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cloud engineer

crontab

Display Instance ID & IP Address

How to display IP address and Instance ID on web page behind a AWS load balancer.

Add these 2 commands in crontab. The job runs every 5 mins.

*/5 * * * * /usr/bin/curl http://169.254.169.254/latest/meta-data/instance-id > /var/www/html/id.txt
*/5 * * * * /usr/bin/curl http://169.254.169.254/latest/meta-data/public-ipv4 > /var/www/html/ip.txt

You can view on the site.

# your domain
http://yourdomain.com/ip.txt
http://yourdomain.com/id.txt
# your server ip address
http://1.1.1.1/ip.txt
http://1.1.1.1/id.txt

Crontab File

How to load crontab from file.

crontab /path/to/file.txt

Save crontab to a file.

crontab -l > /path/to/file.txt

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

Bash Source in Cron

I’m running a “source” command inside a Bash script. The script is running fine on its own, but when I try running it inside a crontab, it doesn’t work. I’m using the source command just to load variables in a file, so I don’t have to edit multiple files. As it turns out, the regular shell in crontab does not quite understand the source command, and it’s the very reason the script fails. Here’s an example.

source /home/user/server.conf

I ended up removing the source line, and hardcoding the variables instead of loading it from a file.

server="servername"