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git-vanity-hash

Overview

git-vanity-hash is a command line tool for creating commit hashes with a specific prefix. Hashes are computed on the cpu using all available cores.

FAQ

Question Answer
Is this a good idea? No
Will this break git tooling? Probably (see how it works)

Usage

git-vanity-hash <mode> [prefix]

mode
    find        Find and print hash (read-only)
    update      Find and update HEAD with found hash
    revert      Revert HEAD back to original commit

prefix
    A hexadecimal string the hash should start with

Examples

Find hash
$ git-vanity-hash find cafe
Found hash: cafe7f3302e66fef6428029563534ff2d8d0bc4f
Update HEAD
$ git-vanity-hash update cafe
Updated HEAD from 6af06aeb70482ba69e5c85225f8c4a0e98cbd942 to cafe7f3302e66fef6428029563534ff2d8d0bc4f
Revert HEAD
$ git-vanity-hash revert
Reverted HEAD from cafe7f3302e66fef6428029563534ff2d8d0bc4f to 6af06aeb70482ba69e5c85225f8c4a0e98cbd942
Full example
$ git init
Initialized empty Git repository in /Users/user/cool-project/.git/

$ git add README.md

$ git commit -m "Add readme"
[master (root-commit) 6af06ae] Add readme
 1 file changed, 1 insertion(+)
 create mode 100644 README.md

$ PAGER=cat git log
commit 6af06aeb70482ba69e5c85225f8c4a0e98cbd942 (HEAD -> master)
Author: Petter Rasmussen <petter@hask.no>
Date:   Sun Feb 23 14:11:19 2020 +0100

    Add readme

$ git-vanity-hash update cafe
Updated HEAD from 6af06aeb70482ba69e5c85225f8c4a0e98cbd942 to cafe7f3302e66fef6428029563534ff2d8d0bc4f

$ PAGER=cat git log
commit cafe7f3302e66fef6428029563534ff2d8d0bc4f (HEAD -> master)
Author: Petter Rasmussen <petter@hask.no>
Date:   Sun Feb 23 14:11:19 2020 +0100

    Add readme

How it works

The commit object is read using git cat-file commit HEAD. An example how it looks:

tree bd2171194b1f31c586da1ee133a845810e303a23
parent 000000757d84e66644069f387f30f657faee4187
author Petter Rasmussen <petter@hask.no> 1582460250 +0100
committer Petter Rasmussen <petter@hask.no> 1582460640 +0100

Add README

A new vanity header and value is added. An example how it looks after the vanity headers is added:

tree bd2171194b1f31c586da1ee133a845810e303a23
parent 000000757d84e66644069f387f30f657faee4187
author Petter Rasmussen <petter@hask.no> 1582460250 +0100
committer Petter Rasmussen <petter@hask.no> 1582460640 +0100
vanity 7-16488

Add README

The string is hashed (see commit_info.rs for more details) and checked if it has the wanted prefix. The vanity value is increased until a matching hash prefix is found.

When a match is found the new commit object is written to the object database with git hash-object -t commit --stdin And then HEAD is changed to point to the new object with git update-ref HEAD <hash>

The vanity header will normally not show when using basic git commands, but can be seen using i.e. git cat-file commit HEAD. Note that the default git tools has support for extra headers, but there is no guarantee that this won't break 3rd party tools.

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