Process Management and Job Control

What is a process?

Every time you run a program, Linux creates a process. A process is one running instance of a program. Your web browser is a process, the terminal window is a process, and the ls command is a process that lasts a fraction of a second.

Each process gets a unique number called the PID (process ID). Linux and you use the PID to refer to one specific process.

Seeing what is running

ps: a snapshot

ps
ps aux
ps aux | grep firefox

• ps lists the processes started from your current terminal.
• ps aux lists processes from all users: a for all users, u for a detailed format and x to include processes that have no terminal.
• Piping into grep filters the list to one program.

Useful columns in ps aux are USER, PID, %CPU, %MEM and COMMAND.

top and htop: a live view

top
htop

top shows a list that updates every few seconds, with the busiest processes at the top. Press q to quit. htop is a friendlier version that you may need to install. When your computer feels slow, this is where to look first.

pgrep: find a PID by name

pgrep firefox

pgrep prints the PID of every process whose name matches.

Stopping processes

kill 1234
kill -9 1234
killall firefox

• kill PID sends the process a polite request to stop, called SIGTERM. The program gets a chance to save its work and clean up.
• kill -9 PID sends SIGKILL, which cannot be ignored. The system ends the process immediately and the program gets no time to save anything.
• killall name sends the signal to every process with that name.

Always try the polite kill first. Use kill -9 only when a program does not respond. A running command in your terminal can also be stopped with Ctrl + C, which is the gentlest option of all.

The order from gentlest to most forceful is: Ctrl + C, then kill PID, then kill -9 PID.

Background jobs

A command normally occupies the terminal until it finishes, which is called running in the foreground. Add & at the end to run it in the background so you get the prompt back at once.

sleep 60 &
jobs
fg %1

• sleep 60 & waits 60 seconds in the background.
• jobs lists the background jobs of this terminal, each with a job number.
• fg %1 brings job number 1 back to the foreground.

Pausing and resuming

If you forgot the &, you can still move a running command to the background:

1. Press Ctrl + Z to pause it.
2. Type bg to resume it in the background.

You can also bring any job back with fg.

• Ctrl + C stops the foreground program.
• Ctrl + Z pauses it.
• Ctrl + D ends input, and on an empty prompt closes the shell.

Keep running after you log out

When you close a terminal, the shell sends its jobs a hangup signal and they stop. nohup ("no hangup") tells a command to ignore that signal.

nohup ./task.sh > out.log 2>&1 &

The command is run in the background (&), keeps running after you close the terminal and writes its output to out.log. Without the redirection, nohup saves output to a file called nohup.out.