How do I read the source code of shell commands?

All these basic commands are part of the coreutils package.

You can find all information you need here:

http://www.gnu.org/software/coreutils/

If you want to download the latest source, you should use git:

git clone git://git.sv.gnu.org/coreutils

To install git on your Ubuntu machine, you should use apt-get (git is not included in the standard Ubuntu installation):

sudo apt-get install git

Truth to be told, here you can find specific source for the ls command:

http://git.savannah.gnu.org/cgit/coreutils.git/tree/src/ls.c

Only 4984 code lines for a command 'easy enough' as ls... are you still interested in reading it?? Good luck! :D

Direct links to source for some popular programs in coreutils:

• cat (767 lines)
• chmod (570 lines)
• cp (2912 lines)
• cut (831 lines)
• date (570 lines)
• df (1718 lines)
• du (1112 lines)
• echo (272 lines)
• head (1070 lines)
• hostname (116 lines)
• kill (312 lines)
• ln (651 lines)
• ls (4954 lines)
• md5sum (878 lines)
• mkdir (306 lines)
• mv (512 lines)
• nice (220 lines)
• pwd (394 lines)
• rm (356 lines)
• rmdir (252 lines)
• shred (1325 lines)
• tail (2301 lines)
• tee (220 lines)
• touch (437 lines)
• wc (801 lines)
• whoami (91 lines)

Full list here.

ls is part of coreutils. You can get it with git :

git clone git://git.sv.gnu.org/coreutils

You'll find coreutils listed with other packages (scroll to bottom) on this page.