Here’s a command that will compare two files and display the only entries in the first file that are missing in the second file. To do this, there’s a compare command in Linux Bash called comm that comes with several options. The option we are interested in is the comm -23 which exactly does that. Let me demonstrate.

Let’s say file a.txt contains:

one
two
three
four
five
six
seven
eight
nine
ten

And file b.txt contains:

one
three
four
five
nine

We then run comm command using the option -23.

comm -23 a.txt b.txt > c.txt

The contents of c.txt is:

two
six
seven
eight
ten