Is there a way to "autosign" commits in Git with a GPG key?

Edit: As of Git version 1.7.9, it is possible to sign Git commits (git commit -S). Updating the answer slightly to reflect this.

The question title is:

Is there a way to “autosign” commits in Git with a GPG key?

Short answer: yes, but don't do it.

Addressing the typo in the question: git commit -s does not sign the commit. Rather, from the man git-commit page:

-s, --signoff
Add Signed-off-by line by the committer at the end of the commit log message.

This gives a log output similar to the following:

± $ git log                                                                                 [0:43:31]
commit 155deeaef1896c63519320c7cbaf4691355143f5
Author: User Name 
Date:   Mon Apr 16 00:43:27 2012 +0200

    Added .gitignore

    Signed-off-by: User Name 

Note the "Signed-off-by: ..." bit; that was generated by the -s flag on the git-commit.

Quoting the release announcement email:

• "git commit" learned "-S" to GPG-sign the commit; this can be shown with the "--show-signature" option to "git log".

So yes, you can sign commits. However, I personally urge caution with this option; automatically signing commits is next to pointless, see below:

Just a side question, maybe commits shouldn't be signed, only tags, which I never create, as I submit single commits.

That's correct. Commits are not signed; tags are. The reason for this can be found in this message by Linus Torvalds, the last paragraph of which says:

Signing each commit is totally stupid. It just means that you automate it, and you make the signature worth less. It also doesn't add any real value, since the way the git DAG-chain of SHA1's work, you only ever need one signature to make all the commits reachable from that one be effectively covered by that one. So signing each commit is simply missing the point.

I'd encourage a browse of the linked message, which clarifies why signing commits automatically is not a good idea in a far better way than I could.

However, if you want to automatically sign a tag, you would be able to do that by wrapping the git-tag -[s|u] in an alias; if you're going to do that, you probably want to setup your key id in ~/.gitconfig or the project-specific .git/config file. More information about that process can be seen in the git community book. Signing tags is infinitely more useful than signing each commit you make.

Note: if you don't want to add -S all the time to make sure your commits are signed, there is a proposal (branch 'pu' for now, December 2013, so no guarantee it will make it to a git release) to add a config which will take care of that option for you.
Update May 2014: it is in Git 2.0 (after being resend in this patch series)

See commit 2af2ef3 by Nicolas Vigier (boklm):

Add the commit.gpgsign option to sign all commits

If you want to GPG sign all your commits, you have to add the -S option all the time.
The commit.gpgsign config option allows to sign all commits automatically.

commit.gpgsign

A boolean to specify whether all commits should be GPG signed.
Use of this option when doing operations such as rebase can result in a large number of commits being signed. It may be convenient to use an agent to avoid typing your GPG passphrase several times.

---

That config is usually set per repo (you don't need to sign your private experimental local repos):

cd /path/to/repo/needing/gpg/signature
git config commit.gpgsign true

You would combine that with user.signingKey used as a global setting (unique key used for all repo where you want to sign commit)

git config --global user.signingkey F2C7AB29!
                                           ^^^

As ubombi suggests in the comments (and explain in "GPG Hardware Key and Git Signing", based on "How to Specify a User Id")

When using gpg an exclamation mark (!) may be appended to force using the specified primary or secondary key, and not to try and calculate which primary or secondary key to use.

Note that Rik adds in the comments:

If you're using something like a YubiKey (as recommended) you don't need to worry about the exclamation point because the only signing key(s) you should have available for a primary key-pair are:

• the primary key itself, which should have a # after it indicating it's not available,
• and the secret subkey with a > after it indicating it's a stub that points to the YubiKey as the only available signing key in its applet.

Only if you keep all your private keys available on your system (bad practice), then probably it would be a good idea to prevent auto-selection between available signing keys

---

user.signingKey was introduced in git 1.5.0 (Jan. 2007) with commit d67778e:

There shouldn't be a requirement that I use the same form of my name in my git repository and my gpg key.
Further I might have multiple keys in my keyring, and might want to use one that doesn't match up with the address I use in commit messages.

This patch adds a configuration entry "user.signingKey" which, if present, will be passed to the "-u" switch for gpg, allowing the tag signing key to be overridden.

This is enforced with commit aba9119 (git 1.5.3.2) in order to catch the case where If the user has misconfigured user.signingKey in their .git/config or just doesn't have any secret keys on their keyring.

Notes:

• By convention, since git 2.4.0 March 2015, it is signingKey, not signingkey, even though the git config keys are case insensitive. That would matter only if you do git config --get-regexp, which is case sensitive, otherwise, it is only a readability convention;
• If you want the git server to check the signature for each push, you will need git 2.2+ (Oct. 2014) at least (commit b945901), as git push --signed failed to consider the user.signingKey config value;
• git 2.9 (June 2016) will use user.signingKey to force signing annotated tags as well as commits: commit 61c2fe0.

git config --global user.signingKey 9E08524833CB3038FDE385C54C0AFCCFED5CDE14
git config --global commit.gpgSign true

Replace 9E08524833CB3038FDE385C54C0AFCCFED5CDE14 by your key ID. Remember: It's never a good idea to use the short ID.

UPDATE: Per a new git edict, all config keys should be in camelCase.