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Documentation/user-manual.txt: example for generating object hashes 

Add a simple example on how object hashes can be generated manually.

Further, because the document suggests to have a look at the initial
commit, clarify that some details changed since that time.

Signed-off-by: Dirk Gouders <dirk@gouders.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Dirk Gouders 2024-03-12 11:41:56 +01:00 committed by Junio C Hamano
parent 945115026a
commit 28636d797f
1 changed files with 34 additions and 2 deletions
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Documentation/user-manual.txt
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@ -4093,7 +4093,38 @@ that not only specifies their type, but also provides size information
about the data in the object. It's worth noting that the SHA-1 hash
that is used to name the object is the hash of the original data
plus this header, so `sha1sum` 'file' does not match the object name
for 'file'.
for 'file' (the earliest versions of Git hashed slightly differently
but the conclusion is still the same).
The following is a short example that demonstrates how these hashes
can be generated manually:
Let's assume a small text file with some simple content:
-------------------------------------------------
$ echo "Hello world" >hello.txt
-------------------------------------------------
We can now manually generate the hash Git would use for this file:
- The object we want the hash for is of type "blob" and its size is
12 bytes.
- Prepend the object header to the file content and feed this to
`sha1sum`:
-------------------------------------------------
$ { printf "blob 12\0"; cat hello.txt; } | sha1sum
802992c4220de19a90767f3000a79a31b98d0df7 -
-------------------------------------------------
This manually constructed hash can be verified using `git hash-object`
which of course hides the addition of the header:
-------------------------------------------------
$ git hash-object hello.txt
802992c4220de19a90767f3000a79a31b98d0df7
-------------------------------------------------
As a result, the general consistency of an object can always be tested
independently of the contents or the type of the object: all objects can
@ -4123,7 +4154,8 @@ $ git switch --detach e83c5163
----------------------------------------------------
The initial revision lays the foundation for almost everything Git has
today, but is small enough to read in one sitting.
today (even though details may differ in a few places), but is small
enough to read in one sitting.
Note that terminology has changed since that revision. For example, the
README in that revision uses the word "changeset" to describe what we