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
$ 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.
- The object's type—
blobfor a regular object,
treefor a tree object, and
commitfor a commit object
- A space
- The (human-readable) number of bytes of data in the object
- A null byte (
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!