Git: Add part of (changed) file to index *by line numbers*

Question

Is there a git command to add differences within a range of line-numbers to the index?

I want to be able to select lines in my editor and run a macro to add any changes in the selection to the index.

Answer 1

If you can persuade your editor to write a version of the file that you want to be staged, you can use the plumbing-level git commands to add it to the index under the right name. You need to bypass "git add" which will always associate path X in the working tree with path X in the index (as far as I know).

Once you have the content you want to stage written to some temporary file $tempfile, run git hash-object -w $tempfile - this will write the object to .git/objects and output the blob id. Then feed this blob id to the index using git update-index --cacheinfo 100644 $blobid $path to associate the path $path with that object.

Here's an example that stages a change to a script called "post_load" in my repo without overwriting the file itself (also demonstrating you can do without a temp file):

git update-index --cacheinfo 100755 $(perl -lne 'print unless (/^#/)' post_load \
                                      | git hash-object -w --stdin) post_load

You don't mention which editor you're planning to do this from, so it's hard to advise you on how to integrate this. As I mentioned, you need to somehow present git with the file as you want it to be staged (remember, git doesn't deal with storing changes). If you can write a macro to just save the file as "$file.tmp", then use something like the above to git update-index --cacheinfo $the_mode $(git hash-object -w $file.tmp) $file (obtaining $the_mode is left as an exercise :p), delete $file.tmp and revert the editor buffer back to $file that would do basically what you're asking for.

For example, the following script takes three arguments: M N path. It will update the index content for the file at "path" so that lines M through N (inclusive) are replaced with the content from stdin:

#!/bin/sh

start_line=$1
end_line=$2
path=$3

mode=$(git ls-files -s $path | awk '{print $1}')
blob_id=$(
    (
        head -n $(expr $start_line - 1) $path
        cat
        tail -n +$(expr $end_line + 1) $path
        ) | git hash-object -w --stdin
    )
exec git update-index --cacheinfo $mode $blob_id $path

for example echo "HELLO WORLD" | ./stage-part 8 10 post_load will replace the three lines from 8-10 with just "HELLO WORLD".

Answer 2

The easiest way to do it currently is with git add in interactive mode:

git add -i path/to/file

It will launch simple UI where you can choose chunks you want to stage and let you edit any chunk to remove lines you don't want to commit.