Linux Tutorial Series – 188 – Logical operators

Here is the video version, if you prefer it:

Let’s talk about logical operators outside of the context of shell scripts, at first.

If I run two commands like this:

touch someFile && emacs someFile

what would happen is that I would create a file named someFile and then I would immediately open it with Emacs (if you don’t have Emacs, you can use less). However, try this:

mislav@mislavovo-racunalo:~/Linux_folder$ ls -l nonExistentFile && emacs someFile

ls: cannot access 'nonExistentFile': No such file or directory

What happened here is that we tried to use ls on a non existent file and we got an error message. But our Emacs didn’t open up as well. What is happening here?

See, when you use the && operator (also known as the “and” operator) you are saying “Execute the first command and then, if it succeeds, execute the second command”. How does the shell know if a command executed successfully? Why, by exit code, of course! (Ward, 2014)⁠

When a command is successful, its exit code is 0. && says “I will execute the second command only if the exit code of the first command (to the left of the && sign) is 0”. ||, another logical operator, says just the opposite: “I will execute the second command only if the exit code of the first command (to the left of the || sign) is not 0”. You can remember it like this, but this makes more sense if you know about logical gates, which you don’t need to to understand Linux, so I won’t explain those.

So if I write:

mislav@mislavovo-racunalo:~/Linux_folder$ ls -l nonExistentFile || emacs someFile

this actually opens up Emacs, because || works in the way I described above.

Hope you learned something useful!

References

Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Page 258