A simple UNIX shell

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Requires flex and bison to build. The GNU Readline
library is required at runtime as well as compile time.

Supports:
  * modes: interactive, file and -c option
  * cd and exit builtins
  * !, && and || operators
  * standard redirections
      -> 2>/dev/null, 2>&1, <&-, <>myfile, etc
      -> << and <<- aren't supported (yet)
  * pipes
      -> echo I hate UNIX | sed s/hate/❤️/ | cowsay

               _____________
              < I ❤️ UNIX >
               -------------
                      \   ^__^
                       \  (oo)\_______
                          (__)\       )\/\
                              ||----w |
                              ||     ||

Doesn't support:
  * basically everything else :p

Just to make sure: DO NOT ACTUALLY USE THIS. This is a
project for me to learn about UNIX systems programming.
Creating a usable shell is a non-goal.

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Author: Jan Doskočil