What is the difference between the -f and -l options of secure-delete srm?

Question

Regarding the secure-delete command in Linux, what is the difference between srm -f, srm -l and srm -fl ? Which is fastest? Which is most secure?

Answer 1

-f fast (and insecure mode): no /dev/urandom, no synchronize mode.

-l lessens the security. Only two passes are written: one mode with 0xff and a final mode random values.^{Ubuntu Manuals}