Description

To learn about UNIX processes, and the command interpreter.

Background

A UNIX shell is a program that makes the facilities of the operating system available to interactive users. There are several popular UNIX shells: sh (the Bourne shell), csh (the C shell), and bash (the Bourne Again shell) are just a few. In this assignment, you will build kapish.

Your Task

You need to create a program named kapish. kapish should be a minimal but realistic killer application interactive UNIX shell.

Initialization and Termination

When first started, kapish should read and interpret lines from the file .kapishrc in your HOME directory, provided that the file exists and is readable. Note the the file name is .kapishrc (with the leading “.”, not kapishrc), and that it resides in the user’s HOME directory (not the current directory). Typically, the .kapishrc file contains commands to specify the terminal type and environment.

To facilitate your debugging and our testing, kapish should print each line that it reads from .kapishrc immediately after reading it. kapish should print a question mark and a space (? ) before each line.

kapish should terminate when the user types Control-D or exit.

Interactive Operation

After startup processing, kapish should read lines from the terminal, prompting with a question mark and a space (? ). Specifically, kapish repeatedly should perform the these actions:

Read a line from standard input.

Lexically analyze the line to form an array of tokens.

Syntactically analyze (i.e. parse) the token array to form command [options] [args] typically fed to the command interpreter.

Execute the command.

Lexical Analysis

Informally, a token should be a word. More formally, a token should consist of a sequence of non-whitespace characters that is separated from other tokens by whitespace characters. kapish should assume that no line of standard input is longer than 512 characters. If a line of standard input is longer than 512 characters, then kapish need not handle it properly; but it should not corrupt memory.

Execution

If the command is a kapish built-in, then kapish should execute it directly (i.e. without forking a child process). kapish should interpret four shell built-in commands:

If environment variable var does not exist, then kapish should create it. kapish should set the value of var to value, or to the empty string if value is omitted. Note: Initially, kapish inherits environment variables from its parent. kapish setenv var should be able to modify the value of an existing environment variable or create

[value] a new environment variable via the setenv command. kapish should be able to set the value of any environment variable; but the only environment variables that it explicitly uses are HOME and PATH.

unsetenv var      kapish should destroy the environment variable var. kapish should change kapish‘s working directory to dir, or to the HOME cd [dir]

directory if dir is omitted.

exit                     kapish should exit.

Note that those built-in commands should neither read from standard input nor write to standard output.

If the command is not an kapish built-in, then kapish should consider the command-name to be the name of a file that contains executable binary code. kapish should use the PATH environment variable to locate the binary, fork a child process and pass the filename, along with its arguments, to the execvp system call. If the attempt to execute the file fails, then kapish should print an error message indicating the reason for the failure.

kapish should print its prompt for the next standard input line only when a command has finished executing.

Process Control

All child processes forked by kapish should run in the foreground; kapish need not support background process control. However, the user must be able to kill the current child processes using Control-C. Control-C should not kill kapish itself.

Error Handling

kapish should handle an erroneous line gracefully by rejecting the line and writing a descriptive error message to standard error. kapish should handle all user errors; it should be impossible for the user’s input to cause kapish to crash.

Memory Management

kapish should contain no memory leaks. For every call to malloc or calloc, there should eventually be a call to free.

History Mechanism (for extra credit)

kapish should support a history mechanism that includes:

A history built-in command. The history command should print a list of all previously issued commands. Note that the history command should write data to standard output.

The ability to re-execute a previously issued command by typing a prefix of that command preceded by an exclamation point (!commandprefix).

kapish need not support editing of the previously issued command. Logistics

Develop on linux.csc.uvic.ca.

The expectation is that you will devote substantial effort to creating and running test cases.

You should submit:

Your source code files.

A makefile. The first dependency rule should build your entire program, compiling with the -Wall and Werror options. The makefile should maintain object (.o) files to allow for partial builds.  A readme file.