http://mylinuxbook.com/linux-processes-part1
Generally, on any operating system, we say we have so many programs running. These running programs introduce the concept of processes. Let’s Define as to what is a process – A process is a program in execution.
Robert Love expresses his definition of a process in one of his books as :
The PID of the process, as the name suggests is its identifier. Hence, most of the operations being done on a process needs the PID to be mentioned.
We shall see in the following sections, how do we see display all the processes with their PID’s and various operations that can be performed on a process.
The Linux command used to view list of processes is ‘ps’ which means ‘process status’ (Some authors also interpret it as ‘process snapshot’).
To see what all this Linux command has to offer in detail, the best source is the man-page.
Let us try out running the command
Getting to know from the ‘ps’ output, let’s walk through it column by column.
Running the above command, we see a huge list of processes. Hence, to read them through reasonably, we pipe the output to ‘more’
It gives an interesting output, showcasing the first ‘init’ process and the other processes spawned out of it.
The process tree snapshot on my ubuntu system is:
Under the umbrella of interactive processes, we have Foreground and Background processes.
Lets discuss them one by one :
Let us create our own program which sleeps for 10 seconds and then ourselves experience what waiting for the foreground process feels like.
The C source looks like:
To check another foreground blocking, open the geditor through the terminal. The terminal launches the geditor, won’t let you input anything to the terminal until we terminate the geditor.
How we use this character, is by suffixing it with the command, as in,
To get these background and foreground handy, linux provides certain commands to view what is running and also switch any foreground process to background and vice versa.
One can use command ‘jobs’ to see what all is running associated with the terminal.
If we want to switch the ‘geditor’ as a foreground process, use linux command ‘fg’
Note: When we suspend the process, it is not running and hence on resuming it will start with the same status, when it was stopped.
There are two linux commands which are provided to create the batch processes:
When we execute the ‘at’ command, we reach the ‘at’ prompt, where we can queue all the tasks/commands/programs in an order. When we are done with the queueing of the task, press key combination ‘Ctrl +d’. On pressing the key combination ‘Ctrl + d’, we see ‘EOT’ displayed at the standard output, following which we are back to our command prompt.
As an example,
It’s usage example is similar to the command ‘at’ (explained above).
Generally, when we code any program in C in Linux and execute it, the terminal becomes its parent process. Hence, in order to develop a daemon process service, the programmer needs to detach the process from its parent process. This is done by killing its parent process, which makes the process independent of the terminal, but controlled by its grandparent process i..e init process.
We shall learn more about coding a daemon process in the second part of the article.
Such processes are called zombie processes, which are killed but still exist.
Zombie processes are generally harmless if there are not much. However, if we have a whole lot of zombie processes lingering, then it could be a pain. Since, the PID’s still taken up by the zombie processes are not available for re-allocation to new processes. Hence, soon the system would be out of available PID’s if the zombie processes keep on increasing, and no new process would be able be launch.
1. Running or Runnable ( R ) – A running state has a broader concept here. Running always does not mean utilising the CPU. Even while a process is ready to run, the state is running state.
Hence, there are two sub-states, when the process is queued in the ready queue to run and when the process is actually being executed, it is in the executing sub-state as has been scheduled by the scheduler.
2. Stopped (T) – If a running process receives a stop signal, it is moved to the stopped state. A process can also be in stopped state if it has been halted by a trace while debugging. .
3. Uninterruptible sleep (D) – It is a sleeping state, process has been blocked. Mostly, process goes into an uninterruptible sleep during an IO operation.
4. Interruptible sleep (S) – It is a sleeping state i.e. a blocking state where the process is waiting for an event to occur.
5. Zombie/Defunct state(Z) – It is the process state in which process has been terminated but not reaped by its parent process.
6. Dead (X) – A process never reaches this state, as as soon as it is dead, it is gone.
Note: For BSD formats and when the stat keyword is used, additional characters may be displayed:
In order to check the current status of the active processes at a moment on the linux system, we again use the ‘ps’ command but with a different set of options.
The ‘top’ usage looks like
To see how and what all it provides, here is an output snapshot from my ubuntu system
The command:
Analysing and trying to understand the ‘top’ command output, let us go line by line.
Now we know what all processes have been launched and are in which states.
Moving on,
On similar lines, the next output states usage of swap space
The usage syntax
The ‘kill’ linux command sends a signal to the specified process. Which process to send the signal is specified by its PID.
There are standard numbers assigned to each set of signals. We can get the information of what number is corresponding to which signal using the same ‘kill’ command through ‘-l’ option.
We’ll use our old program source, and its executable.
http://careers.directi.com/display/tu/Understanding+Processes+in+Linux
Generally, on any operating system, we say we have so many programs running. These running programs introduce the concept of processes. Let’s Define as to what is a process – A process is a program in execution.
Robert Love expresses his definition of a process in one of his books as :
The Process is one of the fundamental abstractions in Unix Operating Systems, the other fundamental abstraction being filesLinux is a multi-user and multi-tasking operating system(seemingly, discussed later in the article). A Linux process is a program in execution on a Linux system. Therefore, whenever a program is executed, a new process is created. A process also consumes resources like the file system, memory or other CPU resources. This gives rise to the need of process management in Linux.
Identifier for Linux Processes
In Linux, every process has a unique process Identifier(ID) associated to it. A process ID( i.e. PID) is a number which is uniquely assigned as soon as the process is created. The PID’s are allocated sequentially as the processes are being created. However, it generally starts from 2, as PID=1 is reserved for ‘init process. As we always expect, there is a maximum limit to the PID value. In a system, the way one can get to know the maximum limit to PID is$ cat /proc/sys/kernel/pid_maxFor me, I got the following value
32768Hence, whenever the sequentially allocated PID reaches the maximum value, it wraps to the lower limit(generally 300) and the next PID’s allocated are the available ones starting from the lower limit.
The PID of the process, as the name suggests is its identifier. Hence, most of the operations being done on a process needs the PID to be mentioned.
We shall see in the following sections, how do we see display all the processes with their PID’s and various operations that can be performed on a process.
Listing Processes
At any moment, the Linux user can view the list of all the processes which have been created and not terminated. There is a reason I don’t say all the processes which are running, as once a process has been started, it can be in any state, not necessarily running. More about process states in further sections.The Linux command used to view list of processes is ‘ps’ which means ‘process status’ (Some authors also interpret it as ‘process snapshot’).
To see what all this Linux command has to offer in detail, the best source is the man-page.
Let us try out running the command
$ ps PID TTY TIME CMD 1779 pts/0 00:00:00 bash 2176 pts/0 00:00:00 psWe just see two processes! Although we are sure there are other processes being running as well. Well, the ‘ps’ command without any options just lists the processes which are created by the current terminal. The first one is the ‘bash’ which is the running linux shell by the terminal and other is the process created by ‘ps’ command itself.
Getting to know from the ‘ps’ output, let’s walk through it column by column.
- PID – The process Identifier which is ‘1779’ for ‘bash’ and ‘2176’ for ‘ps’.
- TTY – stands for terminal-type and is the name of the console/terminal, the process is associated to.
- TIME – The CPU time since the process has started. It is confusing that why the CPU time for ‘bash’ process is ‘00:00:00’? This is because, CPU time is the time for which the process is being executed by the processor. However, when bash runs commands,lets say ‘ls command’ , a child process ‘ls’ is spawned and whatever execution and cpu utilization takes place, goes under the ‘ls’ process and not ‘bash’ Bash process is just the parent process.
- CMD – Command run to create the process.
List All processes
$ps -eIf we look at the ‘ps’ man page, ‘-e’ option means “Select all processes”, which implies now our list of displayed processes is not limited to the ones by the current terminal. Instead, we’ll be able to see all the currently running processes. Note, this option is identical to ‘-A’.
Running the above command, we see a huge list of processes. Hence, to read them through reasonably, we pipe the output to ‘more’
$ ps -e | more PID TTY TIME CMD 1 ? 00:00:00 init 2 ? 00:00:00 kthreadd 3 ? 00:00:01 ksoftirqd/0 5 ? 00:00:00 kworker/u:0 6 ? 00:00:00 migration/0 7 ? 00:00:00 watchdog/0 8 ? 00:00:00 cpuset 9 ? 00:00:00 khelper 10 ? 00:00:00 kdevtmpfs 11 ? 00:00:00 netns 12 ? 00:00:00 sync_supers 13 ? 00:00:00 bdi-default 14 ? 00:00:00 kintegrityd 15 ? 00:00:00 kblockd 16 ? 00:00:00 ata_sff 17 ? 00:00:00 khubd 18 ? 00:00:00 md 21 ? 00:00:00 khungtaskd 22 ? 00:00:01 kswapd0 23 ? 00:00:00 ksmd 24 ? 00:00:00 fsnotify_mark 25 ? 00:00:00 ecryptfs-kthrea 26 ? 00:00:00 crypto 34 ? 00:00:00 kthrotld 35 ? 00:00:00 kworker/u:2 36 ? 00:00:00 scsi_eh_0 38 ? 00:00:00 scsi_eh_1 39 ? 00:00:00 scsi_eh_2 61 ? 00:00:00 devfreq_wq 200 ? 00:00:01 jbd2/sda1-8 201 ? 00:00:00 ext4-dio-unwrit 219 ? 00:00:05 flush-8:0 368 ? 00:00:00 upstart-udev-br 375 ? 00:00:00 udevd 503 ? 00:00:00 kpsmoused 628 ? 00:00:00 upstart-socket- 656 ? 00:00:05 rsyslogd 680 ? 00:00:00 dbus-daemon 708 ? 00:00:00 modem-manager --more--Observe, that for each process we have just following four details in the output, similar to what has been discussed in the section above:
PID TTY TIME CMDIn order to get more details about each process, we use option ‘-f’ alongwith. This option will add more details in the form of more columns.
$ps -efAgain, to read through the huge list of output, page by page, we pipe the output to ‘more’ looks like:
$ps -ef | more UID PID PPID C STIME TTY TIME CMD root 1 0 0 Mar08 ? 00:00:00 /sbin/init root 2 0 0 Mar08 ? 00:00:00 [kthreadd] root 3 2 0 Mar08 ? 00:00:01 [ksoftirqd/0] root 5 2 0 Mar08 ? 00:00:00 [kworker/u:0] root 6 2 0 Mar08 ? 00:00:00 [migration/0] root 7 2 0 Mar08 ? 00:00:00 [watchdog/0] root 8 2 0 Mar08 ? 00:00:00 [cpuset] root 9 2 0 Mar08 ? 00:00:00 [khelper] root 10 2 0 Mar08 ? 00:00:00 [kdevtmpfs] root 11 2 0 Mar08 ? 00:00:00 [netns] root 12 2 0 Mar08 ? 00:00:00 [sync_supers] root 13 2 0 Mar08 ? 00:00:00 [bdi-default] root 14 2 0 Mar08 ? 00:00:00 [kintegrityd] root 15 2 0 Mar08 ? 00:00:00 [kblockd] root 16 2 0 Mar08 ? 00:00:00 [ata_sff] root 17 2 0 Mar08 ? 00:00:00 [khubd] root 18 2 0 Mar08 ? 00:00:00 [md] root 21 2 0 Mar08 ? 00:00:00 [khungtaskd] root 22 2 0 Mar08 ? 00:00:01 [kswapd0] root 23 2 0 Mar08 ? 00:00:00 [ksmd] root 24 2 0 Mar08 ? 00:00:00 [fsnotify_mark] root 25 2 0 Mar08 ? 00:00:00 [ecryptfs-kthrea] root 26 2 0 Mar08 ? 00:00:00 [crypto] root 34 2 0 Mar08 ? 00:00:00 [kthrotld] root 35 2 0 Mar08 ? 00:00:00 [kworker/u:2] root 36 2 0 Mar08 ? 00:00:00 [scsi_eh_0] root 38 2 0 Mar08 ? 00:00:00 [scsi_eh_1] root 39 2 0 Mar08 ? 00:00:00 [scsi_eh_2] root 61 2 0 Mar08 ? 00:00:00 [devfreq_wq] root 200 2 0 Mar08 ? 00:00:01 [jbd2/sda1-8] root 201 2 0 Mar08 ? 00:00:00 [ext4-dio-unwrit] root 219 2 0 Mar08 ? 00:00:05 [flush-8:0] root 368 1 0 Mar08 ? 00:00:00 upstart-udev-bridge --daemon root 375 1 0 Mar08 ? 00:00:00 /sbin/udevd --daemon root 503 2 0 Mar08 ? 00:00:00 [kpsmoused] root 628 1 0 Mar08 ? 00:00:00 upstart-socket-bridge --daemon syslog 656 1 0 Mar08 ? 00:00:05 rsyslogd -c5 102 680 1 0 Mar08 ? 00:00:00 dbus-daemon --system --fork --activation=upstart root 708 1 0 Mar08 ? 00:00:00 /usr/sbin/modem-manager --more--Well, we have a new set of details of the running processes.
- UID – The User ID,is the username of the user, which owns the process.
- PID – Already discusses the Process ID.
- PPID – It is the Parent Process ID. Mostly, all the processes, except the first one, have a parent process i.e. the process which has created our relevant process. Therefore, every process retains the PID of its parent process it calls as the parent process ID i.e. PPID, in its process descriptors. We shall learn about process descriptors in the next part of this article series. Meanwhile, one can understand process descriptors as some related parameters describing the process.
It gives an interesting output, showcasing the first ‘init’ process and the other processes spawned out of it.
The process tree snapshot on my ubuntu system is:
$ pstree
init─┬─NetworkManager─┬─dhclient
│ ├─dnsmasq
│ └─2*[{NetworkManager}]
├─accounts-daemon───{accounts-daemon}
├─acpid
├─at-spi-bus-laun───2*[{at-spi-bus-laun}]
├─atd
├─avahi-daemon───avahi-daemon
├─bamfdaemon───2*[{bamfdaemon}]
├─bluetoothd
├─colord───2*[{colord}]
├─console-kit-dae─┬─63*[{console-kit-dae}]
│ └─{console-kit-dae}Dt\267
├─cron
├─cupsd───dbus
├─2*[dbus-daemon]
├─dbus-launch
├─dconf-service───2*[{dconf-service}]
├─deja-dup───2*[{deja-dup}]
├─firefox───22*[{firefox}]
├─gconfd-2
├─geoclue-master
├─6*[getty]
├─gnome-keyring-d─┬─4*[{gnome-keyring-d}]
│ └─{gnome-keyring-d}Dt\267
├─gnome-terminal─┬─bash───pstree
│ ├─gnome-pty-helpe
│ └─3*[{gnome-terminal}]
├─goa-daemon───{goa-daemon}
├─gvfs-afc-volume───{gvfs-afc-volume}Dt\267
├─gvfs-fuse-daemo───3*[{gvfs-fuse-daemo}]
├─gvfs-gdu-volume
├─gvfs-gphoto2-vo
├─gvfsd
├─gvfsd-burn
├─gvfsd-trash
├─hud-service───2*[{hud-service}]
├─indicator-appli───{indicator-appli}
├─indicator-datet─┬─{indicator-datet}
│ └─{indicator-datet}Dt\267
├─indicator-messa───{indicator-messa}Dt\267
├─indicator-print─┬─{indicator-print}
│ └─{indicator-print}Dt\267
├─indicator-sessi───2*[{indicator-sessi}]
├─indicator-sound─┬─{indicator-sound}
│ └─{indicator-sound}Dt\267
├─lightdm─┬─Xorg
│ ├─lightdm─┬─gnome-session─┬─bluetooth-apple─┬─{bluetooth-apple}
│ │ │ │ └─{bluetooth-apple}Dt\267
│ │ │ ├─deja-dup-monito───2*[{deja-dup-monito}]
│ │ │ ├─gdu-notificatio───2*[{gdu-notificatio}]
│ │ │ ├─gnome-fallback-───2*[{gnome-fallback-}]
│ │ │ ├─gnome-screensav───2*[{gnome-screensav}]
│ │ │ ├─gnome-settings-───2*[{gnome-settings-}]
│ │ │ ├─metacity───3*[{metacity}]
│ │ │ ├─nautilus───2*[{nautilus}]
│ │ │ ├─nm-applet───2*[{nm-applet}]
│ │ │ ├─polkit-gnome-au─┬─{polkit-gnome-au}
│ │ │ │ └─{polkit-gnome-au}Dt\267
│ │ │ ├─ssh-agent
│ │ │ ├─telepathy-indic───2*[{telepathy-indic}]
│ │ │ ├─unity-2d-panel───2*[{unity-2d-panel}]
│ │ │ ├─unity-2d-shell───5*[{unity-2d-shell}]
│ │ │ ├─update-notifier─┬─{update-notifier}
│ │ │ │ └─{update-notifier}Dt\267
│ │ │ └─3*[{gnome-session}]
│ │ └─{lightdm}
│ └─2*[{lightdm}]
├─mission-control─┬─{mission-control}
│ └─{mission-control}Dt\267
├─modem-manager
├─polkitd───{polkitd}
├─pulseaudio─┬─gconf-helper
│ └─2*[{pulseaudio}]
├─rsyslogd───3*[{rsyslogd}]
├─rtkit-daemon───2*[{rtkit-daemon}]
├─system-service-
├─ubuntu-geoip-pr
├─udevd───2*[udevd]
├─udisks-daemon─┬─udisks-daemon
│ └─{udisks-daemon}
├─unity-applicati─┬─{unity-applicati}
│ └─{unity-applicati}Dt\267
├─unity-files-dae─┬─{unity-files-dae}
│ └─{unity-files-dae}Dt\267
├─unity-lens-vide───{unity-lens-vide}
├─unity-music-dae───{unity-music-dae}Dt\267
├─unity-musicstor───{unity-musicstor}Dt\267
├─unity-panel-ser─┬─{unity-panel-ser}
│ └─{unity-panel-ser}Dt\267
├─unity-scope-vid─┬─{unity-scope-vid}
│ └─{unity-scope-vid}Dt\267
├─upowerd───2*[{upowerd}]
├─upstart-socket-
├─upstart-udev-br
├─whoopsie───{whoopsie}
├─zeitgeist-daemo───{zeitgeist-daemo}
├─zeitgeist-datah───{zeitgeist-datah}
└─zeitgeist-fts─┬─cat
└─{zeitgeist-fts}
More information about the ‘pstree’ command can be fetched from its man page- C – The CPU usage and scheduling information. The value is incremented with every tick of the system clock, however degraded by the scheduler by dividing it by two in every second. Therefore, A higher value indicates CPU intensive process.
- STIME – The start time of the process.
- TTY – The terminal type associated with the process. If this value is ‘?’, then it means the process is not associated with any terminal. These are daemon process, which we shall be discussing in the next section.
- TIME – The cumulative CPU time since the process is running.
- CMD – The command which launched the process.
Types of Processes
Although there is no standard classification of types of processes in Linux. The segregation could be in interactive and non-interactive processes, foreground and background processes or daemon or batch processes. It can also be classified based on the status of the processes such as as zombie processes. It is good enough if we comprehend all these various terminologies in the linux system.Interactive processes
An interactive process is one which needs user’s interaction while it is active. For example, when we launch a vi-editor, it is an interactive process. Another example could be the telnet command. Hence, the interactive processes have to be associated to a terminal.Under the umbrella of interactive processes, we have Foreground and Background processes.
Lets discuss them one by one :
Foreground Process
A process is a foreground process if it is in focus and can be given input from the standard input. It blocks the shell until the foreground process is complete. When we run our commands on the terminal, they generally run as foreground processes. They block the terminal until it is complete. Although most of our linux commands are quick enough for us to even realise that.Let us create our own program which sleeps for 10 seconds and then ourselves experience what waiting for the foreground process feels like.
The C source looks like:
#includeNow compile and run the program,#include int main() { int time = 10; sleep(time); printf("Slept for 10 secs\n"); return 0; }
$ gcc wait_process.c -Wall -o wait_process $ ./wait_processWhat do you experience? The terminal is blocked by the running process, and not letting the user to do anything until the program is complete.
To check another foreground blocking, open the geditor through the terminal. The terminal launches the geditor, won’t let you input anything to the terminal until we terminate the geditor.
Background Process
Background processes are ones, that are running, but in the background, not taking any user input from the terminal. It doesn’t block the terminal, and allows us to use the terminal irrespective of the background process is complete or not. They key-character to make any new process to be run in background is ‘&’.How we use this character, is by suffixing it with the command, as in,
&It is time, to run our ‘wait_process’ program to run as a background, so that we can avoid the terminal to get blocked while the process is sleeping.
$ ./wait_process & [1] 2534 $Whoahh! we get the command line back, to be able to use it and not to worry about the sleeping process.
To get these background and foreground handy, linux provides certain commands to view what is running and also switch any foreground process to background and vice versa.
One can use command ‘jobs’ to see what all is running associated with the terminal.
$gedit wait_process.c& [1] 2538 $./wait_process& [2] 2546 $jobs [1]- Running gedit wait_process.c & [2]+ Running ./wait_process & $In the above exercise, we started two background processes – gedit and the wait_process program. Hence, the command ‘jobs’ lists both of them along with their PID’s.
If we want to switch the ‘geditor’ as a foreground process, use linux command ‘fg’
$fgWe see following after running the ‘fg’ command
gedit wait_process.cAnd we again lose the command prompt. No points for guessing, now gedit is running as a foreground process. However, to switch it back as a background process, use key combinations ‘Ctrl + Z’ to suspend the process and then run the linux command ‘bg’
Note: When we suspend the process, it is not running and hence on resuming it will start with the same status, when it was stopped.
^Z [1]+ Stopped gedit wait_process.c $ bg [1]+ gedit wait_process.c & $We got the command prompt back. We need to confirm if the geditor process is still up and running. It can be done by again using the command ‘jobs’
$jobs [1]+ Running gedit wait_process.c &
Batch processes
These are the processes which are queued in and executed one by one in FIFO (First In First Out). Batch processes are not associated with any terminal instead given to the system to run, preferably when the system load is low or at a specific time. Low system load is a relative term, and hence it depends on the system and the type and requirements of the batch processes.There are two linux commands which are provided to create the batch processes:
Linux at command
Usage, taken from man pageat [-V] [-q queue] [-f file] [-mldbv] TIME at [-V] [-q queue] [-f file] [-mldbv] -t time_arg at -c job [job...] atq [-V] [-q queue] atrm [-V] job [job...]The ‘at’ command is used to schedule a process at a specific date/time. The time of today’s day is taken in the format HH::MM. However, it also accepts phrases like ‘today’, ‘tomorrow’, ‘teatime’, etc.
When we execute the ‘at’ command, we reach the ‘at’ prompt, where we can queue all the tasks/commands/programs in an order. When we are done with the queueing of the task, press key combination ‘Ctrl +d’. On pressing the key combination ‘Ctrl + d’, we see ‘EOT’ displayed at the standard output, following which we are back to our command prompt.
As an example,
$ at tomorrow warning: commands will be executed using /bin/sh at> ./wait_process at> job 1 at Sun Mar 10 20:41:00 2013
Linux batch command
Usage, taken from man pagebatchThis command launches the process when the system load is low i.e. load average drops below 0.8, or the value set by atd
It’s usage example is similar to the command ‘at’ (explained above).
Daemon Processes
A daemon process in Linux is also one of its kind which runs in background. However, what is different here is, daemon processes are not associated to any terminal in any way. Therefore such processes don’t take interact with the user. A widespread example of daemon process is a server service of any kind. For example, if we consider a mail server, it just have to listen to the relevant ports and respond with its protocol routines on receiving packages. So, such kind of processes can be run as daemon processes, independent of terminal and user interaction.Generally, when we code any program in C in Linux and execute it, the terminal becomes its parent process. Hence, in order to develop a daemon process service, the programmer needs to detach the process from its parent process. This is done by killing its parent process, which makes the process independent of the terminal, but controlled by its grandparent process i..e init process.
We shall learn more about coding a daemon process in the second part of the article.
Zombie Processes
When a process terminates, there is a proper exit and cleanup routine to be done by the developer of the program. If there is a bug, such that cleanup could not happen appropriately, though the process has been killed. Now, this process do occupy some memory, but will never be scheduled by the scheduler as the process status is ‘terminated’.Such processes are called zombie processes, which are killed but still exist.
Zombie processes are generally harmless if there are not much. However, if we have a whole lot of zombie processes lingering, then it could be a pain. Since, the PID’s still taken up by the zombie processes are not available for re-allocation to new processes. Hence, soon the system would be out of available PID’s if the zombie processes keep on increasing, and no new process would be able be launch.
The init process
The init process is the one which initiates system processes taken from the script/etc/inittaband has been assigned PID = 1. So, these system processes include setting up the user space, mounting file systems, set up everything to get the system up and running. Worth mentioning, it is the init process which is at the apex of the complete process tree. It is run as root, and is the parent process of a user shell. It is the last sequence in the booting and is the one, which launches and controls the shutdown.
Process States
Linux processes generally go through six major states, which are listed below:1. Running or Runnable ( R ) – A running state has a broader concept here. Running always does not mean utilising the CPU. Even while a process is ready to run, the state is running state.
Hence, there are two sub-states, when the process is queued in the ready queue to run and when the process is actually being executed, it is in the executing sub-state as has been scheduled by the scheduler.
2. Stopped (T) – If a running process receives a stop signal, it is moved to the stopped state. A process can also be in stopped state if it has been halted by a trace while debugging. .
3. Uninterruptible sleep (D) – It is a sleeping state, process has been blocked. Mostly, process goes into an uninterruptible sleep during an IO operation.
4. Interruptible sleep (S) – It is a sleeping state i.e. a blocking state where the process is waiting for an event to occur.
5. Zombie/Defunct state(Z) – It is the process state in which process has been terminated but not reaped by its parent process.
6. Dead (X) – A process never reaches this state, as as soon as it is dead, it is gone.
Note: 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.
In order to check the current status of the active processes at a moment on the linux system, we again use the ‘ps’ command but with a different set of options.
$ps axIn my system, I again get a huge list of processes. However, we are interested in the status details of the listed processes, therefore here is a snapshot of the output
PID TTY STAT TIME COMMAND 1779 pts/0 Ss 0:00 bash 1797 ? Sl 0:00 telepathy-indicator 1805 ? Sl 0:00 /usr/lib/telepathy/mission-control-5 1809 ? Sl 0:00 /usr/lib/gnome-online-accounts/goa-daemon 1851 ? Sl 0:03 gnome-screensaver 1891 ? Sl 0:04 update-notifier 1936 ? S 0:00 /usr/bin/python /usr/lib/system-service/system-service-d 1940 ? Sl 0:00 /usr/lib/deja-dup/deja-dup/deja-dup-monitor 2091 ? SNl 0:00 deja-dup --prompt 2129 ? Sl 21:54 /usr/lib/firefox/firefox 2145 ? Sl 0:00 /usr/lib/i386-linux-gnu/at-spi2-core/at-spi-bus-launcher 2724 ? S 0:00 /usr/lib/cups/notifier/dbus dbus:// 2801 ? SNl 0:22 /usr/bin/python /usr/bin/update-manager --no-focus-on-map 3157 ? S 0:00 [kworker/0:1] 3168 ? S 0:00 [kworker/0:2] 3207 ? S 0:00 [kworker/0:0] 3208 pts/0 R+ 0:00 ps axObserver the status of the list of processes under column ‘STAT’. Although, we have more confidence on Linux commands than anything, but it is exciting to confirm from the process ‘ps’ (at the end). Its status is ‘R+’ as we can read from the above output snapshot, which indicates it is running and running as a foreground process. So true!
Real time snapshot of processes
The ‘ps’ command that we just discussed in the previous section, evinces the active process list at a particular moment when the command is executed. In many cases, it is a need of the hour to view dynamic real time running of the processes. For such circumstances, linux comes with the ‘top’ command.The ‘top’ usage looks like
top -hv | -bcHisS -d delay -n iterations -p pid [, pid ...]More details can be found at its man page
To see how and what all it provides, here is an output snapshot from my ubuntu system
The command:
$topOutput:
top - 10:16:34 up 1 day, 17:36, 1 user, load average: 1.51, 0.99, 0.84
Tasks: 136 total, 1 running, 135 sleeping, 0 stopped, 0 zombie
Cpu(s): 28.8%us, 3.3%sy, 0.0%ni, 67.9%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 507536k total, 498516k used, 9020k free, 10488k buffers
Swap: 521212k total, 187996k used, 333216k free, 91572k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2129 ubuntu 20 0 727m 258m 10m S 27.1 52.1 113:12.30 firefox
918 root 20 0 116m 35m 2128 S 3.3 7.1 7:33.90 Xorg
1772 ubuntu 20 0 89352 5264 2940 S 0.7 1.0 0:08.13 gnome-terminal
1496 ubuntu 20 0 142m 3036 1920 S 0.3 0.6 1:20.55 metacity
3782 root 20 0 0 0 0 S 0.3 0.0 0:00.57 kworker/0:0
3800 ubuntu 20 0 2836 1152 880 R 0.3 0.2 0:00.05 top
1 root 20 0 3512 1160 604 S 0.0 0.2 0:00.66 init
2 root 20 0 0 0 0 S 0.0 0.0 0:00.02 kthreadd
3 root 20 0 0 0 0 S 0.0 0.0 0:07.52 ksoftirqd/0
5 root 20 0 0 0 0 S 0.0 0.0 0:00.36 kworker/u:0
6 root RT 0 0 0 0 S 0.0 0.0 0:00.00 migration/0
7 root RT 0 0 0 0 S 0.0 0.0 0:02.89 watchdog/0
8 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 cpuset
9 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 khelper
10 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kdevtmpfs
11 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 netns
12 root 20 0 0 0 0 S 0.0 0.0 0:00.79 sync_supers
13 root 20 0 0 0 0 S 0.0 0.0 0:00.02 bdi-default
14 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 kintegrityd
15 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 kblockd
16 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 ata_sff
17 root 20 0 0 0 0 S 0.0 0.0 0:00.03 khubd
18 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 md
21 root 20 0 0 0 0 S 0.0 0.0 0:00.12 khungtaskd
22 root 20 0 0 0 0 S 0.0 0.0 0:08.45 kswapd0
23 root 25 5 0 0 0 S 0.0 0.0 0:00.00 ksmd
24 root 20 0 0 0 0 S 0.0 0.0 0:00.00 fsnotify_mark
25 root 20 0 0 0 0 S 0.0 0.0 0:00.00 ecryptfs-kthrea
26 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 crypto
34 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 kthrotld
35 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kworker/u:2
36 root 20 0 0 0 0 S 0.0 0.0 0:00.08 scsi_eh_0
38 root 20 0 0 0 0 S 0.0 0.0 0:00.00 scsi_eh_1
39 root 20 0 0 0 0 S 0.0 0.0 0:00.03 scsi_eh_2
Checking out running the linux ‘top’ command, we’ll observe that the
output keeps on changing with every moment, as the process dynamics keep
on changing at real time. This is possible, as the ‘top’ command is
running all this while and monitoring the running tasks. Hence, we need
to terminate the ‘top’ process to get back the command prompt.Analysing and trying to understand the ‘top’ command output, let us go line by line.
top - 10:16:34 up 1 day, 17:36, 1 user, load average: 1.51, 0.99, 0.84Going in order from left to right, it indicates -
- current time
- time since the machine is on
- the number of users logged in
- average load on the system specifying three values for last one minute, last five minutes and last fifteen minutes respectively.
Tasks: 136 total, 1 running, 135 sleeping, 0 stopped, 0 zombieIt helps us know how many total processes are there, which are 136 in our case, which includes one in the running state, 135 are in the sleeping state, zero in the stopped and zero zombie processes.
Now we know what all processes have been launched and are in which states.
Moving on,
Cpu(s): 28.8%us, 3.3%sy, 0.0%ni, 67.9%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%stThis row indicates the CPU usage in percentages. As in,
- 28.8%us – CPU percentage for user processes
- 3.3%sy – CPU percentage for system processes
- 0.0%ni – CPU percentage for processes with ‘nice’ priority
- 67.9%id – idle CPU percentage
- 0.0%wa – CPU percentage for processes waiting for I/O
- 0.0%hi – CPU percentage for processes serving hardware interrupts
- 0.0%si – CPU percentage for processes serving software interrupts
- 0.0%st – It is the steal time i.e. percentage CPU time stolen from a virtual machine i.e. time in an involuntary wait by a virtual machine, while the hypervisor is servicing another process.
Mem: 507536k total, 498516k used, 9020k free, 10488k buffersThe information is in terms of total physical memory available, where how much has been used, how much is free and how much is used for buffers.
On similar lines, the next output states usage of swap space
Swap: 521212k total, 187996k used, 333216k free, 91572k cachedThen following rows list all the details of all the launched processes at real time. What each detail means is:
- PID – The process ID
- USER – The user which owns the process
- PR – Process priority value
- NI – The nice value of a process.
- VIRT – The virtual memory used by the process
- RES – Physical memory used
- SHR – Shared memory of the process
- S – The status of the process where S – Sleeping : R – Running : Z- Zombie
- %CPU – Percentage CPU utilization of the process
- %MEM – Percentage memory usage of the process
- (TIME+) – The total activity time of the process
- COMMAND – The command used to launch the process.
Terminating the processes
The linux system allows its users to terminate any process, of course with due considerations to access permissions. The linux user can terminate a process using command ‘kill’.The usage syntax
kill [ -signal | -s signal ] pid ...
kill [ -L | -V, --version ]
kill -l [ signal ]
More details found in its man pageThe ‘kill’ linux command sends a signal to the specified process. Which process to send the signal is specified by its PID.
There are standard numbers assigned to each set of signals. We can get the information of what number is corresponding to which signal using the same ‘kill’ command through ‘-l’ option.
$ kill -l 1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM 16) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP 21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ 26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR 31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2 63) SIGRTMAX-1 64) SIGRTMAXThe most commonly used signal to terminate processes is
9) SIGKILLNow, as an example experience, we shall be creating a process and terminate it.
We’ll use our old program source, and its executable.
$ ./wait_process & [1] 3315 $ ps PID TTY TIME CMD 1779 pts/0 00:00:00 bash 3315 pts/0 00:00:00 wait_process 3316 pts/0 00:00:00 ps $ kill -9 3315 [1]+ Killed ./wait_process $ ps PID TTY TIME CMD 1779 pts/0 00:00:00 bash 3317 pts/0 00:00:00 psIn the above practical exercise, we ran our waiting process, and killed it before it got complete.
References
http://www.advancedlinuxprogramming.comhttp://careers.directi.com/display/tu/Understanding+Processes+in+Linux
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