Simple-Shell (Multi-Processing)

Objectives:

1. Familiarity with system calls in Unix environment.
2. Introduction to processes and multi-processing.
3. Introduction to signal handling in Unix environment.

It is required to implement a Unix shell program. A shell is simply a program that conveniently allows you to run other programs. The shell supports the following commands:

1. The internal shell command "exit" which terminates the shell
2. A command with no arguments
   Example: ls, cp, rm
3. A command with arguments
   Example: ls –l
4. A command, with or without arguments, executed in the background using &.
   Example: gedit &
   Details: In this case, your shell must execute the command and return immediately, not blocking until the command finishes.
   

Requirements: Show that the opened process will be nested as a child process to the shell program via opening the task manager found in the operating system. Additionally, you have to write in a log file (basic text file) when a child process is terminated (main application will be interrupted by a SIGCHLD signal). So you have to implement an interrupt handler to handle this interrupt and do the corresponding action to it.

5. Shell builtin commands
   Commands: cd & echo
   cd: Cover all the following cases (assume no spaces in path):
   * cd
   * cd ~
   * cd ..
   * cd absolute_path
   * cd relative_path_to_current_working_directory
   echo: Prints the input after evaluating all expressions (assume input to echo must be within double quotations).
6. Expression evaluation
   Commands: export
   Details: Set values to variables and print variables values. No mathematical operations is needed.
   Export Details: Accept input of two forms, either a string without spaces, or a full string inside double quotations.

Compiling and Execution