Crontab [classic]

Crontab fields
Here is a typical crontab entry and the definition of cron fields:
00 03 * * * /tmp/zing.sh

1. minute (00-59)
2. hour (00-23)
3. day of the month (01-31)
4. month of the year (01-12)
5. day of the week (0-6 with 0=Sunday)
6. program or command to run

An asterisk (*) in any field means run every time for this field.
Ranges (X-Y) and steps (X-Y/Z) can also be defined.

Special @reboot option
Recent versions of cron support @reboot instead of time and day settings to run a command after boot. For example:
@reboot /usr/bin/command-to-run

I am not sure if I like this option, compared to using system start up scripts, like /etc/rc.d/rc.local.

User crontabs (including the root user)
To edit a user crontab (including root):
crontab -e
To delete a crontab:
crontab -r

User crontabs are stored as text files in /var/spool/cron/.

System crontab
The system crontab is stored in /etc/crontab. It can be changed (as root) with a text editor.
The system crontab has one extra field before the program to run, which is the user to run the command as (usually root).

Rsync [classic]

Rsync is a file and directory syncronization tool. It can be used to keep local and/or remote files syncronized. What makes rsync powerful is that it can detect changes within large files and only transfer the parts that are different. This is much more efficient than transfering entire files, especially as the file sizes get bigger.

Note: examples that use a shell use ssh with public key authentication

To synchronize a local directory with a remote one (pull), use:

rsync -r -a -v -e "ssh -l username" --delete hostname:/remote/dir/ /local/dir/

To synchronize a remote directory with a local one (push), use:

rsync -r -a -v -e "ssh -l username" --delete /local/dir/ hostname:/remote/dir/

To synchronize a local file with a remote one, use:

rsync -a -v -e "ssh -l username" hostname:/filename /local/filename

To synchronize a remote file with a local one, use:

rsync -a -v -e "ssh -l username" /local/filename hostname:/filename

To synchronize a local directory with a remote rsync server:

rsync -r -a -v --delete rsync://rsync-server.com/stage/ /home/stage/

To synchronize a local directory with a local directory (make a backup), use:

rsync -r -a -v --delete /local/dir/ /backup/dir/