git: get *new* contributors (authors) list in release/tag

Question

I'm preparing changelog for a release and doing some statistic.

It's quite easy to list contributors from previous release:

git shortlog -s -e -n TAG..

reviewers:

git log TAG.. | grep -Ei '(reviewed|acked)-by:' |sed 's/.*by: //' | sort | uniq -c | sort -n -r

committers:

git shortlog -s -e -n -c TAG..

But how to list new contributors (authors) since TAG (e.g. those who haven't been committing before TAG)?

Answer 1

Assuming you can use bash, there are at least two ways:

#!/bin/bash

set -e

OLD_COMMIT="$1"
NEW_COMMIT="$2"
if test -z "$OLD_COMMIT" || test -z "$NEW_COMMIT"; then
    echo 'fatal: must provide OLD_COMMIT and NEW_COMMIT commits' >&2
    exit 1
fi
shift 2

The first, using declarative arrays introduced back in bash 4 (emulates right outer join in imperative fashion):

declare -A OLD_AUTHORS
while read -r OLD_AUTHOR; do
    OLD_AUTHORS["$OLD_AUTHOR"]=1
done < <(git log --pretty=format:%aN "$OLD_COMMIT")

declare -A NEW_AUTHORS
while read -r NEW_AUTHOR; do
    if test -z ${OLD_AUTHORS["$NEW_AUTHOR"]}; then
        NEW_AUTHORS["$NEW_AUTHOR"]=1
    fi
done < <(git log --pretty=format:%aN "$OLD_COMMIT"~1.."$NEW_COMMIT")

for NEW_AUTHOR in "${!NEW_AUTHORS[@]}"; do
    echo "$NEW_AUTHOR"
done | sort

Or, the second, using pipes and somewhat more declarative way:

diff \
    <(git log --pretty=format:%aN "$OLD_COMMIT" | sort | uniq) \
    <(git log --pretty=format:%aN "$OLD_COMMIT"~1.."$NEW_COMMIT" | sort | uniq) \
    | grep '^> ' \
    | cut -c 3-

Both of the above solutions can process the following history (git log --pretty=format:'%h %d% aN'):

c3d766e  (tag: v2) Keith
cffddc6  New John
ee3cc52  Dan
c307f13  (tag: v1) New John
ae3c4a3  New John
9ed948e  Old John
7eb548a  Old John

like this (show-new-authors.sh v1 v2):

Dan
Keith