Uncommited changes in a file are visible in all branches?
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I checked out from master branch to branch feature-foo and made some uncommitted changes in a file foo.py. When I switch back to branch master, I am surprised to find that changes in foo.py are visible in master branch.

I have also thought that changes in a branch belongs to that branch, but it seems that I am terribly wrong. The truth is: until you commit your changes in a certain branch, it will be visible in other branches too.

References
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How to show file version in a specific commit
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We can use git show <revision>:/path/to/file to show the version of file in a specific commit.

<revision> can be a commit hash or something like HEAD or HEAD~2 or tag name that can identify a specific commit. For example, to show file readme.md in current HEAD, use the following command:

git show HEAD:readme.md

To show readme.md in commit 410d12ed0, use git show 410d12ed0:readme.md.

References
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Delete files that have been commited after adding them to gitignore
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We may have commited some files that we do not want to include in the repo to the remote repo. How do we delete these files from remote repo?

First, we need to edit .gitignore file and add the files we want git to ignore. Then use the following command to delete these files from remote repo:

git rm --cached path/to/file
git commit -m "delete unwanted files"
git push origin master

The command git rm --cached test.txt will remove file test.txt from the Git index, but not from your disk. If you omit option --cached, the file will also be removed from your disk.

References
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https://stackoverflow.com/a/7927283/6064933

Show difference between working directory, index and commit tree for a certain file
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Suppose we want to know the difference bewtween working directory, index, and commit tree for file test.txt, we can use the following commands:

git diff test.txt: difference between working directory and index
git diff HEAD test.txt: difference between working directory and HEAD
git diff --staged test.txt or git diff --cached test.txt: difference between index and HEAD.

References
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Difference between `git add -A`, `git add .` and `git add -u`?
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For the commit tree, there are three types of files: modifed, deleted and new files. What these commands do to the three types of files are:

command	modified files	deleted files	new files	description
`git add -A`	yes	yes	yes	stage modified, deleted and new files
`git add .`	yes	yes	yes	stage modified, deleted and new files EndFragment
`git add -u`	yes	yes	no	stage modified and deleted files, but not new files
`git add --ignore-removal .`	yes	no	yes	stage new and modified files, but not deleted files

Using git add -A and git add -u without specifying the path will act on the entire working tree, regardless of your current directory:

If no is given when -u option is used, all tracked files in the entire working tree are updated (old versions of Git used to limit the update to the current directory and its subdirectories).

References
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https://stackoverflow.com/a/26039014/6064933

How to list all file in current repository
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We can use git ls-tree command:

git ls-tree --name-only --full-tree -r HEAD

The meaning of options:

--full-tree: print the whole tree under the working tree root no matter which directory you are in
--name-only: print file names only (do not show file type and hash etc.)
-r: recursively list files under the root directory.

Note that if -r is omitted and only --full-tree and --name-only are used, only files and directories under repo root are listed.

We can also use git ls-files to show all the file in current repo. By default, git ls-files will also show files that are staged but not yet committed, which is different from git ls-tree since it only shows committed files. git ls-files also has more features than that.