Implement a Small UNIX Shell

Implement a Small UNIX Shell Overview and Goal The goal of this task is to write a simple but functional UNIX shell in C. It must: Print a shell prompt ($) Read multiple commands from standard input and execute them Support shell variables and expansion Support the variable $? to print out the return code of the last program Run programs in the background if desired Support redirection of standard input, output and error Support the pipeline operator (|) See expected output below for details. Important Notes Executing any real shell (e.g. /bin/bash) through exec(), popen() or any other means is not a valid solution. All of the functionality must be implemented by yourself in shell.c! Do not use isatty() to check if your shell is running in a terminal (e.g. to display a prompt only in this case). The CI environment is not using a terminal. It is recommended to use fflush() to flush the output streams (otherwise, e.g. CI tests may not work properly) Expected Output Print a shell prompt ($) when ready ... $ Read multiple commands from standard input and execute them $ echo Hello Hello $ expr 1 + 3 4 Support shell variables and expansion $ A=5 $ echo $A 5 $ echo ${A}BC 5BC $ A=$A$A echo $A 55 Support checking the return code of the last program using $? $ true $ echo $? 0 $ false $ echo $? 1 Support running programs in the background with & $ firefox & $ After this, Firefox should be running and the shell accepting commands again. Support redirection of stdin, stdout and stderr with >, < and 2> $ echo Hello > [login to view URL] $ cat - < [login to view URL] Hello Support the pipeline operator | $ echo Hello | tr l L | tr o ? HeLL? Evaluation Total points possible: 7 Minimum number of points required to pass: 5 Points can be obtained as follows: 1 Point: Correct display of shell prompt 1 Point: Read and execute multiple commands 1 Point: Support shell variables and expansion 1 Point: Support $? for displaying the return code of the last program 1 Point: Support & for running programs in the background 1 Point: Support redirection of stdin, stdout and stderr 1 Point: Support pipes with | the project / source has to compile with the attached makefile