What is the difference between git-stash and git-checkout?

Question

I try to go from one local branch to another one. Git tells me that I cannot do it because my local changes to the following files would be overwritten by checkout.

Then I get a "recommendation" Please, commit your changes or stash them before you can switch branches.

I know that I do not need the changes to the mentioned file. It is OK to overwrite them. So, I try to stash. I execute git stash file_name. As a result I get:

Usage: git stash list [<options>]
   or: git stash show [<stash>]
   or: git stash drop [-q|--quiet] [<stash>]
   or: git stash ( pop | apply ) [--index] [-q|--quiet] [<stash>]
   or: git stash branch <branchname> [<stash>]
   or: git stash [save [--patch] [-k|--[no-]keep-index] [-q|--quiet]
               [-u|--include-untracked] [-a|--all] [<message>]]
   or: git stash clear

OK. It does not work. Then I try git checkout file_name. No complains from git. Then I can switch from one branch to another one. So, it seems that I got what I needed (go to the second branch without saving changes to the first branch).

However, I would like to ask why stash did not work, and how the final result would be different in case it had worked?

Answer 1

You cannot stash a single file explicitly. Had you run git-stash it would have stashed all your modifications.

git checkout -- file discards your changes to that file in working dir and checkout the version of that file recorded by the current commit (i.e. the one that HEAD points to).

Answer 2

git stash saves your changes into "stash" - which looks like stack of temporary commits. Can be seen with git stash list.
With git stash it is possible to stash certain file and even chunk of code. Try git stash save -p. It will interactively ask you what you want to save.
Another way is to add all, but one files to index with git add. Then run git stash save -k. It stashes changed file (red in git status) without files prepared to commit (green in git status).