RSync is a powerful utility tool used frequently in Unix and Linux environments. It’s known for efficiently transferring and synchronizing files and directories across systems, while minimizing data transfer by copying only the differing blocks and bytes. Considerably, one of the main uses of RSync can be implementing differential backups.
‘Differential backup’ is a type of data backup method where all changes made since the last full backup are saved. The advantage of a differential backup is that it is faster to create than a full backup.
A typical RSync command looks like this:
`rsync -avz source-directory destination-directory`
The key elements in the command are `-avz`, which, as per their UNIX man pages, stand for:
- `-a`: Archive mode; equals -rlptgoD (no -H,-A,-X)
- `-v`: Verbose; increases the amount of information you are given during the transfer.
- `-z`: Compress file data during the transfer
An RSync command intended to perform a backup will ideally look like:
`rsync -av —delete source-directory destination-directory`
The `—delete` flag deletes files that exist in the destination directory but not in the source directory.
An RSync command for a differential backup looks like this:
`rsync -avz —link-dest=$PWD/PREV_BACKUP source-directory destination-directory`
Where `PREV_BACKUP` refers to the directory of the previous backup. The `—link-dest` option is the secret sauce to differential backups. It creates hard links to the files in the provided directory that are identical to the source files, treating linked files as though they were in the destination dir.
To automate this process, the differential backup might incorporate date in Linux, to create a distinctive directory each time a backup is carried out. Here’s a simple backup script that could work for you:
```
#!/bin/sh
DATE=`date +%Y-%m-%d` #Current Date
SOURCE_DIR=/path/to/source-directory
BACKUP_DIR=/path/to/backup-directory
PREV_BACKUP=$BACKUP_DIR/$(date -d -1day “+%Y-%m-%d”) #Previous Day
rsync -avz —delete —link-dest=$PREV_BACKUP $SOURCE_DIR $BACKUP_DIR/$DATE
```
This script gets the current date, determines the previous backup directory, and runs RSync accordingly.
The simplicity and efficiency of RSync makes it a popular choice for system administrators.
Sources:
- [RSync Man Page](https://linux.die.net/man/1/rsync)
- [Differential Backups using hard links](http://www.admin-magazine.com/Articles/Using-Rsync-for-Backups)
- [RSync examples](http://www.tecmint.com/rsync-local-remote-file-synchronization-commands/)
- [Automating Differential Backups](https://www.thegeekstuff.com/2011/07/rsync-overview/)
