Although the process by which Git calculates SHA-1 hashes is well documented in Pro Git, I had a hard time finding it today and decided to write a blog post that will (hopefully) be a bit easier for myself and others to search for later.

First of all, use the hash-object command as follows to print the SHA-1 hash that Git calculates for an object. (You can also pass a filename as an argument to hash-object.)

$ echo 'test content' | git hash-object --stdin
d670460b4b4aece5915caf5c68d12f560a9fe3e4

Note that, by default, echo prints a trailing newline character so this command is actually computing the SHA-1 hash of "test content\n". Interestingly enough, though, if you try to reproduce this behavior in Ruby by computing the SHA-1 hash of the same string, you get a different result.

$ irb
>> require 'digest/sha1'
=> true
>> puts Digest::SHA1.hexdigest "test content\n"
4fe2b8dd12cd9cd6a413ea960cd8c09c25f19527
=> nil

The reason for this, as explained in Pro Git, is that Git actually prepends the following information to a file's contents before it calculates a hash.

1. The object's type—blob for a regular object, tree for a tree object, and commit for a commit object
2. A space
3. The (human-readable) number of bytes of data in the object
4. A null byte (\0)

In other words, you need to run the following command to generate the appropriate hash.

$ irb
>> require 'digest/sha1'
=> true
>> puts Digest::SHA1.hexdigest "blob 13\0test content\n"
d670460b4b4aece5915caf5c68d12f560a9fe3e4
=> nil    

Hope this helps!