These Unix Spotlights are about some Unix commands that you might not know exist and a few things you can do with them.

TL;DR: bc is a calculator.

One of my favorite commands is bc -l because I'm terrible at arithmetic.  The l (lowercase L) flag allows you to "use the predefined math routines"; practically, this means you get non-integer division and some sweet functions you can use.  I'd recommend always using bc -l over bc.

$ bc -l

# It has some copyright stuff here.

> 22 - (13 / 2)
15.50000000000000000000

> quit

(I've added >'s here for my input in bc.)

Sweet.  Note that to get out of bc you type the word quit and it'll take you back to the shell.

What else can you do with bc?  If you wanted the FULL EXPERIENCE you could do a whole lot of crazy wild calculations and some programming stuff but you're probably better off doing those in your Language Of Choice™ or checking out Wolfram Alpha.  But here's a few quick things I use in bc:

# Square roots
sqrt(2)
1.41421356237309504880

# Natural log
l(10)
2.30258509299404568401

# Exponential
e(10)
22026.46579480671651695790

# Sine, input in radians
s(3.141592/2)
.99999999999994660254

# Cosine, input in radians
c(3.141592/2)
.00000032679489661922

One annoying thing is that constants like $e$ and $\pi$ aren't defined, but it's still pretty okay if you need a basic approximation of something.