A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language, and is used by the operating system to control the execution of the system using shell scripts.

This project is about creating a simple shell.

The shell is a program that that takes keyboard commands and passes them to the operating system to carry out. Almost all Linux distributions supply a shell program from the GNU Project called Bash. The name is an acronym for "Bourne-again shell," in reference to the fact that Bash is an enhanced replacement for "sh," the original Unix shell program written by Steve Bourne.

So how bash actually works? Essentially it breaks down the process into 4 steps: lexer → parser → expander → executor.

The general idea for this shell is reading a string of commands in a prompt using readline.

• Follow the shell syntax.
• Not use more than one global variable.
• Display a promt while waiting for a new commmand.
• Have a working history.
• Find and launch executables (using the PATH variable or an absolute path).
• Handle ' (single quote) which should prevent the shell from interpreting the metacharacters in the quoted sequence.
• Handle " (double quote) which should prevent the shell from interpreting the metacharacters in the quoted sequence except for $ (dollar sign).
• Implement redirections such as:
  • < redirect input.
  • > redirect output.
  • << heredoc (doesn't affect history).
  • >> redirect output in append mode.
• Implement | (pipes).
• Handle environment variables.
• Handle $?.
• ctrl-C, ctrl-D, and ctrl-\ should behave like in bash.
• Recreate the following builtins:
  • echo with option -n.
  • cd with only a relative or absolute path.
  • pwd (no flags).
  • export (no flags).
  • unset (no flags).
  • env (no flags or arguments).
  • exit (no flags).

#!/bin/bash
#
#   This file echoes a bunch of color codes to the 
#   terminal to demonstrate what's available.  Each 
#   line is the color code of one forground color,
#   out of 17 (default + 16 escapes), followed by a 
#   test use of that color on all nine background 
#   colors (default + 8 escapes).
#

T='gYw'   # The test text

echo -e "\n                 40m     41m     42m     43m\
     44m     45m     46m     47m";

for FGs in '    m' '   1m' '  30m' '1;30m' '  31m' '1;31m' '  32m' \
           '1;32m' '  33m' '1;33m' '  34m' '1;34m' '  35m' '1;35m' \
           '  36m' '1;36m' '  37m' '1;37m';
  do FG=${FGs// /}
  echo -en " $FGs \033[$FG  $T  "
  for BG in 40m 41m 42m 43m 44m 45m 46m 47m;
    do echo -en "$EINS \033[$FG\033[$BG  $T  \033[0m";
  done
  echo;
done
echo