Unix Shell

This project contains a simple homemade Unix shell. This project provides simple implementation of basic bash commands and full usage of the Linux (Posix) environment. The shell is the heart of the command-line interface, and thus is central to the Unix/C programming environment. Knowing how the shell itself is built is crucial to built shell proficiency. This shell is a modified version of the Unix Shell project from Operating Systems: Three Easy Pieces.

Overview

Enclosed is an implemtation of a command line interpreter (CLI) or, as it is more commonly known, a shell. The shell should operate in this basic way: when you type in a command (in response to its prompt), the shell creates a child process that executes the command you entered and then prompts for more user input when it has finished.

Running the Program Environment

A Makefile is provided that will perform the following:

• compile This will compile the lsh.c file using gcc.
• doc Rebuild the README.md file that will rebuild as a PDF file.
• test will run a set of unit tests against the program if you are just doing a normal make file compile.
• testJetbrains If you are doing development in CLion (by JetBrains) then use this. The test script provides one extra step and copies the executable from the development subdirectory and places it at the level of the test script.
• clean This will remove the executable and any modified files.

Program Specifications

Basic Shell: lsh

The shell can be invoked with either no arguments or a single argument; anything else is an error. Here is the no-argument way:

prompt> ./lsh
lsh> 

The mode above is called interactive mode, and allows the user to type commands directly. The shell also supports a batch mode, which instead reads input from a batch file and executes commands from therein. Here is how you run the shell with a batch file named batch.txt:

prompt> ./lsh batch.txt

The shell is very simple (conceptually): it runs in a while loop, repeatedly asking for input to tell it what command to execute. It then executes that command. The loop continues indefinitely, until the user types the built-in command exit, at which point it exits. That's it!

Structure

Paths

Important: Note that the shell itself does not implement ls or other commands (except built-ins). All it does is find those executables in one of the directories specified by path and create a new process to run them.

Built-in Commands

• exit: When the user types exit, the shell will simply call the exit system call with 0 as a parameter.

• cd: cd always take one argument, changing directory.

• path: The path command takes 0 or more arguments, with each argument separated by whitespace from the others. A typical usage would be like this: lsh> path /bin /usr/bin, which would add /bin and /usr/bin to the search path of the shell.

Redirection

The shell also supports redirection and parallel commands through > and a & character, similarly to the bash shell. The exact format of redirection is a command (and possibly some arguments)followed by the redirection symbol followed by a filename. Multiple redirection operators or multiple files to the right of the redirection sign are errors. If the output file exists before you run some program, the shell will simplyoverwrite it (after truncating it). an example for running parallel commands is as follows:

lsh> cmd1 & cmd2 args1 args2 & cmd3 args1

Program Errors

The one and only error message. This will print one and only error message whenever encountering an error of any type:

    char error_message[30] = "An error has occurred\n";
    write(STDERR_FILENO, error_message, strlen(error_message));