Lab 5 – UNIX Shell Solution

Purpose and rationale
The purpose of this lab is to allow students to learn a user interface aspect (shell), how to interface to process management and some basic system calls. Please read the document closely.
Lab 5 – UNIX Shell
A simple shell is a basic shell program that supports commands with I/O re-direction. Once, a user enters a command string (ended with a return key), your program should parse the command line and determines whether it has a regular command or contains I/O redirection signified by for output to a file or < for input from a file. The program has also 3 built-in commands: cd (change directory), pwd (print working directory), and exit (exit shell). The built-in functions are not executed by forking and executing an executable. Instead, the shell process executes them itself. All other command must be executed in a child process. Your shell is basically an interactive loop: it repeatedly prints a prompt "csc60msh ", parses the input, executes the command specified on that line of input, and waits for the command to finish. You should structure your shell such that it creates a new process for each new command (except for cd, pwd, and exit). Running commands in a new process protects the main shell process from any errors that occur in the new command.
Please follow these steps:
 On athena, copy all source files from /tmp/nguyendh/hw5/ directory to your hw5 directory. Review the source codes, compile, and execute the programs. Examine the output texts to understand the behavior of each program.
 I have provided you with a simple shell template (csc60mshell.c) that you can choose to work from. Read the template closely to identify the key components and understand its execution flow. You can compile the shell using the following command: gcc csc60mshell.c
 Input processing, fork, execute: work on the basic core functions by completing the code labeled as step 1 in csc60mshell.c. Here, with a new shell, we want to establish the basic functions by executing simple command like ls, cat, etc. The commands are parsed by a provided function called “parseline”.
• Handling built-in Commands: There are three special cases where your shell should execute a command directly itself instead of running a separate process. First, if the user enters "exit" as a command, the shell should terminate. Second, if the user enters "cd dir", you should change the current directory to "dir" by using the chdir system call. Users can run programs with paths relative to the working directory without specifying an absolute path. For example, instead of typing "/a/b/c/myprog", a user could type two commands: "cd /a/b/c" followed by "cd myprog". If the user simply types "cd" (no dir specified), change to the user's home directory. The $HOME environment stores the desired path; use getenv("HOME") to obtain this. Third, if the user enters "pwd", print the current working directory. This can be obtained with getcwd().
 Handling input, output redirection: complete the code for section labeled as step 2. Please also review the sample code for redir.c. You should be able to reuse part of the code here. Below are the high level steps:
Implementing "<" and "":
● Open the file
○ Eg. newfd=open(.....)
● Assign newfd to 0(if "<") or 1(if "") using dup2
○ Eg. dup2(newfd,0);
● Close your file
○ Eg. close(newfd);
● Execute the command using execvp system call
○ The command reads from 0 which is your file now
Note: Pipe function will be handled in the next assignment.
• Handling input errors: Be very careful to check for error conditions at all stages of command line parsing. Since the shell is controlled by a user, it is possible to receive bizarre input. For example, your shell should be able to handle all these errors:
csc60mshell /bin/cat < foo < gub
ERROR - Can’t have two input redirects on one line.
csc60mshell /bin/cat <
ERROR - No redirection file specified.
csc60mshell out.txt
ERROR - No command. Make sure file out.txt is not overwritten.
csc60mshell cat test.c
ERROR - No redirection file specified.
• Handling test cases: Your program should be able to handle the following examples of commands. However, your program will be tested with more than these commands (including error handling cases).
csc60mshell ls ls.out # redirect ls’s stdout to file ls.out
csc60mshell cat foo.txt
csc60mshell wc < ls.out
csc60mshell cd /usr/bin
csc60mshell ls
csc60mshell cd ../
csc60mshell pwd
csc60mshell /usr/bin/ps
csc60mshell cd
csc60mshell find . -name foo1.txt
csc60mshell wc foo1.txt
csc60mshell exit
csc60mshell pwd
csc60mshell cd hw5
csc60mshell cat foo1.txt ls.out out.tx
csc60mshell gcc –o test test.c –g
csc60mshell <carriage-return
csc60mshell
Marks Distribution
Your documented c program: csc60mshell.c with core functions –no I/O redirection, no build-in function
10 points
Your full documented c program: csc60mshell.c – build-in function
3 points
Your full documented c program: csc60mshell.c – I/O redirection
4 points
Your full documented c program: csc60mshell.c – with error checks
3 points
TOTAL
20 points
Resources
Useful Unix System Calls:
getenv/setenv: get/setenv the value of an environment variable
path = getenv("PATH");
cwd = getenv("PWD");
setenv("PWD", tempbuf,1);
getcwd: get current working directory.
chdir: change the current working directory (use this to implement cd)
fork-join: create a new child process and wait for it to exit:
if (fork() == 0) { // the child process
……….
} else { // the parent process
pid = wait(&status);
}
execv: overlay a new process image on calling process
execvp( full_path_name, command_argv_list, 0);
open, close, dup2: for I/O Redirection:
int fid = open(foo, O_WRONLY|O_CREAT);
close(1);
dup2(fid,1);
close(fid);
Exit shell:
_exit(…); - exit without flushing stdio
exit(); - exit with flushing stdio
C Library functions: #include <string.h String compare: int strcmp(const char *s1, const char *s2);
strcmp(….,"cd")
strcmp(….,"exit")
strcmp(….,"pwd")
strcmp(….,"")
strcmp(….,"<")
print a system error message:
perror("Shell Program error");
input of characters and strings:
fgets(cmdline, MAXLINE, stdin);
Compilation & Building your program
To compile and build your executable program, please reuse the makefile, for assignment 2, for this new assignment. Please make appropriate changes where they are applicable. The makefile for assignment 2 is located in the directory /tmp/nguyendh/hw2 .
Partnership
Students may form a group of 2 students (maximum) to work on this project. As usual, please always contact your instructor for question or clarification.
Deliverables
Your source file(s): csc60mshell.c, your program’s output text (with various test cases – please use the UNIX script command to capture your program’s output), and your makefile. Please submit your assignment via SacCT.
Hints Writing your shell in a simple manner is a matter of finding the relevant library routines and calling them properly. Please see the resources section above. Remember to get the basic functionality of your program working before worrying about all of the error conditions and corner cases. For example, for your shell, first get a single command running (probably first a command with no arguments, such as "ls"). Then try adding more arguments. Next, try working on multiple commands. Make sure that you are correctly handling all of the cases where there is miscellaneous white space around commands or missing commands. Finally, support for built-in commands, redirection, and pipes (for next assignment). I strongly recommend that you check the return codes of all system calls from the very beginning of your work. This will often catch errors in how you are invoking these new system calls. And, it's good programming practice. Keep versions of your code. This is in case you need to go back to your older version due to an unforeseen bug/issue.