Hacking Thy Fearful Symmetry

Hacker, hacker coding bright

Git last

October 15, 2015
git

Edit:: Eric Johnson, in the comments, let me know what I partially reimplemented an already existing wheel. The shocking details are in a new last section, "The wheel, reinvented".

This is a quick one about two new aliases I have added to my Git menagerie. It's nothing big, but I think they can be handy.

I don't know if you're like me, but when comes the time to merge branches in Git, I tend to hop between branches like a crazed rabbit. More often than not, the hopping happens between two branches. And so instead of doing git checkout this and git checkout that and git checkout this again, I felt the need to be able to say git checkout... just checkout the last branch, okay?.

Well, that shouldn't be too bad to do, as Git does retain information about the move from one branch to the next:


$ git reflog 
191d461 HEAD@{0}: checkout: moving from d2p2 to d2p2
191d461 HEAD@{1}: commit: working
a939e7a HEAD@{2}: checkout: moving from master to d2p2
a939e7a HEAD@{3}: commit: v1.7.0
aa31c53 HEAD@{4}: merge dzil: Merge made by the 'recursive' strategy.
2cc2660 HEAD@{5}: checkout: moving from dzil to master
8937ae1 HEAD@{6}: commit: up the D2 version
b76596e HEAD@{7}: commit: manifest
6076d3d HEAD@{8}: commit: mailmap for gwg
2cc2660 HEAD@{9}: checkout: moving from master to dzil
2cc2660 HEAD@{10}: commit (amend): merge branch 'pr/7'
893116b HEAD@{11}: merge pr/7: Merge made by the 'recursive' strategy.
5efd217 HEAD@{12}: checkout: moving from pr/7 to master
9dc7d25 HEAD@{13}: commit: changelog
7694211 HEAD@{14}: commit (amend): simplifications
f108d65 HEAD@{15}: reset: moving to f108d65

So it's just a question of munging. And munging is one of the things I do best.

So I added two aliases, last and colast to my .gitconfig:


    last = "!f(){ \
        git reflog \
        | perl -nE'BEGIN{ $i = shift @ARGV }' \
                -E'say $1 if /moving from (.*?) to/ and ( $i eq \"all\" or ! $i-- )' $1 \
        }; f"

    colast = "!sh -c 'git checkout `git last $1`' -"

git last prints out the last banch we were on. Or the nth one if we provide a number. Or the whole history if we give it all:


$ git last all
d2p2
master
dzil
master
pr/7

$ git last
d2p2

$ git last 1
master

And since I wanted this to mostly jump back to those previous branches, git colast add that extra, final functionality:


$ git co master
Switched to branch 'master'
Your branch is up-to-date with 'github/master'.

$ git colast
Switched to branch 'd2p2'

$ git colast
Switched to branch 'master'
Your branch is up-to-date with 'github/master'.

$ git colast
Switched to branch 'd2p2'

Two notes before I leave:

  • It'll be nice to have the times of the branch jumps in the history, but git reflog doesn't (to my knowledge) keep that information. Although we coudl easily use the commit associated with the reflog line as a lower bound for that switch time.

  • I could push the concept one step further and have my shell populate an environment variable $GIT_LAST each time the prompt is generated (the shell fish even provides hooks that are perfect for that). But considering that things would get messy once I switch to a directory under a different repository... nah, I don't think it's worth it.

The wheel, reinvented

A few hours after the initial writing of the blog post, Eric Johnson pointed out in the comment section that there's already some of that functionality baked in in git. git checkout @{-N} will check out the last nth branch, with a special alias git checkout - for the previous branch. The @{-N} notation also work anywhere else where a commit can be given, like for git log @{-1}.

So that pretty much obsolete git colast right there. But git last itself might still be useful. Win some, lose some, learn lots of stuff in between. Heh. I can live with that. :-)

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About the author

Yanick Champoux
Perl necrohacker , ACP writer, orchid lover. Slightly bonker all around. he/him/his