Git Recipes

Useful git tricks I've learnt using Git that might save your life some time.

Introduction

Git is an amazing tool, really powerful, but quite complex as well. Many people I know are sticked to a stupid interface where the options are no more than commit/checkout/fetch/push/pull/stash. Most of them get really embarrassed anytime something unexpected happens, because they're not aware of all the git possibilities.

Each time I do something I don't like, I look for some solution, because you always will find a fix for almost any mess you'd done on git.

Basic Concepts

Difference between HEAD^n and HEAD~n

How to write a commit message

This is not a real trick involving commands, but is one of the most important things I've learned of Git: commit messages matter. I encourage you to read this amazing article explaining why you should really care about it and give some tips to mastering commit messages.

IMHO, git is not only a versioning tool, but a really good documentation on the project. While commenting a method explain how this method works, having a look to the repository history may help a new developer to understand how things are done. Adding a new feature, implementing a new request, fixing a bug... the whole process of developing these things could be better understood taking a look over the repository history. How your code change, tells a story. And good commit messages are the best narrator to deeply understand it and place the reader on the right context.

Retrieving info

Get a list with the historic of commits modifiying a file (first line, the newest commit):

• git rev-list --pretty=oneline HEAD FILENAME

Displays a detailed diff log with the modifications in each commit for a certain file. Notice this is the expanded information of the list displayed with the previous command.

• git log -p [--follow] [-1] FILENAME

Display all the files changed on a commit (both commands prints the same):

• git show --pretty="" --name-only COMMIT_HASH
• git diff-tree --no-commit-id --name-only -r COMMIT_HASH

In which commit some source code was introduced?

• git log --pretty=oneline -S "CODE_STRING_TO_SEARCH" --source --all

  If you need to search with a regular expresion change -S by -G.

Rewritting history

Modify last commit message:

• git commit --ammend -m "New fancy message"
• git commit --ammend Opens editor with the old message for modifying.

Forgot to add some files to last commit? Just add them now and ammend it!:

• First, add the files git add FILENAME, then different options:
• git commit --amend Opens editor with the old message to rewrite.
• git commit --amend -C HEAD Don't open the editor and reuse last message.
• git commit --amend --no-edit Another way to reuse last commit message.

Modify n last commit messages (n = number of commits to change). Also, if you know the number of the eldest commit to be modified, you can point to its parent:

1. git rebase -i HEAD~n or simply git rebase -i COMMIT_ID

• Editor will open. Write reword in the commits you want to modify their messages, and let pick in the ones that are ok.

2. For each commit with 'reword', editor will be opened to let you change the commit message.

Modify n last commits

1. git rebase -i HEAD~n also if you know the first commit git rebase -i COMMIT_ID

• Editor will open. Write edit in the commits you want to modify, and let pick in the ones that are ok.

2. Do whatever changes you want and git add FILENAME (repeat as needed)
3. git commit --amend
4. git rebase --continue
5. Repeat 2,3,4 for each wommit with edit.

Change branch's parent

• git rebase --onto <new-parent> <old-parent> (Further reading)

Rename local branch name:

• git branch -m newBranchName

Recovering files

Delete last commit and recover the modified files. Sometimes you can do temporal commits instead of stashing (which ussually is safer), or maybe you didn't want to change some files, or you want to split a big commit into some smaller chunks.

• git reset --soft HEAD^

Did you fix something in a file on a separate branch and you need it now? Do yo want just one file instead of the whole commit? Cherry-pick a single file from another commit.

• git show [COMMIT_ID]:[PATH_TO_FILE] > [PATH_TO_FILE]

Recovering a single file from stash

Same scenario as last one, but the file you want to bring to your current branch is on the stash (and you don't want to recover the whole stash). You even can take a single file from the stash. stash@{0} represent the latest stashed commit, you can change the 0 for any other number to control which stash you want to use.

• git show stash@{0}:<full filename> Show a single file from stash (to get sure is the file you want)
• git checkout stash@{0} -- <filename> Get a single file from a certain stash
• git show stash@{0}:<full filename> > <newfile> Get a single file from stash to a different filename

If first you would like to check the changes you did on that file, you should remember this. A stash is represented as a commit whose tree records the state of the working directory, and its first parent is the commit at HEAD when the stash was created. stash@{0}^1 shortcut means first parent of given stash.

• git diff stash@{0}^1 stash@{0} -- <filename> Show diff of file (Further reading)

Recover DELETED stash

This really saved my life once. In one sprint I left some fixing half-developed, so I stashed changes to continue later. I didn't have the chance to come back to this work until one month later. In the meantime I was cleaning up my repo, I forgot this precious stashed code, and I deleted that stash. It took me a while to realize this (I knew I had done some changes in the code, but I couldn't notice the changed code; then I remembered I had stashed them and removed the stash). Fortunately I found this answer in SO solving my problem, which I summarize here.

First you would like to find where is your stash. This command will show you all the commits at the tips of your commit graph which are no longer referenced from any branch or tag – every lost commit, including every stash commit you’ve ever created, will be somewhere in that graph.

• git fsck --no-reflog | awk '/dangling commit/ {print $3}'

The easiest way to find the stash commit you want is probably to pass that list to gitk:

1. gitk --all $( git fsck --no-reflog | awk '/dangling commit/ {print $3}' )

• To spot stash commits, look for commit messages of this form:
• WIP on somebranch: commithash Some old commit message

2. git stash apply $stash_commithash

Alias

Get all alias already defined

• git config --get-regexp alias

Create new alias named NAME. The CODE triggered by that command, has to be between quotes if it has spaces (which would happen possibly in all alias you would create). To use it after created, just type git NAME.

• git config --global alias.NAME "CODE"

  For example, creation of comand to display files added/modified on a commit.

  • git config --global alias.whatadded "show --pretty="" --name-only"
  • And to use it: git whatadded COMMIT

Other

While doing a merge, you've applied some wrong changes in a certain file and you want to start again, but just for this file. This command will revoke fixed conflicts of a single file:

• git checkout -m FILENAME