Displaying Processes in the System
- Static view (ps command)
- Dynamic view (top command)
- displays the processes for the current shell.
- We can display all of the processes owned by the current user by using the x option.
State of the Process
- A column in output is shown that is STAT.
- This column shows the current state of the process.
|S||Sleeping or waiting for an event such as keystroke|
|D||Uninterruptible Sleep. Process is waiting for I/O such as a disk drive|
|Z||A difunctional or zombie process|
|<||A high priority process|
|N||A low priority process|
Option with ps command
|-A or -e||Display every active process on a Linux system
|-x||Display all of the processes owned by the user|
|-F||Perform a full-format listing|
|-U||Select by real user ID or name|
|-u||Select by effective user ID or name|
|-p||Select process by pid|
|-r||Display only running processes|
|-L||Show number of threads in a process|
|-G||Show processes by Group|
- Display processes that are related to the user ubuntu
- Select a process with id 1302
- The top command is the traditional way to view your system’s resource usage and see the processes that are taking up the most system resources.
- Top displays a list of processes, with the ones using the most CPU at the top.
- An improved version of top command is htop but it is usually not pre-installed in most distributions.
- When a top program is running, we can highlight the running programs by pressing z.
- we can quit the top program by press q or Ctrl + c.
Option with top command
|-u||display specific User process details|
|-d||To set the screen refresh frequency|
Displaying processes in Tree like structure
- pstree command is used to display processes in tree-like structure.
- showing the parent/child relationships between processes.
Interrupting A Process
- A program can be interrupted by pressing Ctrl + C.
- This will interrupt the given processes and stop the process.
- Ctrl+C essentially sends a SIGINT signal from the controlling terminal to the process, causing it to be killed
Putting a Process in the Background
- A foreground process is any command or task you run directly and wait for it to complete.
- Unlike with a foreground process, the shell does not have to wait for a background process to end before it can run more processes.
- Normally, we start a program by entering its name in the CLI. However, if we want to start a program in background, we will put an & after its name.
- open the gedit program (gedit) as foreground process.
- Now start again gedit as a background process.(gedit &)
- To list the jobs that have been launched from our terminal.
- Using the jobs command, we can see this list.
we first launch two jobs in background(gedit &,firefox &) and then use the jobs command to view the running jobs
Bringing a process to the foreground
- A process in the background is immune from keyboard input, including any attempt to interrupt it with a Ctrl-c.
- fg command is used to bring a process to the foreground.
- start the gedit editor in background(gedit &).
- Then use the jobs command to see the list of jobs launched and then, bring this process to the foreground(fg %1).
Killing a process
- We can kill a process using the kill command.
- To kill a process, we provide the process id as an argument.
we start the gedit program and then kill it using kill command.
- The kill command doesn’t exactly “kill” processes, rather it sends them signals.
- Signals are one of several ways that the operating system communicates with programs.
- Programs listen for signals and may act upon them as they are received.
Common signals with kill command
Following are most common signals that can be send with kill command.
|INT||INT Interrupt. Performs the same function as the Ctrl-c key sent from the terminal. It will usually terminate a program.|
|TERM||Terminate. This is the default signal sent by the kill command. If a program is still “alive” enough to receive signals, it will terminate.|
|STOP||Stop. This signal causes a process to pause without terminating.|
|CONT||Continue. This will restore a process after a STOP signal.|
Pausing a process
- Linux allows you to pause a running process rather than quitting or killing it.
- Pausing a process just suspends all of its operation so it stops using any of your processor power even while it still resides in memory.
- This may be useful when you want to run some sort of a processor intensive task, but don’t wish to completely terminate another process you may have running.
- Pausing it would free up valuable processing time while you need it, and then continue it afterwards.
kill command with STOP option
- We can pause a process by using kill command with STOP option.
- The process id is required as an argument in kill command.
- Start the gedit program in the background(gedit &).
- Find the pid of gedit using ps command(ps).
- Pause the process using kill command(kill –STOP 5008).
- Resume the process by using the kill command with CONT option(kill –CONT 5008).
Changing process priority
- Every running process in Linux has a priority assigned to it.
- We can change the process priority using nice and renice utility.
- Nice command will launch a process with a user defined scheduling priority.
- Renice command will modify the scheduling priority of a running process.
In Linux system priorities are 0 to 139
- 0 to 99 for real time
- 100 to 139 for users.
nice value range is -20 to +19
- -20 is highest
- 0 default
- +19 is lowest.
Relation between nice value and priority is:
PR = 20 + NI
So, the value of PR = 20 + (-20 to +19) is 0 to 39 that maps to 100-139
- we start the gedit program in background and see its nice value using ps command(ps -l).
- Now, we start the gedit program again using nice command. (nice 10 gedit &).
- 3.Then we change its priority using renice command. (sudo renice -n -12 -p 5161)
Change the priority of all of the processes belonging to a user or a group
- We can also use the renice command to change the priority of all of the processes belonging to a user or a group.
- e.g: process belonging to user computer(sudo renice -n -12 -u computer).
- Display all of the process in current shell
- Display every active process in the system
- Provide a full-format listing of process owned by you
- Display a full-format listing of processes owned by user ubuntu
- Display a process with id 1
- Display the dynamic view of the current processes in the system and set the refresh interval 0.5 second
- Display the dynamic view of the processes owned by user with id 999 (or name ubuntu)
- Start the gedit program in background and then bring it foreground
- Suspend the gedit program 1
- Resume the gedit program
- Start the gedit program with priority 90
- Reset the priority of gedit to 65
Related Links to Operating System topics
Operating system Course content
- Operating System-Functions and History
- Generations of Operating System
- Functions of an Operating System
- Components of Operating System
- Types of Operating System
- Services of Operating System
- Properties of Operating System
- Processes in Operating System
- Process Scheduling in Operating System
Lab Practice Task
- Introduction to Linux Ubunto
- Installation with virtual Box
- Writing Linux Commands
- Navigation in File System and Directory Management in Ubunto using CLI
- File Handling and I/O Redirection In Ubunto
- File Access Permission in Linux
- Text Processing Tools and Basic System Configuration Tools in Linux
- Package Management in Linux
- How to manage processes in Linux
- Compiling and Executing C++ programs in Linux
- System Calls
- Introduction To Shell Programming
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