README.md

Process Management

• Process info:

  ps
  # print all running processes for current user
  
  ps aux
  # shows all running processes
  
  ps -eH
  # -e for processes of all users
  # -H for hierarchical format
  
  pstree
  # easier view for process hierarchy
  
  top
  # real-time updated view of processes and resources consumed

• Foreground & background processes:

  • We can run only one foreground process at a time in a shell; however, we can run multiple background processes at the same time:

    xeyes
    # foreground process
    # this does not allow to run other commands
    
    xeyes &
    # background process

  • We can also move a running process from foreground to background, and vice-versa:

    xeyes
    # foreground
    
    # Ctrl+Z to suspend/pause process
    
    jobs
    # shows suspended processes
    
    bg
    # move job to background
    
    jobs
    # xeyes is now running in background
    
    fg
    # move job to foreground
    
    # for fg and bg, we can also use job id
    # in case of multiple jobs

• Managing processes:

  • A process can be in one of these states:

    • Running - CPU executing a process

    • Sleeping - waiting on a resource; when it gets the resource, it will get into running state again

    • Stopped - when stop signal is sent to running process, CPU stops executing the process

    • Terminated - cause a process to die

    • Zombie - remains of the process when it was not cleaned up after dying; does not respond to normal signals used to shut a process down

  • Processes change their states in response to signals; kill command used to send these signals to processes:

    kill -l
    # shows full list of signals
    
    # SIGHUP used to signal to process that it should reread its config
    
    # SIGSTOP used to stop a process from running; Ctrl+Z
    
    # SIGTERM is default signal sent by kill to terminate process
    
    # SIGKILL used if process does not respond to SIGTERM
    
    xeyes &
    # program running in background
    
    ps -ef | grep xeyes
    # shows PID of process
    
    kill 55092
    # SIGTERM sent to process, terminated
    
    kill -9 55092
    # SIGKILL can be sent as well, if process does not respond to SIGTERM
    
    # pkill can be used if we do not know pid
    # kills any process which matches the criteria
    
    pkill xeyes
    # kills multiple instances of program
    
    sleep 5
    # pauses execution in shell for some time, adds delays

• Scheduling processes:

  less /etc/crontab
  # contains processes scheduled at system level
  
  crontab -e
  # personal crontab file
  
  # follow format 'm h dom mon dow  command'
  
  5 1 2 * * touch /home/bob/cron/crontab-ran.txt
  # runs the command at 1:05 every second day of each month
  
  */5 * * * * touch /home/bob/cron/crontab-ran.txt
  # runs the command every 5 minutes
  
  crontab -l
  # prints crontab file contents
  
  crontab -r
  # deletes cronjobs

  # init.d used to run scripts at system bootup
  
  ls -la /etc/init.d
  # view scripts running at startup
  
  ls -la /etc/rc*.d
  # shows different run-levels