You are working for a company that hires interns regularly and develops software in Linux. You are their favorite employee, and they asked you to write a restricted shell interpreter to be used by the interns. In this project, you are going to implement rsh, the restricted shell, which will allow only a handful of commands to be executed from the shell prompt. To be precise, there are twelve commands that can be executed:
1: cp
2: touch
3: mkdir
4: ls
5: pwd
6: cat
7: grep
8: chmod
9: diff
10: cd
11: exit
12: help
The help command should print the list of commands as their appear above. The first 9 commands are programs that you are going to spawn as separate processes using the posix_spawnp() system call. An example program that spawns an echo command is provided in the repository: myspawn.c. By using this example program, you are going to write your own shell interpreter that can execute the commands above. Commands 10, 11, and 12 can be considered as built-in shell commands, which will not spawn a separate process. The handling of commands 11 and 12 by your program is trivial: return from the main function and print a list of commands, respectively. For the "cd" command, you are going to use the chdir() system call to change the directory to a target directory. However, if the user specifies more than one argument to the cd command, similar to bash, you should print the following error message and do not perform any cd action:
-rsh: cd: too many arguments
For all the commands, you can assume that there will not be more than 20 command line arguments.
When the program is run, the shell prompt "rsh>" should be displayed. The shell should be in an infinite loop until the command exit is entered at which point the shell program should be terminated with a "return 0".
If the user enters any dissallowed command or try to execute some other program, you should display the following message and return to the prompt:
NOT ALLOWED!
Similar to project2, a sample executable is provided for convenience in the solution_executable folder.
You are not expected to support background processes, piping, or input/output redirection in this project.
You can use the starter code rsh.c to complete the project. A Makefile is also provided for convenience.
Useful tips:
- You can use fgets() to get a line of input from the rsh prompt. Note that, fgets() also retains the end of line character at the end of the string. You can get rid of that by simply line[strlen(line)-1]='\0'.
- You need to create and argument vector (argv in the sample program myspawn.c) to be passed to the posix_spawn() call. That argument vector is an array of strings terminated with a NULL pointer, which are basically the individual strings that the user enters from the command prompt separated by space. For example, if the user enters "mkdir a b c d", argv should be {"mkdir","a","b","c","d",NULL}. If you want to dynamically resize the argv array while parsing the input, you can use the realloc() function to do so. You can also assume that the argument vector will not contain more than 20 arguments in it.
- strtok() destroys/modifies the original string as it tokenizes it. If you want to restart strtok() a second time on the same string from the beginning, you should first make a separate copy of that string before running strtok() the first time and start strtok() on the copied string the second time.
The setup of the git repo is similar to the one for Project 2. Refer to those instructions for your github repo setup.
For Project 3, create a separate private repository specific for this project under your own user name. For example, I created a private repo named csci210_project3 and my github username is tolgacan. A private repo can be created from the GitHub web site by going to your repositories after clicking on your profile picture (e.g., https://github.com/tolgacan?tab=repositories) and clicking on the "New" button in the top right corner. After that, you can enter a command sequence similar to the one below to clone the starter repo and copy it to your private repo to work with. The project3 repo under the organization CSCI210Mines is the public repository that contains the starter code.
git clone [email protected]:CSCI210Mines/project3.git
cd project3
git remote remove origin
git remote add origin [email protected]:tolgacan/csci210_project3.git # this is my private repo. replace it with your own private repo
git branch -M main
git push -u origin main
After these commands, you should have a copy of your starter code in your own repo and you can update the rsh.c with your solution code and execute the following git commands to push your updates to your own repo:
git add rsh.c
git commit -m "your commit message here"
git push -u origin main
You can also clone your repo and work on it and push the updates from different machines if you add the ssh public keys for these machines to your github profile.
Submission on Gradescope:
After you are logged in to your github account in a browser, if you follow the Gradescope assignment page for this project from Canvas and try to upload a submission, you will be able to select the Github submission options and select your private repository for this project to submit your solution on Gradescope.