git-change-author.sh

#!/bin/bash -eu

git filter-branch --env-filter '

# name, email, and date for author and committer are all set to the preferred
# values unless the override variable is empty or undefined.
# (see git-rewrite-tree(1))
#GIT_AUTHOR_NAME
#GIT_AUTHOR_EMAIL
#GIT_AUTHOR_DATE 
#GIT_COMMITTER_NAME
#GIT_COMMITTER_EMAIL
#GIT_COMMITTER_DATE

declare GIT_PREFERRED_EMAIL="";
declare  GIT_PREFERRED_NAME="";
declare  GIT_PREFERRED_DATE="";

# alternatively, only operate on the committer, not the author
#for GITPTR in GIT_COMMITTER_{NAME,EMAIL,DATE};do
for GITPTR in GIT_{AUTHOR,COMMITTER}_{NAME,EMAIL,DATE};do
  # derive the name of the var contining the preferred value by substituting
  # whichever of the two possible substrings appears in this iteration of the
  # parent loop
  for SUBSTR in "AUTHOR" "COMMITTER";do
    [[ "$GITPTR" =~ "$SUBSTR" ]] && {
      PREFPTR=${GITPTR/$SUBSTR/PREFERRED};
    } || continue;
  done
  # next if preferred value is empty
  [[ "${!PREFPTR-}" ]] || continue ;
  # reassign value of GITPTR var if it does not exactly match preferred
  [[ ${!GITPTR} == ${!PREFPTR} ]] || \
    {
      export eval ${GITPTR}=${!PREFPTR};
    }
done

'