See changes to a specific file using git

I know that I can use the git diff command to check the changes, but, as far as I understood, it is directory based. This means it gives all the changes of all files on the current directory.

How can I check only the changes in one specific file? Say, I have changed files file_1.rb, file_2.rb, ..., file_N.rb, but I am only interested in the changes in the file file_2.rb. How do I check these changes then (before I commit)?

Use a command like:

git diff file_2.rb

See the git diff documentation for full information on the kinds of things you can get differences for.

Normally, git diff by itself shows all the changes in the whole repository (not just the current directory).

Another method (mentioned in this SO answer) will keep the history in the terminal and give you a very deep track record of the file itself:

git log --follow -p -- file

This will show the entire history of the file (including history beyond renames and with diffs for each change).

In other words, if the file named bar was once named foo, then git log -p bar (without the --follow option) will only show the file's history up to the point where it was renamed -- it won't show the file's history when it was known as foo. Using git log --follow -p bar will show the file's entire history, including any changes to the file when it was known as foo.

You can use gitk [filename] to see the changes log

You can use below command to see who have changed what in a file.

git blame <filename>

to list only commits details for specific file changes,

git log --follow file_1.rb

to list difference among various commits for same file,

git log -p file_1.rb

to list only commit and its message,

git log --follow --oneline file_1.rb