Tonight I learned a basic git trick that was not immediately obvious to me, but should have been, I guess.

I've been switching my Spacemacs back to the master branch to try trouble-shoot a performance issue I'm having on the Macintosh where it just hangs occasionally. My master is tracking to Spacemacs master which is still at 0.200.13. I haven't touched it in over a year, and there are some things that I wanted from my develop branch.

I want to merge in the latest version of those few files, but not everything on the branch, so a merge is not the right operation.

Turns out you can use git checkout to do this very easily. The "trick" was: you can specify a path as well as a branch to check out (derp, I knew that). Magit doesn't (so far as I can find) let you check out just one or a few files, so you have to do it from the command line:

git checkout <branch> [<path>...]

Here's an example I did tonight. To get my latest EShell aliases from my Spacemacs develop branch, and merge them into master, I just did this:

$ git checkout master
$ git checkout develop config/eshell/alias
$ git add .
$ git commit -m "merged latest eshell aliases from develop branch"

It also can be used the other way. I fixed a bug on my SUSE laptop where Spacemacs would not start up cleanly. I had a syntax error in my layers/mjl/keybindings.el on a condition that only gets executed outside of a machine running darwin -- clearly I introduced it last year on my Mac and never found out until I tried to use this on a SUSE machine!

Anyway that fix commit is on my master branch. Back to going the other way. One other thing that I wanted from my develop branch is my latest keybindings. When I merge my latest keybindings from develop to master it doesn't have the fix.

I used a combination of git command-line and Magit's cool selective hunk stage/unstage feature (it's like "interactive" mode in the git command line, but not horrible) to get it done very smoothly:

From the command line, on the master branch: git checkout develop layers/mjl/keybindings.el
In Magit, stage the changes, then un-stage the hunk that has the syntax error
In Magit, commit the staged changes on master
Switch to the develop branch (Magit: b b -> develop)
On the command line, in develop branche: git checkout master layers/mjl/keybindings.el
Back in Magit, stage the single hunk with the fix for the syntax error
Commit the change
Push everything up to origin
Switch back to master branch

It's such a simple thing but very handy, I had to blog it while it's fresh in my mind.