Unit 6: Programs Running
Starting, stopping, and did it work?
Unit 6 of 8 in Linux and the command line for kids. Its 5 lessons are A Program With a Number, In the Background, Stopping Things, Did It Work? and Mission Control — below is everything each one explains, and a question or two from it to try.
Every terminal session on this page was replayed before it shipped, on a machine whose every message is tested against real bash and coreutils.
This unit opens with a fortnight’s trial of everything — no card needed — or with a family plan, bought in the iPhone app. The first two units of every track are free for ever. Try it in the app.
🏷️ A Program With a Number
Running, not just sitting there
A program on the disk is a file. A program that is running is something else, and Linux calls it a process.
Every process gets a number when it starts — its PID, or process id. That number is how you talk about it later.
Terminal
kid@alguni:~$ ps
PID TTY TIME CMD
412 pts/0 00:00:00 bash
940 pts/0 00:00:00 ps
Reading the list
ps shows what is running. There is always at least one: the shell itself, which is a program like any other.
And ps appears in its own list, because at the moment it looked, it was running too.
Try it yourself
What is a PID?
- The number Linux gives a running program
- The size of the program
- How long it has been running
- The program's password
Answer it in the app
🌗 In the Background
Waiting for a slow program
Normally the shell starts your command and waits for it to finish. That is fine for ls. It is annoying for something that takes ten minutes.
Put & at the end and the shell starts it and hands you the prompt straight back. The program carries on in the background.
Terminal
kid@alguni:~$ sleep 30 & [1] 940 kid@alguni:~$ jobs [1]+ Running sleep 30 &
What it tells you
[1] 940 is two numbers, not one.
[1] is the job number, which is the shell counting your background jobs: one, two, three.940 is the PID, which is Linux counting every process on the whole machine.
jobs lists yours. ps lists everything.
Terminal
kid@alguni:~$ sleep 60 &
[1] 940
kid@alguni:~$ sleep 90 &
[2] 941
kid@alguni:~$ jobs
[1]- Running sleep 60 &
[2]+ Running sleep 90 &
kid@alguni:~$ ps
PID TTY TIME CMD
412 pts/0 00:00:00 bash
940 pts/0 00:00:00 sleep
941 pts/0 00:00:00 sleep
942 pts/0 00:00:00 ps
Try it yourself
What does the & on the end of a command do?
- Runs it in the background and gives you the prompt back
- Runs it twice
- Runs it as root
- Joins it to the next command
Answer it in the app
🛑 Stopping Things
Ctrl and C
If a program is running in front of you and you want it to stop, hold Ctrl and press C.
That is not typing a command — it is a signal, sent straight to whatever is running. It is the single most useful key combination on a Linux machine, and every terminal in the world understands it.
Stopping something in the background
Ctrl-C only reaches the program in front of you. For a background job you have to say which one, and that is what the number is for.
kill 940 uses the PID. kill %1 uses the job number. Both do the same thing.
Terminal
kid@alguni:~$ sleep 60 & [1] 940 kid@alguni:~$ jobs [1]+ Running sleep 60 & kid@alguni:~$ kill %1 kid@alguni:~$ jobs
A word that sounds worse than it is
kill sounds dramatic. All it really does is send a message to a process asking it to stop, and most programs tidy up and go quietly.
A process that ignores you can be given the message it cannot refuse — kill -9 — but that is a last resort, because it never gets the chance to save anything.
Try it yourself
A program is running in front of you and will not stop. What do you press?
- Ctrl and C
- Escape
- Ctrl and S
- Enter, lots of times
Answer it in the app
✅ Did It Work?
Every command leaves a verdict
When a program finishes it hands back one number. Zero means it worked. Anything else means it did not.
You do not normally see it, but the shell keeps it, and $? is how you ask.
Zero for success looks backwards until you notice there is only one way to succeed and lots of ways to fail — so every other number is free to mean *which* failure.
Terminal
kid@alguni:~$ cat there.txt hi kid@alguni:~$ echo $? 0 kid@alguni:~$ cat missing.txt cat: missing.txt: No such file or directory kid@alguni:~$ echo $? 1
And, and or
You rarely read $? yourself. What you do instead is let the shell read it.
&& — run the next one only if that worked.|| — run the next one only if that failed.
Read them as "and then" and "or else".
Terminal
kid@alguni:~$ cd project && echo "made it" made it kid@alguni:~/project$ cd nowhere && echo "made it" bash: cd: nowhere: No such file or directory kid@alguni:~/project$ cd nowhere || echo "could not" bash: cd: nowhere: No such file or directory could not
Why it matters
This is the difference between a command that is safe and one that is not.
cd photos && rm * deletes things in photos, and does nothing at all if that folder is missing.
cd photos ; rm * deletes things wherever you happen to be standing when cd fails. The semicolon just means "then", with no checking. One character apart.
Try it yourself
What does the last line print?
Terminal
cat real.txt
echo $?What does mkdir photos && cd photos do if mkdir fails?
- Nothing — the `cd` never runs
- It runs the `cd` anyway
- It tries the mkdir again
- It stops the terminal
Answer them in the app
🏆 Mission Control
Everything running at once
You are looking after a machine with jobs on it. You will need all of it: ps to look, & to start, jobs to list, kill to stop, and && to only do the next thing if the last one worked.
Try it yourself
Which of these is safe if the photos folder does not exist?
- cd photos && rm *.jpg
- cd photos ; rm *.jpg
- rm *.jpg ; cd photos
- cd photos || rm *.jpg
Answer it in the app