diff is a handy command. Suppose you have 2 files, which contain almost the same information, but you can't find the difference between the two.
diff will process the files and will tell you what's the difference.
Suppose you have 2 files: dogs.txt and moredogs.txt. The difference is that moredogs.txt contains one more dog name:
diff dogs.txt moredogs.txt will tell you the second file has one more line, line 3 with the line Vanille:
If you invert the order of the files, it will tell you that the second file is missing line 3, whose content is Vanille:
Using the -y option will compare the 2 files line by line:
The -u option however will be more familiar to you, because that's the same used by the Git version control system to display differences between versions:
Comparing directories works in the same way. You must use the -r option to compare recursively (going into subdirectories):
In case you're interested in which files differ, rather than the content, use the r and q options:
There are many more options you can explore in the man page running man diff:
The diff command works on Linux, macOS, WSL, and anywhere you have a UNIX environment