Cron job not working properly on start up

Question

Thanks for reading this.

I have created a bash script to backup my databases. The script will create a directory for the day then create a backup file for each database within that directory.

It runs as expected if I call it from the command line.

I want it to run when the machine starts up so I have created a cron job to run it.

When the cron job runs it creates the directory but it doesn't create the files.

I have checked the syslog and I can see the cron job being processed but there is no output that indicates an error. There is a message which appears to suggest that an email would have been sent if postfix or some such were installed. I don't really want to install postfix unless I really have to.

Is there any other log I could be looking at?

Here's the cron job (it runs as root):

@reboot /usr/bin/db.backup.sh

here's the bash script:

#!/bin/bash

DB_BACKUP="/files/database_backups/`date +%Y-%m-%d`"
DB_USER="root"
DB_PASSWD="a"
HN=`hostname | awk -F. '{print $1}'`

# Create the backup directory
mkdir -p $DB_BACKUP

# Remove backups older than 10 days
find /files/database_backups/ -maxdepth 1 -type d -mtime +10 -exec rm -rf {} \;

# Backup each database on the system
for db in $(mysql --user=$DB_USER --password=$DB_PASSWD -e 'show databases' -s --skip-column-names|grep -viE '(staging|performance_schema|information_schema)');
do mysqldump --max-allowed-packet=1073741824 --user=$DB_USER --password=$DB_PASSWD --events --opt --single-transaction $db | gzip > "$DB_BACKUP/$db-$(date +%Y-%m-%d).sql.gz";
chown -R www-data:www-data $DB_BACKUP
done

Answer 1

Scripts run under cron doesn't have the same environment as when you run it from the command line.

This can be because the PATH defined under cron doesn't include the directory where mysql and mysqldump are located.

Or you use the full path when referencing these commands in your script, like /usr/bin/mysql or you create a new PATH variable at the beginning of your script :

export PATH=/bin:/usr/bin

Or pointing to any other directory where mysql and mysqldump are installed if you did not install MySQL using the default Ubuntu package.

To keep the possible error messages that would have been produced when the script is run from cron, if you don't get a mail locally, you can simply add output redirection to your crontab definition like this :

@reboot /usr/bin/db.backup.sh >/tmp/db-backup.log 2>&1

Any message, information and error, will be later found into /tmp/db-backup.log.

Consider using '>>' instead of '>' in the above line if you want to append the information and not overwrite the log file at each execution.

Update: if the cron job is run before the MySQL server is up

If the script is run in the startup process of the machine, before MySQL server is up, you can implement a small check at the beginning of the script to wait for the MySQL socket file to be present :

while [ ! -e /var/run/mysqld/mysqld.sock]; do
  # if the file doesn't exist, will loop
do

Attention : if the MySQL never comes up for some reason, the script will loop forever.