Build a user shell for an operating system (Unix).

1. Read a unix command from the user, execute it, and repeat. Handle the basic commands (ls, cat, grep, etc.), with their parameters.

2. Before reading each line, print the prompt string specified by shell variable x1, but if x1 is not set, use the default prompt of $$$$.

3. Support background tasks which run without requiring the user to wait before the next command, specified with &.

4. Accept commands from a file if one is specified; ignore lines starting with #

5. Run commands that name a program anywhere in the path.

6. If a command is not found, print an error message and terminate if input is exit

7. If a command fails (with a non-zero exit value n), print "Program terminated: exit code n."

8. Change directories with the cd command.

9. Support redirection of output with > and support redirection of input with < . Also support simple pipes with | .

Hints

For each command, create a child process that uses execve to run it with its parameters.

Project Constraints:

Use only the following python libraries: os,sys,re

From os, use only: pipe(),fork(),dup() or dup2(),execve() ,wait(),open() or create() and close(),read() and write(), and chdir()

Do not use system(), execvp(), or execvpe().