Staged files

git add

To add the previously created Readme.md file, one has to execute

$ git add Readme.md

$ git status
On branch master

No commits yet

Changes to be committed:
  (use "git rm --cached {file}..." to unstage)

	new file:   Readme.md

Why not directly saving a file?

Suppose you have two files, File A and File B, where File B depends on File A.

1. You adjust File A until you are satisfied. In principle you could save File A, however File B wouldn't work anymore.
2. Because you do not want to "save" a not entirely working snapshot of your repo,
   you only git add "File A".
   This can also serve as a reminder of what is already done.
3. Now, you work on File B until you are happy,
   you git add "File B" and save the entirely working snapshot.

Inspecting

You now know how to add files and their modifications. Next you will learn how to inspect their history.

To see the repository history, use git log
It is helpful to use the --pretty flag for output styling options

$ git log --pretty="[%ci](%h) %an: %s"
[2018-04-06 15:14:02 +0200](3003795) Christopher Koerber: Added .gitignore for tex products
[2018-04-06 13:33:23 +0200](44aab06) Christopher Koerber: Added article and adjusted readme
[2018-04-04 18:03:54 +0200](db35944) Christopher Koerber: Committed empty readme file.

The strings in parenthesis are commit identifiers. You need them, e.g., for comparing different versions.

$ git diff db35944 44aab06 Readme.md

diff --git a/Readme.md b/Readme.md
index e69de29..e6da3a6 100644
--- a/Readme.md
+++ b/Readme.md
@@ -0,0 +1 @@
+Compile article.tex by running pdflatex on it

Comparing files with git diff

$ git diff {options} {commit1} {commit2} {path}

Using git checkout to walk back in time

You can also revisit old commits

$ git checkout db35944
...
HEAD is now at db35944... Committed empty readme file.

$ cat Readme.md

$ ls
Readme.md  article.aux	article.log  article.pdf

$ git checkout master
Previous HEAD position was db35944... Committed empty readme file.
Switched to branch 'master'

Using git checkout to revert changes

Lets say we make some unwanted changes to a file

$ echo "I'm doing something stupid" >> article.tex
$ cat article.tex 
\documentclass{minimal}\begin{document}Hell World\end{document}
I'm doing something stupid

$ git status
Changes not staged for commit:
	modified:   article.tex

$ git checkout article.tex
$ cat article.tex 
\documentclass{minimal}\begin{document}Hell World\end{document}

$ git status
nothing to commit, working tree clean

Updating local repos with git pull

Whenever a remote repository has changed, you should consider updating your local copy. There are several strategies for doing so. The most direct strategy is git pulling. Note that git warns you before it changes files irreversibly.

$ git pull
Already up to date.

$ git status
On branch devel
Your branch is up to date with 'origin/devel'.

nothing to commit, working tree clean

Conflicts when pulling

If you and a collaborator work on the same line in the same file at the same time, you create a conflict. Without further input, git does not know what to keep.

$ cd ..
$ git clone git_tutorial git_tutorial_B
Cloning into 'git_tutorial_B'...

$ cd git_tutorial
$ echo "Some notes" >> Readme.md
$ git add Readme.md
$ git commit -m "Updated readme"

$ cd git_tutorial_B
$ echo "Instruction B" >> Readme.md
$ git add Readme.md
$ git commit -m "New instruction"

$ git pull
error: Pulling is not possible because you have unmerged files.
hint: Fix them up in the work tree, and then use 'git add/rm {file}'
hint: as appropriate to mark resolution and make a commit.
fatal: Exiting because of an unresolved conflict.

Updating remote repos with git push

Once are ready to share your (accumulated) commits, use git push.

$ echo "See the LICENSE for usage agreement" >> Readme.md
$ git add Readme.md
$ git commit -m "Mentioned LICENSE in Readme"
$ git status
On branch devel
Your branch is ahead of 'origin/devel' by 1 commit.
  (use "git push" to publish your local commits)

$ git push
...
To https://github.com/ckoerber/git_tutorial.git
   3003795..73ab23c  devel -> devel

Conflicts when pushing

If you and a collaborator work on the same line in the same file at the same time, you create a conflict. Without further input, git does not know what to keep.

$ cd ..
$ git clone git_tutorial git_tutorial_B
Cloning into 'git_tutorial_B'...

$ cd git_tutorial
$ echo "Some notes" >> Readme.md
$ git add Readme.md
$ git commit -m "Updated readme"

$ cd git_tutorial_B
$ echo "Instruction B" >> Readme.md
$ git add Readme.md
$ git commit -m "New instruction"

$ git push
To ~/git_tutorial/
! [rejected]        devel -> devel (fetch first)
error: failed to push some refs to '~/git_tutorial/'
...

Branching

Branches correspond to independent lines of development. You have already seen examples for different branches: the local master branch is an independent line of development based on the origin/master branch. You can also create local independent lines of development.

1. Create a new branch

   $ git branch new-feature master
   

   The syntax is "git branch {branch-name} {start-point}"
2. Switch to new branch and add stuff

   $ git checkout new-feature
   Switched to branch 'new-feature'
   

   
   $ echo "import numpy" > newFeature.py
   $ echo "started to work on new feature" >> Readme.md
   $ git add newFeature.py Readme.md
   $ git commit -m "Added new feature"
   
3. Go back to old branch

   $ git checkout master
   Switched to branch 'master'
   Your branch is up to date with 'origin/master'.
   

   $ ls
   LICENSE  Readme.md  article.tex
   

Fast forward merge

You have a linear path from current branch to target branch head

$ git merge new-feature 
Updating 841377f..5321d8f
Fast-forward
 newFeature.py | 0
 ...

3-Way merge without conflict

You do not have a linear path from current branch to target branch head but it was worked on different parts of files.

1. Add another new file and commit it

   $ touch logo.png
   $ git add logo.png
   $ git commit -m "Added logo"
   
2. Merge new-feature branch into current branch

   $ git merge new-feature -m "Merge branch 'new-feature'"
   Merge made by the 'recursive' strategy.
    Readme.md     | 1 +
    newFeature.py | 1 +
    2 files changed, 2 insertions(+)
    create mode 100644 newFeature.py
   

   This creates a new commit

Git setup

Git basics

Nearly all commands come with additional flags, e.g., use git commit -am "{message}" to automatically stage and commit all unstaged files. Use -h to find the flag you are looking for.

Git branches

Git collaborating

Nearly all commands come with additional flags, e.g., use git push --set-upstream {remote} {branch} to define the default push remote for specified branch (after this git push without options automatically pushs the {branch} to {remote}). Use -h to find the flag you are looking for.