Generate a git patch for a specific commit

Try:

git format-patch -1 <sha>

For generating the patches from the topmost commits from a specific sha1 hash:

git format-patch -<n> <SHA1>

The last 10 patches from head in a single patch file:

git format-patch -10 HEAD --stdout > 0001-last-10-commits.patch

Say you have commit id 2 after commit 1 you would be able to run:

git diff 2 1 > mypatch.diff

where 2 and 1 are SHA hashes.

This command (as suggested already by @Naftuli Tzvi Kay):

git format-patch -1 HEAD

^{Replace HEAD with specific hash or range.}

will generate the patch file for the latest commit formatted to resemble UNIX mailbox format.

-<n> - Prepare patches from the topmost commits.

Then you can re-apply the patch file in a mailbox format by:

git am -3k 001*.patch

See: man git-format-patch.

If you want to be sure the (single commit) patch will be applied on top of a specific commit, you can use the new git 2.9 (June 2016) option git format-patch --base

git format-patch --base=COMMIT_VALUE~ -M -C COMMIT_VALUE~..COMMIT_VALUE

# or
git format-patch --base=auto -M -C COMMIT_VALUE~..COMMIT_VALUE

# or
git config format.useAutoBase true
git format-patch -M -C COMMIT_VALUE~..COMMIT_VALUE

See commit bb52995, commit 3de6651, commit fa2ab86, commit ded2c09 (26 Apr 2016) by Xiaolong Ye (``).
^{(Merged by Junio C Hamano -- gitster -- in commit 72ce3ff, 23 May 2016)}

format-patch: add '--base' option to record base tree info

Maintainers or third party testers may want to know the exact base tree the patch series applies to. Teach git format-patch a '--base' option to record the base tree info and append it at the end of the first message (either the cover letter or the first patch in the series).

The base tree info consists of the "base commit", which is a well-known commit that is part of the stable part of the project history everybody else works off of, and zero or more "prerequisite patches", which are well-known patches in flight that is not yet part of the "base commit" that need to be applied on top of "base commit" in topological order before the patches can be applied.

The "base commit" is shown as "base-commit: " followed by the 40-hex of the commit object name.
A "prerequisite patch" is shown as "prerequisite-patch-id: " followed by the 40-hex "patch id", which can be obtained by passing the patch through the "git patch-id --stable" command.

To generate path from a specific commit (not the last commit):

git format-patch -M -C COMMIT_VALUE~1..COMMIT_VALUE