General Tips

1. If 2 files are formatted differently, you can first format them and then check the differences between them. An extreme way is to get rid of all white spaces in the 2 files and then compare them.

   sed -i 's/ //g' file_1 file_2
   

Command-line Tools

git-diff

git diff is a generally useful command. It can not only show you the changes of files in side a Git repository but can also be used to compare files/directories outside of a Git repository.

1. Show differences between 2 files. The option --no-index can be omitted if at least one of the two files is outside a Git repository. Output of git diff is colorized.

   git diff --no-index file_1 file_2
   

2. Show differences between 2 directories, which is similar to that of showing differences between 2 files.

   git diff --no-index dir_1 dir_2
   

colordiff/diff

It is suggested that you use colordiff instead of diff as colordiff colorizes the output of diff. However, both colordiff and diff can be replaced by git diff as shown above.

vimdiff

All of git diff, colordiff and diff show line-leve changes. vimdiff is great command-line tool that is able to show differences inside lines.

Online Tools

text-compare

text-compare is a good one.

diffnow

References

https://stackoverflow.com/questions/16683121/git-diff-between-two-different-files

https://wiki.dandascalescu.com/reviews/software/diffuse_-_the_one_best_visual_diff_tool_for_linux

https://coderwall.com/p/ydluzg/better-git-diff

https://stackoverflow.com/questions/5326008/highlight-changed-lines-and-changed-bytes-in-each-changed-line

https://stackoverflow.com/questions/18069611/fastest-way-of-finding-differences-between-two-files-in-unix

https://stackoverflow.com/questions/4544709/compare-two-files-line-by-line-and-generate-the-difference-in-another-file

https://unix.stackexchange.com/questions/11128/diff-within-a-line

https://www.networkworld.com/article/3190407/linux/nine-ways-to-compare-files-on-unix.html