Using Git / Git Cheatsheet

07 May 2019 - tsp

Putting an existing project under version control
Removing an remote
Signing and verifying git commits using GPG
Displaying history
How to get git(hub) to work with SSH keys
- Windows, PuTTY and PLink, GUI
Cloning an project (Fetching it for the first time from a remote repository)
Staging local changes
Commiting staged changes to local repository
Pushing local commits to an remote repository
Modifying author or email address on an previous commit
Showing configuration options
Step by step - how to apply to your own small personal project
Good practice

Note that at the end there is a short step by step guide on how to apply version control to your first short projects. Please note that - to use all features of git - you should use plain text source files, not MS Word or similar binary content. Although git can of course also manage binary files you loose the ability of viewing differences, merging changes, etc. You keep the ability to revert and look at previous versions with checkout of course.

The TL;DR:

Initializing a new project repository:

git init

and if not done globally

git config user.email "example@example.com"
git config user.name "example"

Adding files to the local stage:
```
git add WHATEVER
```

Commiting to local repository - one of:

git commit -S -m "Log Message"
git commit -S
git commit -m "Log Message, not signing commit"
git commit

Pushing to a remote repository:
```
git push origin master
```
or remembering the origin and master:
```
git push -u origin master
```
and after that only
```
git push
```
Pulling changes from the remote
```
git pull
```

Adding an remote

git remote add origin git@github.com:USERNAME/reponame.git

Creating a new branch

git branch branchName
git checkout branchName

Switching branches

git checkout master

git checkout branchName

Merging two branches:

git checkout targetBranch
git merge sourceBranch

Deleting a branch
```
git branch -d branchName
```
Displaying repository (and conflict) status:
```
git status
```
Conflicts. They are shown after a merge or a pull has been executed and are also shown via status. After resolving just commit the resolved status. For every conflict the diff is embedded in the files that have conflicted in the way
```
<<<<<<< TARGETBRANCHNAME
Theirs
=======
Yours
>>>>>>> SOURCEBRANCHNAME
```
You simply edit the files to contain only the correct value (without the target and source branch lines), add and commit as usual to resovle the conflict.
Rebase. This is a really powerful tool for various tasks. The easiest way is to use the interactive editor
```
git checkout TARGETBRANCH
git rebase -i SOURCEBRANCH
```
or
```
git rebase -i PREVIOUSCOMMIT
```
This lets one pick (commit will be a single commit), squash (commit will be combined with the next commit in the file), fixup or edit the commit. This allows squashing, moving commits to another branch or rewriting history. Since rebase is such a powerful tool it’s highly recommended to learn the behaviour for example with a git simulation.

Putting an existing project under version control

If you want your project to be hosted on github - first create a new repository on github.
Change your working directory into your existing projects directory
Initialize the git repository:
```
git init
```
Add all local files (as wanted) to the repository
```
git add .
```

If you want you can now set your user.name and user.email. When using github this is mandatory

git config user.email "example@example.com"
git config user.name "example"

One can also do that globally by adding the --global option:

git config --global user.email "example@example.com"
git config --global user.name "example"

Commit your changes to your local repository (do not follow that step if you want to sign your commits. This is described in the next section!)
```
git commit -m "Commit log message"
```
If you want to write a longer logmessage just leave the -m line. And editor will popup and let you write your log message. Write expressive log messages that describe what you’ve changed.
```
git commit
```
Now we have to add the remote origin if we don’t want to use the repository only locally. This should be either the https or ssh repository URI. When using github you can grab yours from the repositories page. It should either look like
```
https://github.com/USERNAME/reponame.git
```
or
```
git@github.com:USERNAME/reponame.git
```
To add the repository reference to the origin:
```
git remote add origin git@github.com:USERNAME/reponame.git
```
You can verify the remote with
```
git remote -v
```
Now push your changes like you would do after any other commit
```
git push origin master
```
If you want to use that repository as upstream you can set the -u flag and use git push afterwards instead of specifying the remote during the push:
```
git push -u origin master
```

Removing an remote

If one wants to remove an remote to unlink a repository from their upstream or change an invalid reference on can

git remote remove origin

To list all remotes one can use

git remote -v

Signing and verifying git commits using GPG

Since git is an distributed version cotrol system the basic assumption is that no remote repository is authorative (even if used so) and that no remote repository is trusted. Commits of people can (and should) be signed with GPG - the certificates should certify authenticity of the set user.email. One can then verify if the commits have really been made by the authors that claim to have done the changes (see displaying history) as well as they can be checked during a pull to only download changes that have a valid signature attached (see creating a trusted keyring only for GIT when one just wants to trust a small subset of people to make modifications to a repository)

Creating new GPG key, using for signing commits on git(hub)

Generate a new key:
```
gpg --gen-key
```
Select “RSA and RSA” when asked for type of keys
Decide which key length you want to use
Select validity of the key. Normally keys should expire after a maximum of a few years
You can use any Name and Mailadress you want. If you use github’s mail privacy feature it’s a good idea to use the same name as your github username and the same mailadress as the generated noreply address
Now you can also list all keys and their associated private keys:
```
gpg --list-secret-keys --keyid-format LONG
```
This command outputs 3 lines per key - these are sec, uid and ssb. Locate the key you want to use to sign your commits and determine the key id. If you see a line
```
sec 4096R/5198ACD84912ED02  YYYY-MM-DD [expires: ]
```
then the key-ID is 5198ACD84912ED02.
Extract the public key:
```
gpg --armor --export 5198ACD84912ED02
```
You can upload the public key block (including begin and end lines) to your github profile settings.

For later backup export private and public keys:

gpg --armor --export-secret-keys 5198ACD84912ED02 > signerkey.priv
gpg --armor --export 5198ACD84912ED02 > signerkey.pub

You should also upload your key to a keyserver to make it easier to for third parties to fetch your public key and verify your identity (also based on the web of trust which is explained later).
```
gpg --keyserver pool.sks-keyservers.net --send-keys 4912ED02
```
To tell git to use this key for signing either globally or on a per repository basis, set user.signingkey property to the given key id
```
git config --global user.signingkey 5198ACD84912ED02
```
Depending on your system configuration you may have to tell git about the location of your gpg executable (this has been mostly seen on MS Windows). An example for MS windows would be:
```
git config --global gpg.program "c:\Program Files (x86)\gnu\gnupg\gpg2.exe"
```
If you now want to sign your commit, set the -S switch during commit:
```
git commit -S -m "commit message"
```
or
```
git commit -S
```

Signing commits

Basically you just have to supply the -S switch during commit.

git commit -S

git commit -S -m

Creating a trusted keyring only for GIT

If one extensivly uses gpg one should use a private keyring of trusted keys that are trusted for commits on the git repositories - if one would use the default keyring every known key with a valid signature (including those used for mail, etc.) would be trusted for signing git commits.

The idea is to create an own keyring just for git (or specific git repositories). To do that simply export the authorized keys to a keyring:

gpg --export KEYSPECIFIER | gpg --no-default-keyring --keyring=~/git.pgp --import -

To list the keys inside an keyring use

gpg --no-default-keyring --keyring=~/git.pgp --list-keys

Note that signatures with keys from your main keyring will not be validated if they are not included inside the git keyring. One has to build a full local web of trust or trust the local keys.

To interactively change the trustlevel one can use

gpg --no-default-keyring --keyring=~/git.pgp --edit-key KEYSPECIFIER

To change the trust level one can use the trust command inside this shell.

sec  rsa4096/06572BE1C0A8062D
     created: 2018-10-03  expires: 2026-10-01  usage: SC  
     trust: unknown       validity: ultimate
ssb  rsa4096/08D2E9AC73B82E95
     created: 2018-10-03  expires: 2026-10-01  usage: E   
[ultimate] (1). tspspi <11219971+tspspi@users.noreply.github.com>

gpg> trust
sec  rsa4096/06572BE1C0A8062D
     created: 2018-10-03  expires: 2026-10-01  usage: SC  
     trust: unknown       validity: ultimate
ssb  rsa4096/08D2E9AC73B82E95
     created: 2018-10-03  expires: 2026-10-01  usage: E   
[ultimate] (1). tspspi <11219971+tspspi@users.noreply.github.com>

Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)

  1 = I don't know or won't say
  2 = I do NOT trust
  3 = I trust marginally
  4 = I trust fully
  5 = I trust ultimately
  m = back to the main menu

Your decision? 5

Of course you should only trust ultimately your own keys. Other keys should maximally be trusted fully - if you trust that this person is really who they are and that they are only signing valid keys themselves.

Leve the shell with

gpg> quit

To use the trusted keyring with git one has to provide a wrapper around the gpg binary for github. This is easily achieved by using a shellscript (called /usr/local/bin/gitgpg.sh in this example) and specifying this script in the gpg.program option of git.

The shellscript is pretty simple:

#!/bin/sh
exec /usr/local/bin/gpg --no-default-keyring --keyring=~/git.pgp "$@"

Do not forget to set execute permissions and protect the script against modifications

chmod 555 /usr/local/bin/gitgpg.sh

Now one can either globally (for all repositories) or locally (for a specific repository) configure git to use gitgpg.sh

git config --global gpg.program /usr/local/bin/gitgpg.sh

git config gpg.program /usr/local/bin/gitgpg.sh

Now one can verify signatures on upload by using

git pull --verify-signatures

Changing local trust level for a given key

First get the key ID from the local key store

gpg --list-keys --keyid-format LONG

If - for example - the output looks like this:

pub   4096R/06572BE1C0A8062D 2018-10-03 [verfällt: 2026-10-01]
uid               [ uneing.] tspspi <11219971+tspspi@users.noreply.github.com>
sub   4096R/08D2E9AC73B82E95 2018-10-03 [verfällt: 2026-10-01]

you can now use gpg –edit-key to set the trust level:

gpg --edit-key 06572BE1C0A8062D

after entering the interactive GPG shell you can now use

trust

and set the trust level.

Uploading a public key to a keyserver

First determine the key ID by using –list-keys, then upload it via the –send-keys option to the keyservers. It’s a good idea to use the sks-keyservers pool. Note that it takes a while for the keys to propagate to all keyservers on the pool - so don’t be impatient if you cannot import them on other machines immediately

gpg --list-keys
gpg --keyserver pool.sks-keyservers.net --send-keys C0A8062D

Signing keys for the web of trust

If you want to certify the authenticity of another key, first download it from the keyserver (for example by key id):

gpg --keyserver pool.sks-keyservers.net --recv C0A8062D

Then the other person should tell you the fingerprint of their key on their machine.

gpg --finger C0A8062D

Compare the fingerprint you are reading with the fingerprint the owner of the key tells you (do this carefully) during the following step and verify the identity of the owner of the key (using official documents, etc.). To start the signature process you can use one of the following commands. The second one is required to sign if you have multiple private keys with which you want to sign (read the comment about trust levels below before sigining!)

gpg --sign-key <keytosign>
gpg --default-key <yoursigningkeyID> --sign-key <keytosign>

You can sign with various certification levels. There are:

0: No indication
1: Personal belief but no verification (for example for pseudonyms)
2: Casual verification (you’ve seen an government issued ID card, passport, etc.)
3: Extensive verification

Normally gpg uses certification level 2. If you want to certify with another level you can add the –ask-cert-level option:

gpg --ask-cert-level 3 --sign-key <keytosign>
gpg --ask-cert-level 3 --default-key <yoursigningkeyID> --sign-key <keytosign>

Take signing keys seriously. The people who assign your keys trust depend on your certifications to be valid. Also keep in mind that it’s valid for third parties who you trust - the web of trust helps to identify people to verify the identity of people they don’t know directly but only over one or more hops.

Displaying history

To display the history (in your local repository) use

git log --show-signature

If you don’t care about GPG signatures of commits you can leave the --show-signature option. Verification of GPG signatures requires you to have the public keys of the authors - you can download a key with a given key ID if it has been uploaded to a keyserver by

gpg --keyserver pool.sks-keyservers.net --recv C0A8062D

How to get git(hub) to work with SSH keys

Windows, PuTTY and PLink, GUI

After installing PuTTY and git:

Create a new SSH key with puttygen (use ED25519 format) and store private and public key somewhere on your machine.
Upload the “Public key for pasting into OpenSSH authorized_keys file” into your github Settings (add new SSH key)
Modify the users environment to include GIT_SSH pointing to your plink.exe.
Tell the git client who you are (this can also be set on a per repository basis, just don’t use –global)
- git config –global user.name “exampleusername”
- git config –global user.email “example@example.com”
Load your private key into pageant (the PuTTY authentication agent). As long as the keys are loaded into pageant SSH applications are capable of authenticating with that key. As you don’t require the keys any more remove them from the agent.
Testing SSH connection. This can be done using plink:
```
plink git@github.com
```
If you see any error messages there is a problem with your local installation, if github greets you with your username everything is setup correctly

Cloning an project (Fetching it for the first time from a remote repository)

Simply run

git clone git@github.com:USERNAME/reponame.git

to clone via git over SSH or

git clone https://github.com/USERNAME/reponame.git

to clone via HTTPS. When using github the URIs can be retrieved from the respositories webpage - upper right corner “clone or download”.

One can additionaly specify the target directory. If one wants to clone into the currently empty directory one could use:

git clone git@github.com:USERNAME/reponame.git .
git clone https://github.com/USERNAME/reponame.git .

Staging local changes

Everytime a local change should be added to a commit into the local repository one has to stage the change. To stage one uses

git add FILENAME

to add the changed files. One can of course also add a whole directory

Commiting staged changes to local repository

To comit locally staged changes to the local repository one can use

git commit
git commit -S
git commit -m "Message"
git commit -S -m "Message"

The -S flag can be used to sign the commit with an GPG key (see above), -m allows to specify the commit logmessage on the command line. If not specified the git client will open the configured editor and present an default commit message that should be changed

Pushing local commits to an remote repository

git push
git push origin master

Modifying author or email address on an previous commit

If you’ve commited with an invalid author name or your private mail address even when using githubs mail protection feature you might discover that error not until you try to push the changes to the remote repository. In this case you can re-write history (beware that this is nearly always a bad thing - do this only if you change the last locally non pushed commit):

git commit --amend --author="Example User <example@example.com>"

Showing configuration options

To show all configuration options that have been configured (for example via git config)

git config --list

Step by step - how to apply to your own small personal project

If you want to use git just for your own personal project (to see what has changed, have an offsite backup on github, being capable of reverting changes and optionally using branches the following section lists the required steps after installing git.

Initializing version control. Change into the directory your work is contained inside (using the cd shell command) and initialize the gti SCM:
```
git init
```
Add all currently present work or only the parts you want to add
```
git add .
```
Do your initial commit
```
git commit -m "Initial commit"
```
or for a longer message
```
git commit
```
If you also want to sign your commits (see above) use -S additionally
If you want to use github create the repository and use the remote command shown on the webpage
```
git remote add origin git@github.com:USERNAME/reponame.git
```
and push initially
```
git push -u origin master
```

Without branching (just a linear chain of changes).

Modify whatever you want
List what has changed
```
git status
```
Stage the changes
```
git add .
```
or only for single files
```
git add WHATEVER
```
Review what you’re going to commit
```
git status
```
Commit into your local repository
```
git commit
```
or with -m and optionally -S as above
If you’re using a remote push
```
git push
```

In case you are working on another machine you can clone the remote repository on the first time

git clone git@github.com:USERNAME/reponame.git

Before resuming work and modifying content it’s a good idea to always pull the most recent version from the remote:

git pull

If you have any conflicting changes git shows you the conflicts during the pull and also with

git status

In case of conflicts just edit the conflicting files and search for a section looking like

<<<<<<< TARGETBRANCHNAME
Theirs
=======
Yours
>>>>>>> SOURCEBRANCHNAME

Edit the whole section to just contain the parts you want to be contained inside the file:

Yours and Theirs

Then git add the file or all files and git commit them.

If you want to show a the list of log messages

git log

or the differences between the current and previous version

git diff FILENAME

To revert to the current head on the repository (i.e. delete uncommited changes)

git reset FILENAME

And to undo the last commit just do

git revert UNWANTEDCOMMITHASH

Good practice

Some practice tips:

Writing meaningful log messages
Keep your commits small
Each commit should change exactly one thing (implement one feature, add one topic, etc.)
If multiple commits in your local repository are logically related and change the same feature squash before pushing