap is taken, and how much time the system is spending in different CPU states.

The second section of the output is very similar to the ps command output.However it shows you the amount of memory used by a program, and also the amount of needed memory for a specific process.