DAR (Disk ARchive) is a very capable cross-platform command-line backup tool.
The program has a vast amount of functionality. There's support for full, differential, incremental and decremental backup types; selective compression; strong encryption via Blowfish, Twofish, AES256, Serpent256 or Camellia256; archive splitting, merging and a host of file system features (hard links, sparse files, OS X file forks, Linux ACL, more).
Making this work from the command line isn't always straightforward, as you can see from the first example in the official DAR tutorial.
dar -c /mnt/zip/linux_full -s 100M -S 95M -p -b -z -R / -X "*~" -X ".*~" -P dev/pts -P sys -P proc -P mnt/zip -D
Still, there is a lot of documentation - even if it is largely using Linux examples - and if you're looking for a backup tool you can use in your own scripts then DAR could be a good choice.
Complex but very full-featured, DAR is a useful tool for running automated backups from your own scripts.