There are some variances between linux/BSD, and other flavours of unix, but generally speaking these are the tools you can expect to have at your fingertips while you are operating a unix inspired operating system.
this page is a mess, should be split out into a page per program
compare BSD/GNU tail, add notes
coreutils chmod coreutils chown coreutils chroot coreutils dd coreutils cut coreutils seq coreutils sort coreutils tee coreutils date
atat 09:00 27.07.14 <<END > echo "Don't forget to wish a Happy Birthday to Raphaël!" \ > | mail firstname.lastname@example.org > END
watch# run a command every 5 seconds watch -n 5 <command>
repeat# repeat a command a particular number of times repeat 300 <command> && sleep 5