Loop through all the forked repos, update the remote upstream and rebase it with the current local HEAD. With '-f' flag to push after performing the tasks above, for each repo.

rebase_upstream

#!/usr/bin/env sh

push=0

if [[ $# -eq 1 && $1 -eq '-f' ]]; then
  push=1
fi

T1=$(date +%s)
cwd=$(pwd)
repoCount=0

for d in *; do
  if [ -d "${d}" ]; then
    echo "\n<<<<<<<<<< Updating [${d}] & Rebase with upstream >>>>>>>>>>"
    cd ${d}
    HEAD=$( git rev-parse --abbrev-ref HEAD )
    git fetch upstream && git checkout ${HEAD}
    git rebase upstream/${HEAD}
    [ $push -eq 1 ] && git push origin ${HEAD}
    repoCount=$((repoCount + 1))
    cd ${cwd}
  fi
done

T2=$(date +%s)
echo "\n<<<<<<<<<< Total Took $(($T2 - $T1))s to update ${repoCount} repos >>>>>>>>>>"