Jobs are programs only but they hold the terminal in which they run “hostage” since they may run for a long time, we can’t use the terminal for the period in which they are running in the foreground. We can send them to background, or make them keep running even after the terminal is closed.
Ctrl + Z suspends (stops) a running program and sends it to background, such that the terminal is free to be interacted to by the user.
jobs list all jobs attached to the current terminal/shell
$ jobs
[1]- Running xeyes &
[2]+ Running xclock &
[3]- Stopped xeyes
The + in the left side tells us that the job will be acted upon if no number is supplied with %<n> in job related commands. The & at the rightmost side tells us that the job is running in the background.
bg %1 send job number 1 to background; the terminal becomes usable
fg %1 bring job number 1 to foreground; the terminal becomes un-usable
<command> & run the command in background
nohup <command> keep running the command even if the parent terminal is closed (no hangup)
kill %1 kill job number 1; sends a termination request to it (default)
ps list all running processes in the current shell and their info
ps -aux same as above but has more info like CPU usage, memory usage, etc…
top shows real-time stats on processes (refreshes every 10 sec)
top -p 1 show info for only process ID 1
kill <PID> send a termination request to the process (SIGTERM = 15)
kill -9 <PID> force kill the process (SIGKILL = 9)
ps -p 1 is the first process (PID = 1) that runs on the system. It is nothing but init! Either Systemd or SysV.
nice -n 15 <PID> adjust niceness value of a process (in range [-20, 19]; lesser the niceness, more resources will be allowed to be hogged by the process; only sudo users can assign a negative niceness to a process)
renice -n 15 <PID> reassigning niceness value; see nice value in NI field of the top or ps command output
run programs/commands with a niceness value set:
$ ls # niceness = 0 (default)
$ nice ls # niceness = 10
$ nice -n 1 ls # niceness = 1
It is a technique used to create multiple terminals in a primary one. We can detach (a lot like bg), reattach, and split terminals.
screen launches the current terminal as a “screen”; press Enter and you’ll notice no difference but we can press shortcuts now
Ctrl + A D: detach; create a new screen
screen -ls list all screens
screen -r <screen_name>: re-attach
tmux (newer and better, but often not pre-installed on distros)