Git

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References

Git cheat sheet

Introduction

Git Features:

  • Reliability
  • Performance
  • Distributed

Distributed

Originally from BitKeeper. Other distributed SCM is Mercurial.

  • No single repository. Everybody always has his own copy of the repository. The repository content is pulled from other people's repository.
  • No politics, no commit access control. All work is always done locally, so there is no need to define such politics.

Reliability

Every change, file, directory, etc. is cryptographically hashed (sha1sum).

  • Easy corruption detection. Any tampering to a file or directory content (either malicious or because of hardware failure) is immediately detected.
  • Easy distribution. Moreover because the repository is distributed all over the place, it is very easy to repair a given repository. You only need to drop all broken objects, and get all missing objects from a remote copy.

Performance

Very fast commit. Local repository

Terminology and Concepts

commit
A commit is a snapshot of your working tree at some point in time. There are different ways to name a commit:
  • branchname — a branch name is an alias for most recent commit on that branch
  • tagname — similar to a branch alias, but that does not change in time
  • HEAD — currently checked out commit
  • c82a22c — the SHA-1 hash id of the commit (can be truncated as long as it remains unique)
  • name^ — the parent of commit name
  • name^^ — the grand-parent of commit name (and so on)
  • name^2 — the 2nd parent of commit name (and so on)
  • name~10 — the 10th ancestor of commit name (same as name^^^^^^^^^^)
  • name:path — reference a specific file/directory in a given commit
  • name^{tree} — reference the tree held by a commit
  • name1..name2 — a commit range, i.e. all commits reachable from name2 back to, but no including, name1 (if either name is omitted, use HEAD instead)
  • name1...name2 — refers to all commits referenced by name1 or name2, but not by both. For git diff, refers to all commits between name2 and the common ancestor of name1 and name2.
  • master.. — to review changes made to the current branch
  • ..master — after a fetch, to review all changes occured since last rebase or merge
  • --since="2 weeks ago" — all commits since a certain date
  • --until=”1 week ago” — all commits up to a certain date
  • --grep=pattern — all commits whose commit message matches the regular expression pattern.
  • --committer=pattern — all commits whose committer matches the pattern
  • --author=pattern — all commits whose author matches the pattern
  • --no-merges — all commits in a range that have only one pattern (i.e. ignore all merge commits)
detached head
When HEAD is no longer a reference to anything (like ref: refs/heads/branch), but instead contains the actual hash of a commit.
git checkout -b newbranch           # To attach HEAD back on a new branch...
hunk
individual change within a file (basically a file diff output is made of a sequence of one or more hunks).

Install

Install the following essential packages:

  • git-core — the main program
  • git-gui — a gui front-end

Optionally install also:

  • git-doc — documentation
  • gitweb — Web interface
  • ViewGit — Another web interface
  • gitosis — Project management:
  • tig — a text-mode repository browser interface to git and color pager.
tig                                # launch browser
git show | tig                     # Use as pager. Colorize output of git-show
  • gitview — Git Repository browser
  • gitg — a Git repository browser targeting Gtk+ / GNOME
    • Version delivered with Lucid/Maverick is a very old one. Compile from the sources to get the latest version. Alternatively the repository ppa:pasgui/ppa contains a more recent version.
    To install the repository, create a file /etc/apt/sources.list.d/pasgui-ppa-lucid.list (change lucid as necessary): 
    deb http://ppa.launchpad.net/pasgui/ppa/ubuntu lucid main 
deb-src http://ppa.launchpad.net/pasgui/ppa/ubuntu lucid main

    Then add the apt key: 

    sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys F599ACE3
  • qgit — A graphical interface to git repositories using QT

Configuration

References:

Global per-user configuration settings are stored in file ~/.gitconfig

  • Add color to git output for all commmands:
    • git config --global color.ui true
  • Define author/email
    • git config --global user.name "Your Name"
git config --global user.email you@example.com
  • Add some frequently used aliases:
    • git config --global alias.st 'status'
git config --global alias.ci 'commit'
git config --global alias.co 'checkout'
git config --global alias.br 'branch'
git config --global alias.last 'log -1 HEAD'

How-To

Here we shall describe how to perform some tasks in Git.

Cloning to/from a Server using SSH

Reference: [1]

Clone a local repository to remote server griffin, through ssh. Repositories are all stored in a directory repositories/ in home directory of remote user git : 

git clone --bare myproject myproject.git          # Create a bare clone of your repository, if not available yet
scp -r myproject.git/ git@griffin:repositories/   # Copy the repository to server - requires SSH access
rm -rf myproject.git                              # Delete local bare clone

Now any other user that has SSH access to git@griffin may get a copy of that repository with 

git clone git@griffin:repositories/myproject.git  # Clone repository and create working tree in myproject/

Now, the user that created the repository at the first place can

  • either delete his own repository and clone the remote one as any other user,
  • or more safely, he can tell git to add the remote repository and set up tracking branch for master:
git remote add -f origin git@griffin:repositories/my_project.git  # Add remote repository and fetch automatically
git remote set-head -a origin                                     # Set origin/HEAD automatically - see man git-remote, set-head
git branch --set-upstream master remotes/origin/master            # Set master to track remote branch master from origin

See git-clone below for more details.

Mirroring

Reference: [2]

Cloning from a Server using git: Protocol over a Proxy

Reference: [3], [4]

The referenced links propose some script. Here another variant. Add this script to your path (say ~/bin/proxygit): 

#!/bin/bash
# proxygit - git through http proxy
#
# Usage:  proxygit [options] COMMAND [ARGS]
#   Setup Git HTTP proxy variable GIT_PROXY_COMMAND and call git with the given parameters.
#   The proxy settings are read from env variable $http_proxy
#
# Note:
# - Requires package socat
# - $GIT_PROXY_COMMAND must not be defined

if [ -n "$GIT_PROXY_COMMAND" ]; then
	PROXY=$(echo $http_proxy|sed -r 's!(http://)?([^/:]*):([0-9]*)/?!\2!')
	PROXYPORT=$(echo $http_proxy|sed -r 's!(http://)?([^/:]*):([0-9]*)/?!\3!')
	exec /usr/bin/socat - "PROXY:$PROXY:$1:$2,proxyport=$PROXYPORT"
else
	export GIT_PROXY_COMMAND="$0"
	exec git "$@"
fi

Commands

Here we'll summarize how to use some of the Git commands

git-clone

The command git clone /dir/repo/project.git is identical to running the following commands: 

git init                                            # Create an empty repo
git remote add -f origin /dir/repo/project.git      # Add a remote repo called 'origin' and fetch
git set-head -a                                     # Set default remote branch for remote 'origin' automatically
git checkout --track origin/master                  # Create a tracking branch 'master', and update working tree

Another equivalent option for last command is git checkout -b origin origin/master (since the start-point is remote, git creates a tracking branch).

Some variants:

  • To get a local copy of a remote repository, but without changing the working tree (i.e. keeping all local changes), just change the last command to:
git init
git remote add -f origin -m master /dir/repo/project.git
git branch --track master origin/master             # Branch 'master' set up to track remote branch 'master' from 'origin'
  • To merge remote branch locally, but without creating a tracking branch, change the last command to :
# ...
git merge origin/master                             # Merge

git-remote

git-remote set-head
Sets / deletes the default branch for a remote. For remote origin, this creates the reference refs/remotes/origin/HEAD with content ref: refs/remotes/origin/master if default branch is master.
Use set-head to follow the changes in another branch than the default one:
git remote set-head origin exoticbranch
git diff remotes/origin                             # Show diffs with remote branch exoticbranch from origin
                                                    # ... shortcut for git diff remotes/origin/exoticbranch

git-reset

git reset resets the current HEAD to the new state (hence also moving branch tip if you are currently on a branch). There are 3 different reset modes:

  • soft
    Only change the HEAD reference to a different commit. Working tree files and index are left untouched.
  • mixed (default)
    Like soft, but also reset the index (working tree files are left untouched). This actually clear the index from any staged changes.
  • hard
    Like mixed, but also erases all changes in the working tree, so that it matches the contents of the new HEAD. This is a dangerous command, so better use some alternatives that avoid data loss, like:

    Commit any changes first 

    git commit -a -m "snapshot WIP"
git reset --hard~3

    Stash the changes 

    git stash
git reset --hard HEAD~3
# ...
git reset --hard HEAD@{1} # or ORIG_HEAD
git stash apply

    Idem & don't change master too early 

    git stash
git checkout -b new-branch HEAD~3
...
git branch -D master
git branch -m new-branch master

Note:

  • git reset copies the old head to ORIG_HEAD.
  • Caution! — git reset moves the tip of the current branch (when HEAD is a ref to a branch). Don't do this on changes that have been published.


Some use cases (see man git-reset for details):

  • Undo a commit and redo
git commit ...
git reset --soft HEAD^
edit
git commit -a -c ORIG_HEAD # or -C
  • Undo commits permanently
git commit ...
git reset --hard HEAD~3
  • Undo a commit, making it a topic branch
git branch topic/wip
git reset --hard HEAD~3 # or --mixed
git checkout topic/wip  # or -m topic/wip
  • Undo a merge or pull
git pull                # conflicts
git reset --hard
git pull . topic/branch # no conflict
git reset --hard ORIG_HEAD
  • Undo a merge or pull inside a dirty work tree
git pull
git reset --merge ORIG_HEAD
  • Interrupted workflow
git checkout feature
#work work work
git commit -a -m "snapshot WIP"
git checkout master
#fix fix fix
git commit
git checkout feature
git reset HEAD^     # or --soft
  • Reset a single file in the index
git add foo.c
git reset -- foo.c

Tips

Frequently Used Commands

git commit -a                     # Add all changes and commit in one pass
git commit --amend                # Amend tip current branch (message, add some files) - also for merge commits

Working the Git Way

  • Check project diff before commit -a:
    • git diff                          # First see what's in the working tree (or git status)
git commit -a                     # Commit all changes
  • Give git commit a directory argument instead of using -a:
    • git commit fs/                     # Commit all changes in directory fs
  • Clean up an ugly sequence of commits ([5]).
    • Better than hunk-based commit because (1) each stage can be tested individually, (2) intermediate commits may contain changes that is not in the final one.
    1. First make sure that the ugly sequence is on some temporary branch target (what we aim for), and that end result is good and clean.
    2. Switch back to starting point, and do:
      3. git diff -R target > diff             # diff to target
    3. Edit diff file, to select only those changes we want to include in a first commit. Then do a git-apply diff
      4. vi diff
git-apply diff                        # Must be in project root dir
    4. Test, finalize the last changes before commits, and diff against target if necessary.
      5. # test test test
git diff -R target > diff             # if necessary
    5. Commit, and repeat from step 2.
    6. When done, branch target can be removed
  • Use gitk to get a graphical visualisation of current commit, or some subsets. For instance
    • gitk                                 # View current commit and all ancestors
gitk master..                        # View changes to current branch (i.e. reachable from HEAD, excluding master)
  • Use git stash to save the current state of the working tree (see [6]).
    • git stash                            # Save current work in working tree
...                                  # (whatever, including git reset --hard...)
git stash apply                      # Bring back changes in working tree
  • Forgot to add some files in the previous commit? Mistyped the commit message? Use git commit --amend:
    • git commit                           # Oups! forgot one file
git add somefile                     # ... Add the missing file
git commit --amend                   # ... and replace the previous commit
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