Which command can be used to compare two files line by line?

The command that can be used to compare two files line by line is 'diff'. This command analyzes two files and outputs the differences between them, making it a powerful tool for understanding changes between text files. When using 'diff', it presents a clear view of what lines are added, removed, or altered from one file to the other, and it does so in a format that is easy to read for users concerned with textual content.

While other commands like 'cmp', 'comm', and even the non-existent 'compare' may have their specific uses, they do not perform the same line-by-line comparison task in the way 'diff' does. 'cmp' focuses on a byte-by-byte comparison and reports when the first difference is found, which is less useful for detailed examination of changes. 'comm' is designed to compare two sorted files and shows their common lines, while 'compare' is not a standard Unix/Linux command, making it not applicable in this context. Thus, 'diff' is indeed the most suitable command for line-by-line file comparison.