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
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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().
