Use Flock In Shell
Let’s say you have a crontab entry where scripts are called and these scripts could take a while to run. When the cron job comes back around, you don’t want multiple instances of the scripts starting up.
Enter flock. Flock will “lock” the script (or app) and not allow the script to execute again until the previous instance is completed.
By way of example, here is a partial of one of my crontab entries:
# rclones for Google Drive
0 * * * * /usr/bin/flock -n /tmp/.rclone.CFBArtwork /var/www/bin/rclone-sync-CFBArtwork >/dev/null 2>&1
0 * * * * /usr/bin/flock -n /tmp/.rclone.CFBDocuments /var/www/bin/rclone-sync-CFBDocuments >/dev/null >&1
0 * * * * /usr/bin/flock -n /tmp/.rclone.CFBSales /var/www/bin/rclone-sync-CFBSales >/dev/null 2>&1
0 * * * * /usr/bin/flock -n /tmp/.rclone.CFBTaproom /var/www/bin/rclone-sync-CFBTaproom >/dev/null 2>&1
0 * * * * /usr/bin/flock -n /tmp/.rclone.CFBProduction var/www/bin/rclone-sync-CFBProduction >/dev/null >&1
0 * * * * /usr/bin/flock -n /tmp/.rclone.CFBtmp /var/www/bin/rclone-sync-CFBtmp >/dev/null 2>&1