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`Unix / Linux - Processes Management
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`In this chapter, we will discuss in detail about process management in Unix. When you
`execute a program on your Unix system, the system creates a special environment for that
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`2
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`

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`program. This environment contains everything needed for the system to run the program
`as if no other program were running on the system.
`
`Whenever you issue a command in Unix, it creates, or starts, a new process. When you
`tried out the Is command to list the directory contents, you started a process. A process, in
`simple terms, is an instance of a running program.
`
`The operating system tracks processes through a five-digit ID number known as the pid or
`the process ID. Each process in the system has a unique pid.
`
`Pids eventually repeat because all the possible numbers are used up and the next pid rolls
`or starts over. At any point of time, no two processes with the same pid exist in the system
`because it is the pid that Unix uses to track each process.
`
`Starting a Process
`
`When you start a process (run a command), there are two ways you can run it —
`
`Foreground Processes
`
`o Background Processes
`
`Foreground Processes
`By default, every process that you start runs in the foreground. It gets its input from the
`keyboard and sends its output to the screen.
`
`You can see this happen with the Is command. If you wish to list all the files in your
`current directory, you can use the following command —
`
`$1s ch*.doc
`
`This would display all the files, the names of which start with ch and end with .doc —
`
`ch01-1.doc ch010.doc ch02.doc
`
`ch03-2.doc
`
`ch04-1.doc ch040.doc ch05.doc
`
`ch06-2.doc
`
`ch01-2.doc ch02-1.doc
`
`The process runs in the foreground, the output is directed to my screen, and if the Is
`command wants any input (which it does not), it waits for it from the keyboard.
`
`While a program is running in the foreground and is time-consuming, no other commands
`can be run (start any other processes) because the prompt would not be available until the
`program finishes processing and comes out.
`
`Background Processes
`A background process runs without being connected to your keyboard. If the background
`process requires any keyboard input, it waits.
`
`3
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`

`

`The advantage of running a process in the background is that you can run other
`commands; you do not have to wait until it completes to start another!
`
`The simplest way to start a background process is to add an ampersand (&) at the end of
`the command.
`
`$1s ch*.doc &
`
`This displays all those files the names of which start with ch and end with .doc —
`
`ch01-1.doc
`
`ch010.doc ch02.doc
`
`ch03-2.doc
`
`ch04-1.doc
`
`ch040.doc ch05.doc
`
`ch06-2.doc
`
`ch01-2.doc ch02-1.doc
`
`Here, if the Is command wants any input (which it does not), it goes into a stop state until
`we move it into the foreground and give it the data from the keyboard.
`
`That first line contains information about the background process - the job number and the
`process ID. You need to know the job number to manipulate it between the background
`and the foreground.
`
`Press the Enter key and you will see the following —
`
`[1] + Done
`
`is ch*.doc &
`
`$
`
`The first line tells you that the Is command background process finishes successfully. The
`second is a prompt for another command.
`
`Listing Running Processes
`
`It is easy to see your own processes by running the ps (process status) command as
`follows -
`
`$ps
`
`PID
`
`TTY
`
`TIME
`
`CMD
`
`18358
`
`ttyp3
`
`00:00:00
`
`sh
`
`18361
`
`ttyp3
`
`00:01:31
`
`abiword
`
`18789
`
`ttyp3
`
`00:00:00
`
`ps
`
`One of the most commonly used flags for ps is the -f ( f for full) option, which provides
`more information as shown in the following example —
`
`$ps -f
`
`UID
`
`PID PPID C STIME
`
`TTY TIME CMD
`
`amrood 6738 3662 0 10:23:03 pts/6 0:00 first_one
`
`amrood 6739 3662 0 10:22:54 pts/6 0:00 second_one
`
`amrood
`
`3662 3657 0 08:10:53 pts/6 0:00 -ksh
`
`amrood 6892 3662 4 10:51:50 pts/6 0:00 ps -f
`
`4
`
`

`

`Here is the description of all the fields displayed by ps -f command —
`
`Sr.No.
`
`Column & Description
`
`1
`
`2
`
`3
`
`4
`
`5
`
`6
`
`7
`
`8
`
`UID
`
`User ID that this process belongs to (the person running it)
`
`PID
`
`Process ID
`
`PPID
`
`Parent process ID (the ID of the process that started it)
`
`C
`
`CPU utilization of process
`
`STIME
`
`Process start time
`
`TTY
`
`Terminal type associated with the process
`
`TIME
`
`CPU time taken by the process
`
`CMD
`
`The command that started this process
`
`There are other options which can be used along with ps command —
`
`Sr.No.
`
`1
`
`Option & Description
`
`-a
`
`Shows information about all users
`
`5
`
`

`

`2
`
`3
`
`4
`
`-x
`
`Shows information about processes without terminals
`
`-u
`
`Shows additional information like -f option
`
`-e
`
`Displays extended information
`
`Stopping Processes
`
`Ending a process can be done in several different ways. Often, from a console-based
`command, sending a CTRL + C keystroke (the default interrupt character) will exit the
`command. This works when the process is running in the foreground mode.
`
`If a process is running in the background, you should get its Job ID using the ps
`command. After that, you can use the kill command to kill the process as follows -
`
`$ps -f
`
`UID
`
`PID PPID C STIME
`
`TTY TIME CMD
`
`amrood 6738 3662 0 10:23:03 pts/6 0:00 first_one
`
`amrood 6739 3662 0 10:22:54 pts/6 0:00 second_one
`
`amrood
`
`3662 3657 0 08:10:53 pts/6 0:00 -ksh
`
`amrood 6892 3662 4 10:51:50 pts/6 0:00 ps -f
`
`$kill 6738
`
`Terminated
`
`Here, the kill command terminates the first_one process. If a process ignores a regular
`kill command, you can use kill -9 followed by the process ID as follows —
`
`$kill -9 6738
`
`Terminated
`
`Parent and Child Processes
`
`Each unix process has two ID numbers assigned to it: The Process ID (pid) and the Parent
`process ID (ppid). Each user process in the system has a parent process.
`
`Most of the commands that you run have the shell as their parent. Check the ps -f
`example where this command listed both the process ID and the parent process ID.
`Zombie and Orphan Processes
`
`6
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`

`

`Normally, when a child process is killed, the parent process is updated via a SIGCHLD
`signal. Then the parent can do some other task or restart a new child as needed. However,
`sometimes the parent process is killed before its child is killed. In this case, the "parent of
`all processes," the init process, becomes the new PPID (parent process ID). In some
`cases, these processes are called orphan processes.
`
`When a process is killed, a ps listing may still show the process with a Z state. This is a
`zombie or defunct process. The process is dead and not being used. These processes are
`different from the orphan processes. They have completed execution but still find an entry
`in the process table.
`
`Daemon Processes
`
`Daemons are system-related background processes that often run with the permissions of
`root and services requests from other processes.
`
`A daemon has no controlling terminal. It cannot open /dev/tty. If you do a "ps -ef" and
`look at the tty field, all daemons will have a ? for the tty.
`
`To be precise, a daemon is a process that runs in the background, usually waiting for
`something to happen that it is capable of working with. For example, a printer daemon
`waiting for print commands.
`
`If you have a program that calls for lengthy processing, then it's worth to make it a
`daemon and run it in the background.
`
`The top Command
`
`The top command is a very useful tool for quickly showing processes sorted by various
`criteria.
`
`It is an interactive diagnostic tool that updates frequently and shows information about
`physical and virtual memory, CPU usage, load averages, and your busy processes.
`
`Here is the simple syntax to run top command and to see the statistics of CPU utilization
`by different processes -
`
`$top
`
`Job ID Versus Process ID
`
`Background and suspended processes are usually manipulated via job number (job ID).
`This number is different from the process ID and is used because it is shorter.
`
`In addition, a job can consist of multiple processes running in a series or at the same time,
`in parallel. Using the job ID is easier than tracking individual processes.
`
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