To add a crontab job, type the following command at a UNIX/Linux shell prompt:
$ sudo crontab -e
Add the following line:
1 2 3 4 5 /path/to/script
where
1: Minutes (0-59)
2: Hours (0-23)
3: Days (1-31)
4: Month (1-12)
5: Day of the week(1-7)
/path/to/script - your own shell script
In your case it would be:
55 23 * * * /path/to/yourShellScript