Question

Is there a way to efficiently read previous versions of files in Git? (I’m using Git as a database for Content Management System, and need it do display history).

Git doesn’t store full versions of files, it only store differences, so, if You need previous version – You can’t just read it from disk, You should ask Git to calculate it using differences.

It seems that GitHub somehow does that, for example, You can see previous version of file. Does it really calculates it for every HTTP request or somehow optimizes it?

Answer 1

Erratum: git ALWAYS stores full versions of files. Thus viewing any revision is equally efficient.

This is in marked contrast to some other revision systems which only store diffs (patches). Cvs in particular was hideous for accessing deep history or non-trunk branches for this very reason (for a large repository with many users).

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For reference, To conveniently access a particular file at a particular version (sha/reference):

git show HEAD:full/path/to/file

Replace head with a tag, branch or git sha (the long hex number)

The path is the full path relative to the base of the git repository-not the file system root. I only mention this because it has bitten me a few times-you can’t cd into a directory and expect to not specify the full path.

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Wikipedia (home of all that is true and good) backs me up:

Git stores each revision of a file as a unique blob object. The relationships between the blobs can be found through examining the tree and commit objects. Newly added objects are stored in their entirety using zlib compression.

In case Wikipedia isn’t your bag, a careful reading of the git internals manual also verifies it.