Fedora Linux

the uppercase letter for a field to shift the field left on the display, or type the lowercase letter to shift it right. Press Enter to exit this display.

To end a process, type k (for kill ). Type in the process ID and press Enter; top will prompt you for the signal to be used. Press Enter to accept the default (15). If the process does not terminate within a few seconds, repeat the procedure with the signal 9.

4.9.1.4. Displaying process information from the shell prompt

Instead of using top to continuously monitor information, you can use the ps (process status) command to display a snapshot of the process table at a particular point in time.

By default, ps shows only processes executed by you on the current terminal:

$ ps
PID TTY TIME CMD
14797 pts/1 00:00:00 bash
22962 pts/1 00:00:00 ps

This shows the process ID, terminal device ( pts/1 means /dev/pts/1 ), total amount of CPU time consumed (less than one second in this example), and the command executed. This information alone is rarely useful, so ps is almost always used with some arguments.

ps uses options to select the processes to be displayed. The most useful ones are:

-A-e

All (or everyone's) processes

-u user

Processes owned by user (which can be a username or numeric user ID)

Other options are used to control the output format:

-f

Displays full information, including the UID, PID, PPID, start time (STIME), terminal (TTY), total CPU time used (TIME), and command (CMD).

-F

Displays extra-full information: everything included in -f , plus the processor number of the CPU the program is running on (PSR) and the approximate kilobytes of RAM used (RSS).

Like ls, the ps command has dozens of options. The Fedora version of ps can use Unix System V syntax or BSD syntax, so many option letters have two meanings; the one that is used depends on whether the option is specified with or without a hyphen!

To see the full documentation for ps, view the manpage but be prepared to take some time; it's over 16 pages long!

4.9.1.5. Terminating processes from the shell prompt

You can terminate processes by command name or by PID. When you terminate a process by name, you save yourself the hassle of looking up the PID, but if there is more than one process of the same name running, you will kill them all.

To kill by command name:

$ killall xclock

If the process doesn't terminate within a few seconds, add the -KILL argument:

$ killall -KILL xclock

Note that this will kill only processes of that name that are owned by you ; you don't have permission to kill other users' processes unless you are root . You will see an error message if other users have a process of the same name running, but this will not affect the killing of the processes that you own.

To kill PID 48292:

$ kill 48292

Again, if that doesn't work within a reasonable period of time, add the -KILL argument:

$ kill -KILL 48292

4.9.2. How Does It Work?

The Linux kernel has only two basic functions for starting processes: fork( ) and exec( ) .

fork( ) makes an exact copy of the current process and starts it running. exec( ) replaces the currently running program with a new program, running in the same process. So to get a new program running, the shell uses fork( ) to create a child process (a copy of the shell) and then uses exec( ) to change the program running in the child process.

When a child process is created, a number of variables are inherited from the parent process, including the real and effective user IDs, the real and effective group IDs, the umask , the terminal, the current working directory, and the environment variables.

Processes are generally permitted to run on a CPU until their timeslice the amount of time allocated to them by the scheduling algorithmis over, at which point another process is scheduled to be run.

However, processes frequently give up the CPU early because they reach a point when they need a resource to continue; this is called blocking . This is often due to slow input/output operations; no matter how fast your disk drive is, the CPU is still faster, so when one process is waiting for disk data, another process can be executing.

The difference between your typing speed and your CPU speed is even greater; most people type six characters per second or less, so on a 3 GHz PC, the CPU will average at least 500 million operations between keystrokes.