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Emily Dunham: Creating and deleting a Git branch named -D

do, 13/10/2016 - 09:00
Creating and deleting a Git branch named -D

git branch -D deletes a Git branch. Yet someone on IRC asked, “I accidentaly got a git branch named -D. How do I delete it?”. I took this as a personal challenge to create and nuke a -D branch myself, to explore this edge case of one of my favorite tools.

Making a branch with an illegal name

You create a branch in Git by typing git branch branchname. If you type git branch -D, the -D will be passed as an argument to the program by your shell, because your shell knows that all things starting with - are arguments.

You can tell your shell “I just mean a literal -, not an argument” by escaping it, like git branch \-D. But Git sees what we’re up to, and won’t let that fly. It complains fatal: '-D' is not a valid branch name.. So even when we get the string -D into Git, the porcelain spits it right back out at us.

But since this is Unix and Everything’s A File(TM), I can create a branch with a perfectly fine name to get through the porcelain and then change it later. If I was at the Git wizardry level of Emily Xie I could just write the files into .git without the intermediate step of watching the porcelain do it first, but I’m not quite that good yet.

So, let’s make a branch with a perfectly fine name in a clean repo, then swap things around under the hood:

$ mkdir dont $ cd dont $ git init $ git commit --allow-empty -am "initial commit" [master (root-commit) da1f6b6] initial commit $ git branch * master $ git checkout -b dashdee switched to a new branch 'dashdee' $ git branch * dashdee master $ grep -ri dashdee .git/ .git/HEAD:ref: refs/heads/dashdee .git/logs/HEAD:da1f6b67446e83a456c4aeaeef1e256a8531640e da1f6b67446e83a456c4aeaeef1e256a8531640e E. Dunham <> 1476402564 -0700 checkout: moving from master to dashdee $ find -name dashdee ./.git/refs/heads/dashdee ./.git/logs/refs/heads/dashdee

OK, so we’ve got this dashdee branch. Time to give it the name we’ve wanted all along:

$ find .git -type f -print0 | xargs -0 sed -i 's/dashdee/\-D/g' $ mv .git/refs/heads/dashdee .git/refs/heads/-D $ mv .git/logs/refs/heads/dashdee .git/logs/refs/heads/-D Look what you’ve done...

Is this what you wanted?:

$ git branch * -D master

You are really on a branch named -D now. You have snuck around the guardrails, though they were there for a reason:

$ git commit --allow-empty -am "noooo" [-D 18dac23] noooo Try to make it go away $ git branch -D -D fatal: branch name required

It won’t give up that easily! You can’t escape:

$ git branch -D \-D fatal: branch name required $ git branch -D '-D' fatal: branch name required $ git branch -D '\-D' error: branch '\-D' not found.

Notice the two categories of issue we’re hitting: In the first 2 examples, the shell was eating our branch name and not letting it through to Git. In the third case, we threw so many escapes in that Bash passed a string other than -D through to Git.

As an aside, I’m using Bash for this. Other shells might be differently quirky:

$ echo $0 bash $ bash --version GNU bash, version 4.3.46(1)-release (x86_64-pc-linux-gnu) Copyright (C) 2013 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <> This is free software; you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Succeed at making it go away

Bash lets me nuke the branch with:

$ git branch -D ./-D Deleted branch ./-D (was broken). $ git branch master

However, if your shell is not so eaily duped into passing a string starting with - into a program, you can also fix things by manually removing the file that the branch -D command would have removed for you:

$ rm .git/refs/heads/-D $ git branch master Clean up $ cd .. $ rm -rf dont

Please don’t do this kind of silly thing to any repo you care about. It’s just cruel.

Categorieën: Mozilla-nl planet