A lot, as it turns out! 2.11 has been released with a bunch of new features and usability improvements, particularly for those of you working on projects with deep histories, large files, or submodules. Here’s the awesome stuff that piqued our interest on the Bitbucket team: Git Auto-sized SHA-1 abbreviations Alternates for submodules Submodule diffs git ls-files --recurse-submodules Experimental diff improvements Simpler stash IDs Long running filter processes git cat-file --filters receive.maxInputSize Merge comparison shorthand Auto-sized SHA-1 abbreviations Git uses SHA-1 hashes, usually displayed as big ugly hexadecimal strings, to uniquely identify commits and other objects. When developers switch from version control (where monotonically incrementing IDs are easy because they are centrally generated) to version control (where each contributor generates their own IDs) the question of “what would happen if two different objects create the same hash?!” sometimes comes up. Fortunately, the chance of a SHA-1 collision is disregardably tiny. Scott Chacon, author of , puts it best: centralized distributed Pro Git If all 6.5 billion humans on Earth were programming , and every second, each one was producing code that was the equivalent of the entire Linux kernel history (3.6 million Git objects) and pushing it into one enormous Git repository, it would take roughly 2 years until that repository contained enough objects to have a 50% probability of a single SHA-1 object collision. A higher probability exists that every member of your programming team will be attacked and killed by wolves in unrelated incidents on the same night. However, to save on terminal real estate, up until Git 2.10 many common commands abbreviate SHAs to the first 7 hexdigits of the hash. Abbreviated SHAs This can be a problem as abbreviated SHAs are short enough that they may eventually lose their uniqueness. I say because Git will always output enough characters to ensure the abbreviated SHA is unique at the time the command is run. But if you record a short SHA in a commit message, an email, or another external system, it may no longer be unique when you copy and paste it back into a Git command run against the repository at some future date. eventually Abbreviating from 40 hexdigits to 7 brings the number of possible SHAs down from the unthinkably large to the still quite large . However, Git creates a of objects that each require their own SHA-1: one for each commit and annotated tag, one for each version of each file, and one for each tree object (which are created each time a directory is created or its contents change). Even worse, the means that you only need a relatively small number of objects — just in fact — to have a 50% chance of two of them having the same leading 7 hexdigits. 1.46 x 10⁴⁸ 268,435,456 lot birthday paradox 19,290 Git 2.11 introduces a new feature where SHA abbreviation length is calculated based on an estimate of the number of objects in your repository. The Linux kernel has just passed 5 million objects in it’s Git repository, and defaults to twelve hexdigits under Git 2.11: linux.git Git’s own repository, with just over 220,000 objects, defaults to nine hexdigits: git.git If you’re in the habit of recording short SHAs for your own projects, you may wish to future proof yourself by overriding the SHA abbreviation length manually: $ git config --global core.abbrev 12 As a bonus, Git 2.11 has also learnt to list conflicting objects referenced by an abbreviated SHA: So if you do have the misfortune to run into an ambiguously abbreviated SHA-1, Git now gives you enough information to resolve the ambiguity yourself. Submodules, submodules, submodules Git submodules have seen a raft of improvements in recent Git releases: parallelized fetching in Git 2.8 in Git 2.9 git clone --shallow-submodules “shallow” flags in 2.10 .gitmodules I still recommend submodules only as a last resort, as dependency management systems are typically more effective for combining projects. However, Git 2.11 does have three notable improvements that make submodules significantly easier to use if you do need them. Feel free to skip to the if you have no interest in submodules. section on [git diff](#892c) improvements Alternates for submodules The option can be used with to specify another local repository as an "alternate" object store, to save recopying objects over the network that you already have locally. The syntax is: --reference git clone $ git clone --reference <local repo> <url> As of Git 2.11, you can use in combination with to set up alternates pointing to submodules from another local repository. The syntax is: --reference --recurse-submodules submodule $ git clone --recurse-submodules --reference <local repo> <url> This can potentially save a huge amount of bandwidth and local disk, but will fail if the referenced local repository does not have all the required submodules of the remote repository that you’re cloning from. Fortunately the handy option will fail gracefully and fall back to a normal clone for any submodules missing from the referenced local repository: --reference-if-able $ git clone --recurse-submodules --reference-if-able <local repo> <url> Submodule diffs Previously Git had two modes for displaying diffs of commits that updated your repository’s submodules: displays the old commit and new commit from the submodule referenced by your project (this is also the default if you omit the option altogether): git diff --submodule=short --submodule git diff --submodule=short is a bit more useful, and displays the summary line from the commit message of any new or removed commits in the updated submodule: git diff --submodule=log git diff --submodule=log Git 2.11 introduces a third option: . This displays a full diff of all changes in the updated submodule: --submodule=diff git diff --submodule=diff git ls-files --recurse-submodules learnt the option. Traditionally listed tracked files in your repository, and as of Git 2.11 can list files tracked by your submodules as well: git ls-files --recurse-submodules git ls-files git ls-file --recurse-submodules This perhaps isn’t a command that you’d run every day, but it’s handy if you want to write a script that walks every file in your repository its submodules. and Experimental diff improvements can produce some slightly confusing output when the lines before and after a modified section are the same. This can happen when you have two or more similarly structured functions in a file. For a slightly contrived example, imagine we have a simple file that contains a single function: git diff /* @return {string} "Bitbucket" */function productName() {return "Bitbucket";} Now imagine we’ve committed a change that prepends function that does something similar: another /* @return {string} "Bitbucket" */function productId() {return "Bitbucket";} /* @return {string} "Bitbucket" */function productName() {return "Bitbucket";} You’d expect to show the top five lines as added, but it actually incorrectly attributes the very first line to the original commit: git diff The wrong comment is included in the diff. Not the end of the world, but the couple of seconds of cognitive overhead from the every time this happens can add up. whaaat? Git 2.11 introduces a new experimental diff option, , that attempts to produce more aesthetically pleasing diffs: --indent-heuristic git diff --indent-heuristic Under the hood, cycles through the possible diffs for each change and assigns each a “badness” score. This is based on heuristics like whether the diff block starts and ends with different levels of indentation (which is aesthetically bad) and whether the diff block has leading and trailing blank lines (which is aesthetically pleasing). Then the block with the lowest badness score is output. --indent-heuristic This feature is experimental, but you can test it out ad-hoc by applying the option to any command. Or if you like to live on the bleeding edge, you can enable it across your system with: --indent-heuristic git diff $ git config --global diff.indentHeuristic true Simpler stash IDs The command is a nifty little tool for temporarily shelving changes while you work on something else. If you're a fan, you probably know that you can shelve multiple changes, and view them with : [git stash](https://www.atlassian.com/git/tutorials/git-stash/) git stash list git stash list However you may not know why Git’s stashes have such awkward identifiers — , , etc. — and may have written them off as “just one of those Git idiosyncrasies”. It turns out that like many Git features, these weird IDs are actually a symptom of a very clever use (or abuse) of the Git data model. stash@{1} stash@{2} Under the hood, the command actually creates a set of special commit objects that encode your stashed changes, and maintains a that holds references to these special commits. This is why the output from looks a lot like the output from the command. When you run , you're actually saying “apply the commit at position 1 from the stash reflog”. git stash reflog git stash list git reflog git stash apply stash@{1} As of Git 2.11, you no longer have to use the full syntax. Instead, you can reference stashes with a simple integer indicating their position in the stash reflog: git stash@{n} $ git stash show 1$ git stash apply 1$ git stash pop 1 And so forth. If you’d like to learn more about how stashes are stored, I wrote a little bit about it in . How git stash works Long running filter processes Bitbucket supports the popular extension to help users who need to efficiently track large binary files in their repositories. Git 2.11 comes with a couple of improvements to make Git LFS faster and more pleasant to use. The most important change is that Git now supports long-running clean and smudge filter processes for transforming LFS pointers, rather than having to invoke a new process each time. Git LFS (Large File Storage) much When you a file, can be used to transform (or ) the file’s contents before being written to the Git object store. Git LFS reduces your repository size by using a clean filter to squirrel away large file content in the LFS cache, and adds a tiny “pointer” file to the Git object store instead. git add clean filters clean The Git LFS filter converts large files into tiny pointer files. clean are the opposite of clean filters — hence the name. When file content is read from the Git object store during a , smudge filters have a chance to transform it before it’s written to the user’s working copy. The Git LFS smudge filter transforms pointer files by replacing them with the corresponding large file, either from your LFS cache or by reading through to your Git LFS store on Bitbucket. Smudge filters git checkout The Git LFS filter converts pointer files back into the large file content. smudge Traditionally smudge and clean filter processes were invoked once that was being added or checked out. So a project with 1,000 files tracked with Git LFS invoked the command 1,000 times for a fresh checkout! While each operation is relatively quick, the overhead of spinning up 1,000 individual smudge processes is costly. per file git-lfs-smudge As of Git 2.11, smudge and clean filters can be defined as long running processes that are invoked once for the first filtered file, then fed subsequent files that need smudging or cleaning until the parent Git operation exits. Lars Schneider, who contributed long running filters to Git, nicely summarized the impact of the change on Git LFS performance: Filter process is 💥 80x faster 💥 on macOS and 💥 58x faster 💥 on Windows for the test repo with 12k files. On Windows that means the tests runs in 57 seconds instead of 55 minutes ! That’s a seriously impressive performance gain! Note that you’ll need to upgrade to both Git 2.11 Git LFS 1.5 to take advantage of these speed improvements. and git cat-file --filters Another small improvement for users of Git LFS and other filter-based extensions is the new option for the command. lets you inspect objects in your Git object store (the is for “pretty print”). For example, I can view the contents of the file on the branch with: --filters git cat-file git cat-file -p -p images/bitbucket.png v2 The contents of a Git LFS pointer file, courtesy of git cat-file -p However, happens to be tracked with Git LFS, and skips the Git LFS smudge filter (and any other filters that we've defined), so rather than getting our image back we just get the contents of the Git LFS pointer file. bitbucket.png git cat-file As of Git 2.11, you can ensure the appropriate filters are applied with the option. For Git LFS files, this will dereference the pointer and look up the actual content, typically printing an impressive amount of binary junk to your terminal: --filters The actual LFS file content, courtesy of git cat-file --filters For binary content, it’s usually better to pipe the content to a temporary file instead of your terminal. Or, if you’re on macOS, you can pipe it straight to an appropriate editor in one step with open -a <app> -f: git cat-file --filters <commit-ish>:<path> | open -a Preview.app -f Alternatively, Atlassian’s free Git client has superb support for Git LFS, including binary previews of LFS-tracked content. And I don’t use the term lightly: SourceTree’s founding developer—Steve “Sinbad” Streeting — also happens to have . SourceTree superb contributed 31 KLOC to the Git LFS project receive.maxInputSize Git 2.11 also introduces a server-side setting for restricting the number of bytes that can be transferred in a single push. This is handy for repo administrators who want to prevent users from accidentally pushing large binaries (that are better tracked with Git LFS) to the upstream repository. The setting is useful if you're running your own homegrown Git server or otherwise administering your server config by hand. Running the following command will limit the push size to 10 megabytes: receive.maxInputSize on your Git server $ git config --global receive.maxInputSize 10485760 If you’re using Bitbucket Server, I’d recommend using a pre-receive hook add-on like to constrain pushes. The gives you much more flexibility over things file size and type, naming conventions, and enforcing commit provenance, rather than just limiting the total number of bytes pushed. ScriptRunner Bitbucket hook API Merge comparison shorthand Git has a bunch of neat shorthand for traversing your commit history. One popular command for showing the changes that were merged in a merge commit is: $ git log <merge commit>^..<merge commit> which roughly translates to: Show me all the new commits that were merged with _<merge commit>_ ! The in the command means “parent of”, and the double-dot syntax specifies that we're interested in commits that are reachable by the merge commit, but reachable by its first parent. (The first parent of a merge is the commit that was at the tip of the checked out branch when the merge was performed.) ^ not git log <merge>^..<merge> shows the commits merged into the target branch by <merge> Git 2.11 introduces a shorthand for this syntax: $ git log <merge commit>^-1 You can also replace with to compare the merge commit to its second parent, to see commits that were not on the merged branch. ^-1 ^-2 git log <merge>^-n shows commits from all branches that aren’t the nth parent of <merge> Or, if you’re into octopus merges (merges involving more than two parents), you can use to compare against the n_th_ parent. ^-n This new form works anywhere you can specify a revision range! For example, you can use: $ git diff <merge commit>^-1 to generate a diff of all the changes introduced by a merge commit. </tome> You made it, thanks for reading! This is a long post, but we only managed to get through of the 100+ features, fixes, and performance improvements shipped by Junio and his merry band of Git wizards in version 2.11. Check out , and then (or upgrade using your favorite package manager). ten the full release notes install the latest version of Git And if you ever want to chat about Git or , grab me on Twitter: I’m . Bitbucket @kannonboy

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