Running and testing

• Run sudo make gsh on a unix-based terminal (if using for the first time), followed by make run to run the custom shell.
• The shell should automatically revert all changes made to system upon exiting. In case of faliure to do so, run make clean to forcefully remove all project files.

Basic Linux/Unix Shell

Linux (and other Unix like OSes), have “shells” or programs which present a command line interface to users to type commands in. In this assignment you need to use standard C libraries, including Linux system calls such as fork(), exec() family system calls and wait() family of system calls.

There are two kinds of commands – “internal” and “external”. Internal commands are those which are interpreted by the shell program itself, without requiring a different program to handle the expected operations (of the said command). Examples of internal commands are like cd, pwd, exit etc. External commands on the other hand relate to commands which are not handled directly by the shell program but by an external program. Common examples include ls, cat, grep etc.

In this assignment, you have two tasks:

1. You need to design a simple shell that can handle three, internal commands – cd, echo and pwd. These commands would be handled directly by the shell. Your shell should also be able to handle five external commands – ls, cat, date, rm and mkdir. For these external commands you need to write individual programs to handle these commands. To handle these external commands, the shell should typically create a new process, using the fork() system call and within each process you need to use the execl() family system call to run the individual program. The parent program must also wait for the child program to terminate using the wait() family of system calls. For each of these commands, you need not handle all the command line options. Two options per command is sufficient. You need to document which two options you are handling and need to demonstrate correct functioning of the command with respect to (atleast) your chosen options. You also need to handle corner cases such as invalid options (graceful degradation).

2. The second task would be the achieve the above functionality of the shell using pthread_create() (instead of fork()) and system() (instead of execl() family of functions). The thread based execution would be performed if the command is followed by the characters, &t. The rest of the functionalities should remain the same. Note: you only need one set of external command programs which could be used with either versions of the shell, be it the that uses fork()/execl() or the one that uses pthread_create()/system().