From 28636d797f57a9ad87a0b0872521e4ab81e5720e Mon Sep 17 00:00:00 2001 From: Dirk Gouders Date: Tue, 12 Mar 2024 11:41:56 +0100 Subject: [PATCH] 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 Signed-off-by: Junio C Hamano --- Documentation/user-manual.txt | 36 +++++++++++++++++++++++++++++++++-- 1 file changed, 34 insertions(+), 2 deletions(-) diff --git a/Documentation/user-manual.txt b/Documentation/user-manual.txt index 6433903491..90a4189358 100644 --- a/Documentation/user-manual.txt +++ b/Documentation/user-manual.txt @@ -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