Comparing two files in linux terminal

if you have vim installed,try this:

vimdiff file1 file2

or

vim -d file1 file2

you will find it fantastic.

Try sdiff (man sdiff)

sdiff -s file1 file2

If you prefer the diff output style from git diff, you can use it with the --no-index flag to compare files not in a git repository:

git diff --no-index a.txt b.txt

Using a couple of files with around 200k file name strings in each, I benchmarked (with the built-in timecommand) this approach vs some of the other answers here:

git diff --no-index a.txt b.txt
# ~1.2s

comm -23 <(sort a.txt) <(sort b.txt)
# ~0.2s

diff a.txt b.txt
# ~2.6s

sdiff a.txt b.txt
# ~2.7s

vimdiff a.txt b.txt
# ~3.2s

comm seems to be the fastest by far, while git diff --no-index appears to be the fastest approach for diff-style output.

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Update 2018-03-25 You can actually omit the --no-index flag unless you are inside a git repository and want to compare untracked files within that repository. From the man pages:

This form is to compare the given two paths on the filesystem. You can omit the --no-index option when running the command in a working tree controlled by Git and at least one of the paths points outside the working tree, or when running the command outside a working tree controlled by Git.

Sort them and use comm:

comm -23 <(sort a.txt) <(sort b.txt)

comm compares (sorted) input files and by default outputs three columns: lines that are unique to a, lines that are unique to b, and lines that are present in both. By specifying -1, -2 and/or -3 you can suppress the corresponding output. Therefore comm -23 a b lists only the entries that are unique to a. I use the <(...) syntax to sort the files on the fly, if they are already sorted you don't need this.