Processes and jobs

Processes

• Process: instance of 1 or more related tasks (threads) executing on computer. Can be independent of or dependent on other processes. Kernel allocates share of resources (memory, CPU, peripheral devices) to processes

Types of processes

Type	Description	Example
Interactive	Need to be started by user	bash, firefox
Batch	Automatic - scheduled & disconnected from terminal Queued and work on first-in, first-out (FIFO) basis	updatedb
Daemon	Run continuously. May launch during system startup	httpd, xinetd, sshd
Thread	Runs under umbrella of a main process, sharing memory & other resources, but scheduled & run by system on individual basis Individual thread can end without ending entire process New threads can be created at any time. Many programs now multithreaded	gnome-terminal, firefox
Kernel thread	Kernel tasks that users can't start or terminate Perform actions like moving a thread from 1 CPU to another, making sure I/O operations to disk are completed	kswapd, migration, ksoftirqd

Process scheduling and states

• When process in running state: either currently executing instructions on a CPU or waiting for a share time slice) from CPU in which to run.
• Critical kernel routing aka scheduler constantly shifts processes in and out of CPU, sharing time according to relative priority, how much time needed & how much already granted to a task
• Run queue for each CPU: each running process is in one of these queues

State	Description
Running	Process currently being executed by CPU
Sleeping	Process waiting for input from another process. Sits in a wait queue
Waiting	Process has received input, ready to execute & waiting for attention from CPU
Stopped	Process halted. Will not run again till a signal received
Zombie	Process dead. Takes up no resources. Retained until parent process allows it to disappear

Process and thread IDs

• OS gives each process a unique process ID (PID). Tracks state, CPU & memory usage, precisely where resources allocated in memory etc.
• New PIDs usually assigned in ascending order as processes spawned so PID = 1 is usually for init (initialization) or systemd process.

ID type	Description
Process ID (PID)	Unique process ID number
Parent process ID (PPID)	Process (parent) that started this process
Thread ID (TID)	Same as PID for single-threaded process For multi-threaded, each thread shares same PID but has unique TID

User and group IDs

• OS identifies different users' processes with Real User ID (RUID) assigned to each user
• Users can be grouped: Real Group ID (RGID)
• Effective owner given by Effective UID (EUID) (for user) and EGID (for group)
• ps auxw - shows user identity of running processes

The ps command

• ps -ef displays all processes
  • -e for every process
  • -f for full details
• ps -eLf also shows theads
• Can also use ps aux, ps GXO <list of properties to show>
• Look for a running process ps -ef | grep -i <search criteria>
• ps -U root shows processes run by root, -G to list processes by owning group

pstree

• pstree shows running processes in a tree format, showing relationships between child & parent processes. Threads displayed in curly braces

Terminate a running process

• kill -SIGKILL <pid> or kill <pid>
  • Have various different options such as SIGKILL, SIGTERM etc. See more info in man kill
• Or use top, see below

Terminate a running process graphically using xkill

• Kills connections to X Server (GUI). Good for e.g. applications that have hung, or undesired windows displayed
• xkill on its own brings up a pointer - select window to kill. Can also kill by X Identifier xkill -id <X resource number>

pidof and pgrep

• Get PID of specific process pidof cupsd
• pgrep -U root shows all processes owned by root, use -G to show for a particular group and use -l at the end to also show names of the processes

Process priority

• Nice values: -20 'highest' priority, 19 'lowest'
• 'Higher' priority means gets attention first from CPU, lower means has to wait for CPU attention
• Most system-started processes have value 0 priority
• Child processes inherit parent process' nice value
• ps -l displays nice values, in top shown in the NI column
• Renice a process: nice -n <value> <pid of process>
• Use nice -2 top to increase niceness of top (make it lower priority) or nice --10 top to give it a lower value/higher priority

Signals

• Show available signals with kill -l or kill -L
  • Can use the name or the number
  • Format: kill -<number>|-s <name>|--signal <name>
• Owning users can kill own processes, root can kill all processes
• Can kill multiple at once: kill <signal value> <pid1> <pid2> <pidN> or pkill <signal value> <name> or killall <name>
  • If you don't specify signal value, it uses default 15
• Most commonly used kill signal values:

Signal number	Signal name	Description
1	SIGHUP	Causes process to disconnect from a closed terminal it was tied to Also used to instruct running daemon to re-read config
2	SIGINT	Issues ^C (Control + C) to interrupt execution of process
9	SIGKILL	Kills process abruptly
15	SIGTERM	Sends soft termination signal to stop process in 'orderly fashion', but process can ignore it. Default

• https://unix.stackexchange.com/questions/2107/how-to-suspend-and-resume-processes

nohup signal

• To execute command (or program launched at command line) immune to hangup signals and will continue running in the background even if you log out:

nohup <command or program or script> &

• Can also use screen command

More info on signals

• https://en.wikipedia.org/wiki/Signal_(IPC)

Kill user sessions

• Use w to find active sessions & corresponding pts then e.g. pkill -9 -t pts/1
• Kill all sessions of specific user killall -u john
• https://linuxiac.com/how-to-terminate-user-session-in-linux/

Background and foreground processes (jobs)

• Foreground jobs (i.e. commands run from shell): only 1 can run at a time
• Can also run jobs (commands) in the background, usin the shell, so that can run other commands at the same time
  • E.g. updatedb &
  • View running jobs jobs
  • bg (background) and fg (foreground) commands - for graphical applications e.g. firefox bg. This will result in messages related to the running process being generated in the terminal screen so can be useful for troubleshooting
  • Cancel a job by using disown or kill %1 where %1 is job 1, see job number using jobs
• Background jobs run in a lower priority
• Control + Z suspends foreground job (equivalent to SIGTSTP signal), use fg to bring it back, Control + C cancels it (SIGINT). Can also use suspend
• Jobs are connected to the terminal window they are started from, will not show in a new or separate terminal
• Example:

pidstat -dhlrsu 2 >> pidstat.txt &

disown