A recent OS upgrade rendered the crontab to malfunction on macOS Monterey. It turned out the system just needed a reset of System Preferences > Security & Privacy > Privacy tab, and to make sure cron has full access to disks. Once you flipped that, your crontab should start working. Hope that helps.
cron
AWS LightSail Delete Snapshots
Here’s my script to delete AWS LightSail snapshots. It can delete daily or weekly backups and scheduled via crontab. If you want longer or shorter retention, adjust the expired value. It’s in seconds. 604800 is 1 week. 2592000 is 1 month.
#!/bin/bash current=$(date +%s) if [ $# -eq 0 ]; then exit 1 fi if [[ $1 = 'daily' ]]; then prefix='daily' expired=$(($current-604800)) elif [[ $1 = 'weekly' ]]; then prefix='weekly' expired=$(($current-2592000)) else exit 1 fi snaps='/root/snapshots/snapshots.json' names='/root/snapshots/names.txt' parse='/root/snapshots/parse.txt' logfile='/root/snapshots/snapshots.log' /usr/local/bin/aws lightsail get-instance-snapshots > $snaps cat $snaps | jq -r '.instanceSnapshots[] | .name' > $names cat $names | grep $prefix > $parse while read -r line; do snapshot=$(echo $line | cut -d_ -f3) snapshotname=$line if [ `expr $snapshot + 1 2> /dev/null` ] ; then if [ $snapshot -le $expired ]; then echo 'Deleted: '$snapshotname >> $logfile /usr/local/bin/aws lightsail delete-instance-snapshot \ --instance-snapshot-name $snapshotname else echo 'Nothing: '$snapshotname >> $logfile fi else echo $snapshot is not numeric > /dev/null fi done < $parse echo 'Current time: '$current >> $logfile echo 'Expired time: '$expired >> $logfile echo '-----------------------------------' >> $logfile |
Schedule deletes via crontab.
# run daily at 5am 0 5 * * * /bin/bash /root/snapshots/delete-snapshot.sh daily 2>&1 # run weekly every sunday at 6am 0 6 * * 0 /bin/bash /root/snapshots/delete-snapshot.sh weekly 2>&1 |
AWS LightSail Create Snapshots
Here’s my script to create snapshots of a LightSail instance. Create daily or weekly snapshots.
#!/bin/bash if [ $1 != '' ]; then prefix=$1 timestamp=$(date +%s) snapshotname=$prefix'_web-server_'$timestamp /usr/local/bin/aws lightsail create-instance-snapshot \ --instance-snapshot-name $snapshotname \ --instance-name web-server else echo 'Need one argument. e.g. daily or weekly' fi |
Crontab. Job can be scheduled daily or weekly.
# run daily at 4am 0 4 * * * /bin/bash /root/snapshots/create-snapshot.sh daily 2>&1 # run weekly every sunday at 5am 0 5 * * 0 /bin/bash /root/snapshots/create-snapshot.sh weekly 2>&1 |
Adding to Crontab in Bash
Here’s a neat command to append a job to a crontab within a Bash script.
crontab -l | { cat; echo "0 1 * * * /dir/script.sh > /logs/script.cronlog 2>&1"; } | crontab - |
To validate, perform crontab -l to see if your changes are there.
crontab -l |
Create a Crontab via Script
How do you create a crontab entry via a Bash script? That’s a good question. Here is one solution:
(crontab -l 2>/dev/null; echo "* 11 * * * /path/to/job -with args") | crontab - |
If you want to run it as a specific user, use the following:
(crontab -l -u user 2>/dev/null; echo "* 11 * * * /path/to/job -with args") | crontab - |
Cronjob Not Running
I had issues with cron. The jobs were not running, so it seemed. It turned out that they were in fact running. The jobs were firing off based on schedule. The problem was, cron didn’t have the correct path to the commands. I realized this when I looked at a couple of log files, in /var/mail and /var/log/syslog to look for clues. The fix was to include the path in cron.
Edit cron.
crontab -e |
Add the shell and path at the top of the file.
SHELL=/bin/sh PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin |
Cron Allow
Some Linux distributions don’t turn on cron automatically for users. To give certain users access to cron, add them to cron.allow.
Edit cron.allow.
$ sudo nano /etc/cron.allow |
Add one user per line.
johndoe |
Save and close.
Implementing Surfcali.com
I have a website called Surfcali.com. It’s a website that provides tide table information for select California beaches. The tide information on the website is auto-generated by the xtide program which is available on most Linux distributions.
You can install xtide on Ubuntu by typing this command from the Terminal.
sudo apt-get install xtide |
Instead of explaining how the website is implemented using lots of words, it’s probably much easier to explain it via video.
So, here’s a short video of how Surfcali.com was put together.