Things a shell does

Execute scripts in PATH in foreground or background
Give error message if input isn't intelligible
Execute different functions based on special characters
- $ - command substitution
- ' ' - literal evaluation
- \ - escape character
- > - pipe output to something (create/overwrite file)
- >> - pipe output to something (append/overwrite file)
- < - pipe input to something
- | - pipe output to command's input
- & - execute in background
- && - execute command and command
- !! - last command
- *,? - globs
- Interrupts
  - ctrl+c - kill
  - ctrl+z - suspend

Things a shell has

History
Environment Variables (PATH)
Prompt
Input buffer
Output buffer
Builtin functions
- cd
- exit
- complete
directory location

Design

History file
Possible customization via .shellrc
Builtin functions
Input parsing

Feature Ranking