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Process Management

Every process have

  • PID - process-id
  • PPID - parent process id

The shell process:

  • $$ - current shell PID

ps - process status

  • ps - process status
  • ps -f Full listing showing PPID of each process
  • ps [-e|-A] All processes including user and system processes
  • ps -u usr Processes of user usr only
  • ps -a Processes of all users exxcluding processes not associated with terminal
  • ps -l Long listing showing memory related information
  • ps -t term Processes running on terminal

Mechanism of process creation

  • fork - system call fork is called to fork current process. PID changed.
  • exec - the memory image is overwritten with new program.
  • wait - the parent executed wait system call for the child process to complete.

If the parent process dies earlier then the child process is called orphan. They are the adopted by init and killed. Processes whose parent don't wait for their death moves to zombies state. It remains in this state until there parent picks up there exit code. Deamons are the process who don't have a controlling terminal. Usually are system processes.

Types of commands

  • External commands - cat, ls
  • Shell scripts
  • Internal commands - cd, echo - can change the directory

Running jobs in background

  • & - using the & symbol at the end of a command. The process runs in the background.

e.g. sort emp.list &

  • nohup - no hangup - command to deattach a process from a terminal

e.g. nohup sort emp.list &

Job execution with low priority

  • nice command - runs a job at low priority
  • nice -n [level] command - increase the priority of the command by level which can be between 1 to 19.

Killing process with signals

  • kill PID - kills a process
  • kill -s [Signal] PID - send the signal to the process
    • Signal can be as folows:
    • KILL
    • kill -l - list all the signals

Job Control

A job is the name given to a group of processes.

Easiest way to create a job is by using pipeline of two or more commands.

  • bg - relegate a job to the background
  • fg - bring it back to the foreground
  • jobs - list the active jobs
  • Ctrl+z - suspend a foreground job
  • kill - kill a job

Use

  • suspend the job - use Ctrl+z
  • use bg to send it to background
  • now your job is running in the background
  • list the jobs using jobs
  • to bring a job to foreground use the fg command

fg

  • fg % - brings first job to foreground
  • fg %sort - brings sort job to foreground
  • bg %?perm - send to the background job containing string perm

  • bg %2 - sends second job to background

  • stty -a - see the current terminal config

    • start = ^q; stop = ^s; susp = ^z; dsusp = ^y;

  • lsof - List all open files and the process using them

  • lsof -itcp:4000 - Return the process listening on port 4000

  • jobs List all background jobs

  • jobs -p List all background jobs with their PID

Process priority

  • nice -n -20 foo Change process priority by name
  • renice 20 PID Change process priority by PID
  • ps -o ni PID Return the process priority of PID

Stopping Processes

  • CTRL+C Kill a process running in the foreground
  • kill PID Shut down process by PID gracefully. Sends TERM signal.
  • kill -9 PID Force shut down of process by PID. Sends SIGKILL signal.
  • pkill foo Shut down process by name gracefully. Sends TERM signal.
  • pkill -9 foo force shut down process by name. Sends SIGKILL signal.
  • killall foo Kill all process with the specified name gracefully.

Scheduling Tasks

* * * * *
Minute Hour Day of month Month Day of the week
  • crontab -l List cron tab
  • crontab -e Edit cron tab in Vim
  • crontab /path/crontab Load cron tab from a file

  • crontab -l > /path/crontab Save cron tab to a file

  • * * * * * foo Run foo every minute

  • */15 * * * * foo Run foo every 15 minutes
  • 0 * * * * foo Run foo every hour
  • 15 6 * * * foo Run foo daily at 6:15 AM
  • 44 4 * * 5 foo Run foo every Friday at 4:44 AM
  • 0 0 1 * * foo Run foo at midnight on the first of the month

  • 0 0 1 1 * foo Run foo at midnight on the first of the year

  • at -l List scheduled tasks

  • at -c 1 Show task with ID 1
  • at -r 1 Remove task with ID 1
  • at now + 2 minutes Create a task in Vim to execute in 2 minutes
  • at 12:34 PM next month Create a task in Vim to execute at 12:34 PM next month
  • at tomorrow Create a task in Vim to execute tomorrow