Refname is ambiguous

Question

Yesterday, I created a branch from a branch, pushed it to origin, and merged it back into master. You can see that here:

$ history |less  8358  git co master  8359   commit # shortcut that adds all and commits  8360  push # shortcut that `git push`'s  8361  git lg # shortcut that logs my output  8362  git co 1600  8363  git co -b 1601  8364  npm run test  8365  npm run test  8366  npm run test  8367  npm run test  8368  npm run test  8369  npm run test  8370  npm run test  8371  npm run test  8372  npm run test  8373  npm run test  8374  npm run test  8375  npm run test  8376  npm run test  8377  ./release.sh  8378  ./release.sh  8379  commit  8380  push  8381      git push --set-upstream origin 1601  8382  git lg  8383  git co master  8384  git merge --no-ff -  8385  git st  8386  commit   8387  push  

I'm including irrelevant stuff in this list so I don't accidentally cut out something that is relevant; as you can see, all of this history is sequential. Here is everything in my history that refers to 1601:

$ history | grep 1601  1601  npm run test  8363  git co -b 1601  8381      git push --set-upstream origin 1601  8438  git co 1601  8445  git co 1601  8446  git show-ref 1601  8447  history | grep 1601  8449  git show-ref | grep 1601  8458  git show-ref --heads --tags | grep 1601  8460  history | grep 1601  8461  history | grep (1601|merge)  8462  history | grep -p (1601|merge)  8464  history | grep -E (1601|merge)  8466  history | grep -E -e (1601|merge)  8467  history | grep -E -e \(1601|merge\)  8468  history | grep -E -e '(1601|merge)'  8469  history | grep -E -e '(1601|merge|master)'  8470  history | grep -E -e '(1601|1601|merge|master)'  8472  history | grep -E -e '(1601|1601|merge|master)'  8474  history | grep -E -e '(1601|1601|merge|master)'  8480  history | grep 1601 

Somehow, after these steps, whenever I git co 1601, it gives me this warning:

$ git co 1601 warning: refname '1601' is ambiguous. Already on '1601' Your branch is up to date with 'origin/1601'. 

I've been googling for a while, looking for others that have had this issue, but none of them seem to have the same situation. Here is what I've looked at that hasn't helped:

1. Git warning: refname 'xxx' is ambiguous
2. Git: refname 'master' is ambiguous
3. git refname 'origin/master' is ambiguous
4. http://thesimplesynthesis.com/post/git-error-warning-refname-originbranch-name-is-ambiguous

The following output is to help with troubleshooting:

$ git show-ref | grep 1601 137b74c8ff6807f07bb32ba0d2f76a3ba287e981 refs/heads/1601 137b74c8ff6807f07bb32ba0d2f76a3ba287e981 refs/remotes/origin/1601 

$ git tag -l  

$ git show-ref --heads --tags | grep 1601 137b74c8ff6807f07bb32ba0d2f76a3ba287e981 refs/heads/1601 

cat .git/config # this is the only place where it mentions 1601 [branch "1601"]         remote = origin         merge = refs/heads/1601  $ cat .git/config | grep 1601 [branch "1601"]         merge = refs/heads/1601  

$ git checkout 1601 -- warning: refname '1601' is ambiguous. Already on '1601' Your branch is up to date with 'origin/1601'. 

$ git ls-remote . \*1601* beea6ee68d6fe6b4f5222c0efe0e206e23af993c        refs/heads/1601 beea6ee68d6fe6b4f5222c0efe0e206e23af993c        refs/remotes/origin/1601 

$ git show 1601 # this only shows one log message warning: refname '1601' is ambiguous. commit beea6ee68d6fe6b4f5222c0efe0e206e23af993c (HEAD -> 1601, origin/1601) ... 

$ git --version git version 2.31.1 

$ type git git is hashed (/usr/bin/git) 

FWIW, I am using cygwin on Windows.

Why am I getting this warning, how do I get rid of it, and how do I prevent it in the future?

Answer 1

I can get this behavior by giving a branch a name that matches the SHA1 prefix of a commit.

$ git rev-list @ | egrep '^[0-9]{4}' 7802d6937673dbff4d26dc906c714a054c9e883e 81070af68e1b254c7f36eccb1261050a5a4a133a 7537dac08fd165845b6300f32c19a52fc7fa7299 $ git branch 7802 $ git branch 9999 $ git checkout 7802 warning: refname '7802' is ambiguous. Switched to branch '7802' $ git checkout 9999 Switched to branch '9999' $ 

Since object id prefixes at least four digits long are legitimate ways of referring to objects, making a ref name that's also a four-or-more-long hex string is likely to produce this kind of ambiguity. So, don't do that. If you want to number something, include a type marker, like bugfix/1601 or something.

Answer 2

Why am I getting this warning ...

As you figured out via jthill's answer, it is because 1601 matches some suitable object's hash.

There are some other ways to get the same error, including making a branch and tag name that have the same spelling (e.g., git branch abc; git tag abc).

how do I get rid of it ...

Use unambiguous names. For branch-vs-tag, that means don't use the same name as a branch and tag name; for branch-or-tag vs commit hash, that means avoid commit-hash names that might occur here. As TTT commented, commit hash abbreviations must contain at least four characters, so abc is OK as a branch or tag name even though all three characters in abc are legitimate in hash IDs.

To see if a branch name might potentially collide with a hash ID, check whether it consists entirely of characters from the set [0-9][a-f][A-F] and is at least four characters long. If so, add some character that is outside the set, or shorten the name. A quick:

egrep '^[a-f]{4,}$' /usr/share/dict/words 

turns up some interesting branch names to avoid, such as accede, aface, beaded, deed, and facade. (These are of course the sources of various fake hash IDs I like to use as examples, such as feedc0ffee, deadbeef, cafedad, and so on—some require a bit of 11ce45e¹ with the spelling.)

and how do I prevent it in the future?

A really handy system is to systematize your branch names to include / or - or something: for instance, feature/c0ffee never runs into any issues like this.

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¹"License", in something vaguely resembling 1337-5p34k.