Re: Alter parent ID of existing commit object

[Jonathan Nieder <jrnieder@gmail.com>]

Hi Paul,

Paul Richards wrote:

> Is it possible to edit an old commit object and only alter the ID of
> the parent commit but otherwise leave all the other information intact
> (tree, message, authors, date, etc).
> 
> I'd expect such a command to return the new hash of the modified commit.

The standard answer to this question is to say “use grafts and
filter-branch”.  The git-filter-branch(1) man page explains this
approach.  It is very powerful, but sometimes I do not want to have
that much power.

So I will tell a secret: in the scenarios when I wanted something like
this (actually, what I have occasionally wanted is to transform a
single-parent commit into a merge), I did something like the following:

  $ git cat-file commit $rev
  tree dcd2cc4b76f8756423f5c1ab7d2c62d458a8b15f
  parent 5f1e6d9ce35e212708f9adc55e6b9a7e0d296df4
  author Will Palmer <wmpalmer@gmail.com> 1272275407 -0500
  committer Jonathan Nieder <jrnieder@gmail.com> 1272275443 -0500
  
  pretty: Respect --abbrev option
  
  Prior to this, the output of git log -1 --format=%h was always 7
  characters long, without regard to whether --abbrev had been passed.
  
  Signed-off-by: Will Palmer <wmpalmer@gmail.com>
  Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
  $ git cat-file commit $rev >tmp
  $ sed -i -e "s/parent .*/parent $(git rev-parse othercommit)/" tmp
  $ git hash-object -t commit -w tmp
  ca55c560685284ac6d121939b2cd881f426e7074

Easy.  Still, I would be happy to see this packaged in a command, so I
could recommend it in combination with ‘git replace’ to people who are
scared of sed.

Thanks for bringing it up.
Jonathan