Update collection of git repositories quickly in parallel

update_repos.sh

#!/bin/bash
# $1 - a folder containing multiple git repos; i.e. ~/work
# The Mac way to get number of logical cores
# You can run more parallel processes than the number of cores,
# but at some point the process will slow to a crawl
cores=$( sysctl -n hw.ncpu )
# list all paths below $1 in single column, and pipe to xargs:
#   ls -d1 $1/*/ |
# Run a bash command on each path, using $cores number of processes:
#   xargs -I % -n 1 -P $cores bash --login -c
# cd to the path
#   cd % ;
# discard changes to package.json (if any)
#   git checkout -- package.json
# pull updates
#   git pull ;
# if '.nvmrc' exists, then use that version of npm, else use npm version 4
#   [ -f .nvmrc ] && nvm use || nvm use 4 ;
# update node dependencies
#   npm install ;

ls -d1 $1/*/ | xargs -I % -n 1 -P $cores bash --login -c 'cd % ; git checkout -- package.json ; git pull ; [ -f .nvmrc ] && nvm use || nvm use 4 ; npm install ;'