Process state codes

man ps

PROCESS STATE CODES
       Here are the different values that the s, stat and state output specifiers (header "STAT" or "S") will display to describe the state of a
       process.
       D    Uninterruptible sleep (usually IO)
       R    Running or runnable (on run queue)
       S    Interruptible sleep (waiting for an event to complete)
       T    Stopped, either by a job control signal or because it is being traced.
       W    paging (not valid since the 2.6.xx kernel)
       X    dead (should never be seen)
       Z    Defunct ("zombie") process, terminated but not reaped by its parent.

       For BSD formats and when the stat keyword is used, additional characters may be displayed:
       <    high-priority (not nice to other users)
       N    low-priority (nice to other users)
       L    has pages locked into memory (for real-time and custom IO)
       s    is a session leader
       l    is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)
       +    is in the foreground process group

http://nixcraft.com/linux-software/431-what-i-o-wait-under-ps-command.html. Retrieved on 2007-05-11 11:18.

Generally, a process is put into D state when waiting for some sort of I/O to complete. The process has made a call into the kernel and is waiting for the result. During this period, it is unable to be interrupted, as doing so might jeopardize the state of the driver and hardware.

http://studentweb.tulane.edu/~jchrist1/Linux%20Unleashed,%20Third%20Edition/ch34/609-612.html. Retrieved on 2007-05-11 11:18.

ps Command Output The output of the ps command is always organized in columns. The first column is labeled PID, which means “Process ID” number. Every process on the system has to have a unique identifier so Linux can tell which processes it is working with. Linux handles processes by assigning a unique number to each process, called the process ID number (or PID). PIDs start at zero when the system is booted and increment by one for each process run, up to some system-determined number (such as 65,564) at which point it starts numbering from zero again, ignoring those that are still active. Usually, the lowest number processes are the system kernel and daemons, which start when Linux boots and remain active as long as Linux is running. When you are working with processes (such as terminating them), you must use the PID. The TTY column in the ps command output shows you which terminal the process was started from. If you are logged in as a user, this will usually be your terminal or console window. If you are running on multiple console windows, you will see all the processes you started in every window displayed. The STAT column in the ps command output shows you the current status of the process. The two most common entries in the status column are S for “sleeping” and R for “running.” A running process is one that is currently executing on the CPU. A sleeping process is one that is not currently active. Processes may switch between sleeping and running many times every second. The TIME column shows the total amount of system (CPU) time used by the process so far. These numbers tend to be very small for most processes because they require only a short time to complete. The numbers under the TIME column are a total of the CPU time, not the amount of time the process has been alive. Finally, the COMMAND column contains the name of the command line you are running. This is usually the command line you used, although some commands start up other processes. These are called “child” processes, and they show up in the ps output as if you had entered them as commands.

http://www.comfsm.fm/computing/UNIX/ps.html. Retrieved on 2007-05-11 11:18.

There are a few pieces of information about each process that need some explanation, expecially the cyptic ones like the STAT column. USER: The owner of the process, typically the user who started it PID: The processes unique ID number. These are assigned sequentially as processes start. When they reach 30,000 or so, the number starts over again at 0. 0-5 are usually low-level operating system processes which never exit, however.  %CPU: Percentage of the CPU’s time spent running this process.  %MEM: Percentage of total memory in use by this process VSZ: Total virtual memory size, in 1K blocks. RSS: Real Set Size, the actual amount of physical memory allocated to this process. TTY: Terminal associated with this process. A ? indicates the process is not connected to a terminal. STAT: Process state codes. Common states are S – Sleeping, R – Runnable (on run queue), N – Low priority task, Z – Zombie process START: When the process was started, in hours and minutes, or a day if the process has been running for a while. TIME: CPU time used by process since it started. COMMAND: The command name. This can be modified by processes as they run, so don’t rely on it abolutely!

http://www.slackbook.org/html/process-control-ps.html. Retrieved on 2007-05-11 11:18.

Second, there is a new column: STAT. It shows the status of the process. S stands for sleeping: the process is waiting for something to happen. Z stands for a zombied process. A zombied processes is one whose parent has died, leaving the child processes behind. This is not a good thing. D stands for a process that has entered an uninterruptible sleep. Often, these processes refuse to die even when passed a SIGKILL. You can read more about SIGKILL later in the next section on kill . W stands for paging. A dead process is marked with an X. A process marked T is traced, or stopped. R means that the process is runable.

… is a really interesting, more visual alternative to ps.