For using a shell commands through a terminal program, where can I find documentation or help or man pages for the commands as implemented in Android? While it would be nice to have them on the device via man, PDF or web-based would be fine, too. When I run "man" from terminal, I get an error and I haven't found a good source online.

Android is based on Linux and has a Linux kernel underneath. Is there any way I can use shell commands and interact directly with the Linux system? E.g. A shell app that has access to the Linux system?