Re: Alter parent ID of existing commit object

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 [off-list ref] 1272275407 -0500
  committer Jonathan Nieder [off-list ref] 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 [off-list ref]
  Signed-off-by: Jonathan Nieder [off-list ref]
  $ 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