Simple way to trap and clean up temporary files in BASH - This script is useful as a cron job to keep an Ubuntu machine upgraded automatically

system-upgrade.sh

#!/bin/bash

# Simple way to trap and clean up temporary files in BASH - This script is useful as a cron job to keep an Ubuntu machine upgraded automatically
# The following cron job would keep the system up-to-date 4 times a day: 0 */4 * * * /root/bin/system-upgrade.sh

pidfile="/root/bin/system-upgrade.pid"

janitor() {
    rm -f $pidfile
}

#0  success
#1  incorrect invocation or permissions
#2  system error (out of memory, cannot fork, no more loop devices)
#4  internal mount bug or missing nfs support in mount
#8  user interrupt

trap janitor 0 1 2 4 8

if [[ -e $pidfile ]]; then
    exit 1
else
    touch $pidfile
fi

apt update && apt -y upgrade