Tagging

This Post is part of a series about Git, Git concepts, commands and usage patterns to remind me and to help me learn. The first post of the series is Git - A New Years Resolution.

Like most VCSs, Git has the ability to tag specific points in history as being important. Typically people use this functionality to mark release points (v1.0, and so on).

Listing Your Tags

Listing the available tags in Git is accomplished with git tag (with optional -l or --list):

$ git tag
v0.1
v1.3

This command lists the tags in alphabetical order; the order in which they appear has no real importance.

You can also search for tags that match a particular pattern. The Git source repo, for instance, contains more than 500 tags. If you’re only interested in looking at the 1.8.5 series, you can run this:

$ git tag -l "v1.8.5*"
v1.8.5
v1.8.5-rc0
v1.8.5-rc1
v1.8.5-rc2
v1.8.5-rc3
v1.8.5.1
v1.8.5.2
v1.8.5.3
v1.8.5.4
v1.8.5.5

Creating Tags

Git supports two types of tags: lightweight and annotated.

• A lightweight tag is very much like a branch that doesn’t change; it’s just a pointer to a specific commit
• Annotated tags are stored as full objects in the Git database. They’re checksummed; contain the tagger name, email, and date; have a tagging message; and can be signed and verified with GNU Privacy Guard (GPG).

It’s generally recommended that you create annotated tags so you can have all this information; but if you want a temporary tag or for some reason don’t want to keep the other information, lightweight tags are available too.

Annotated Tags

Creating an annotated tag in Git is simple. The easiest way is to specify -a when you run the tag command:

$ git tag -a v1.4 -m "my version 1.4"
$ git tag
v0.1
v1.3
v1.4

The -m specifies a tagging message, which is stored with the tag. If you don’t specify a message for an annotated tag, Git launches your editor so you can type it in.

You can see the tag data along with the commit that was tagged by using the git show command:

$ git show v1.4
tag v1.4
Tagger: Richard Thomas <[email protected]>
Date:   Sat May 3 20:19:12 2014 -0700

my version 1.4

commit ca82a6dff817ec66f44342007202690a93763949
Author: Martin Pickering <[email protected]>
Date:   Mon Mar 17 21:52:11 2008 -0700

    changed the version number

That shows the tagger information, the date the commit was tagged, and the annotation message before showing the commit information.

Lightweight Tags

Another way to tag commits is with a lightweight tag. This is basically the commit checksum stored in a file; no other information is kept. To create a lightweight tag, don’t supply any of the -a, -s, or -m options, just provide a tag name:

$ git tag v1.4-lw
$ git tag
v0.1
v1.3
v1.4
v1.4-lw
v1.5

This time, if you run git show on the tag, you don’t see the extra tag information. The command just shows the commit:

$ git show v1.4-lw
commit ca82a6dff817ec66f44342007202690a93763949
Author: Martin Pickering <[email protected]>
Date:   Mon Mar 17 21:52:11 2008 -0700

    changed the version number

Tagging Later

You can also tag commits after you’ve moved past them. Suppose your commit history looks like this:

$ git log --pretty=oneline
15027957951b64cf874c3557a0f3547bd83b3ff6 Merge branch 'experiment'
a6b4c97498bd301d84096da251c98a07c7723e65 beginning write support
0d52aaab4479697da7686c15f77a3d64d9165190 one more thing
6d52a271eda8725415634dd79daabbc4d9b6008e Merge branch 'experiment'
0b7434d86859cc7b8c3d5e1dddfed66ff742fcbc added a commit function
4682c3261057305bdd616e23b64b0857d832627b added a todo file
166ae0c4d3f420721acbb115cc33848dfcc2121a started write support
9fceb02d0ae598e95dc970b74767f19372d61af8 updated rakefile
964f16d36dfccde844893cac5b347e7b3d44abbc commit the todo
8a5cbc430f1a9c3d00faaeffd07798508422908a updated readme

Now, suppose you forgot to tag the project at v1.2, which was at the “updated rakefile” commit. You can add it after the fact. To tag that commit, you specify the commit checksum (or part of it) at the end of the command:

$ git tag -a v1.2 9fceb02

You can see that you’ve tagged the commit:

$ git tag
v0.1
v1.2
v1.3
v1.4
v1.4-lw
v1.5

$ git show v1.2
tag v1.2
Tagger: Martin Pickering <[email protected]>
Date:   Mon Feb 9 15:32:16 2009 -0800

version 1.2
commit 9fceb02d0ae598e95dc970b74767f19372d61af8
Author: Andy Dunn <[email protected]>
Date:   Sun Apr 27 20:43:35 2008 -0700

    updated rakefile
...

Sharing Tags

By default, the git push command doesn’t transfer tags to remote servers. You will have to explicitly push tags to a shared server after you have created them. This process is just like sharing remote branches; you run git push origin <tagname>.

$ git push origin v1.5
Counting objects: 14, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (12/12), done.
Writing objects: 100% (14/14), 2.05 KiB | 0 bytes/s, done.
Total 14 (delta 3), reused 0 (delta 0)
To [email protected]:martinp/simplegit.git
 * [new tag]         v1.5 -> v1.5

If you have a lot of tags that you want to push up at once, you can also use the --tags option to the git push command. This will transfer all of your tags to the remote server that are not already there.

$ git push origin --tags
Counting objects: 1, done.
Writing objects: 100% (1/1), 160 bytes | 0 bytes/s, done.
Total 1 (delta 0), reused 0 (delta 0)
To [email protected]:martinp/simplegit.git
 * [new tag]         v1.4 -> v1.4
 * [new tag]         v1.4-lw -> v1.4-lw

Now, when someone else clones or pulls from your repository, they will get all your tags as well.

Checking out Tags

If you want to view the versions of files a tag is pointing to, you can do a git checkout, though this puts your repository in “detached HEAD” state, which has some ill side effects:

$ git checkout 2.0.0
Note: checking out '2.0.0'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch>

HEAD is now at 99ada87... Merge pull request #89 from martinp/appendix-final

$ git checkout 2.0-beta-0.1
Previous HEAD position was 99ada87... Merge pull request #89 from martinp/appendix-final
HEAD is now at df3f601... add atlas.json and cover image

In “detached HEAD” state, if you make changes and then create a commit, the tag will stay the same, but your new commit won’t belong to any branch and will be unreachable, except for by the exact commit hash. Thus, if you need to make changes (say you’re fixing a bug on an older version, for instance) you will generally want to create a branch:

$ git checkout -b version2 v2.0.0
Switched to a new branch 'version2'

If you do this and make a commit, your version2 branch will be slightly different than your v2.0.0 tag since it will move forward with your new changes, so do be careful.

Last modified on 2018-01-10