Use git diff

1. Compare the local working directory with the remote warehouse (all modified files)

Git diff head

2. Compare the given file

Git diff -- filename

3. Comparison with Historical submission

Git diff commit-ascidfjutt23892929921

Working tree: it is the directory where you work. Every time you modify the code, the status of the working tree changes.
Index file: it is an index file that connects the working tree and commit. The content of the index file changes every time we use the git-add command for registration, in this case, the index file is synchronized with the working tree.
Commit: it is the final stage. Only after commit, our code has actually entered the GIT repository. We use Git-commit to submit the content in index file to commit.