The flag "-e" lets us know the program entrypoint.
$ rabin -e /bin/ls 0x08049a40
Again, if we mix it with -v we get a better human readable output.
$ rabin -ev /bin/ls [Entrypoint] Memory address: 0x08049a40
With -vv we will get more information, in this case the memory location as well as the file offset.
$ rabin -evv /bin/ls [Entrypoint] Memory address: 0x08049a40 File offset: 0x00001a40
Combined with -r radare will create a new flag space called "symbols", and it will add a flag named "entrypoint" which points to the program's entrypoint. Thereupon, radare will seek it.
$ rabin -er /bin/ls fs symbols f entrypoint @ 0x08049a40 s entrypoint