Write a command shell called rshell.
Your shell will perform the following steps:
-
Print a command prompt (e.g.
$) -
Read in a command on one line. Commands will have the form:
cmd = executable [ argumentList ] [ connector cmd ]
connector = || or && or ;
where executable is an executable program in the PATH and argumentList is a list of zero or more arguments separated by spaces.
The connector is an optional way you can run multiple commands at once.
If a command is followed by ;, then the next command is always executed;
if a command is followed by &&, then the next command is executed only if the first one succeeds;
if a command is followed by ||, then the next command is executed only if the first one fails.
For example:
$ ls -a
$ echo hello
$ mkdir test
is equivalent to:
$ ls -a; echo hello; mkdir test
There should be no limit to the number of commands that can be chained together using these operators, and your program must be able to handle any combination of operators. For example, you should be able to handle the command:
$ ls -a; echo hello && mkdir test || echo world; git status
-
Execute the command. This will require using the syscalls
fork,execvp, andwaitpid. Previous cs100 students created a written tutorial and two video tutorials ( a fun cartoon tutorial ; more serious explanation). You should also refer to the man pages for detailed instructions. -
You must have a special built-in command of
exitwhich exits your shell. -
Anything that appears after a
#character should be considered a comment. For example, in the commandls -lR /, you would execute the program/bin/lspassing into it the parameters-lRand/. But in the commandls # -lR /, you would execute/bin/ls, but you would not pass any parameters because they appear in the comment section.
IMPORTANT:
Most bash commands are actually executables located in /bin, /usr/bin/ (e.g. ls).
But some commands are built-in to bash (e.g. cd).
So while the ls command should "just work" in your shell, the cd command won't.
You'll be adding this feature in a later homework.
HINT:
Pay careful attention to how you parse the command string the user enters.
There are many ways to mess this up and introduce bugs into your program.
You will be adding more parsing features in future assignments, so it will make your life much easier if you do it right the first time!
I recommend using either the strtok function from the C standard libraries (see this tutorial by former cs100 student Dave MacPherson) or the Tokenizer class provided in the boost library (see this tutorial by former cs100 students Chelsey Calingo and Virgilio Garcia).
Students often don't do this section of the assignment well and end up having to redo all of assignment one in order to complete the future assignments.
Create a new project on github called rshell.
Create a branch called exec.
Do all of your work under this branch.
When finished, merge the exec branch into the master branch, and create a tag called hw0.
Remember that tags and branches in git are case sensitive!
NOTE: git push will not automatically push tags to your repository.
Use git push origin hw0 to update your repository to include the hw0 tag.
To download and grade your homework, the TA will run the following commands from the hammer server:
$ git clone https://github.com/yourusername/rshell.git
$ cd rshell
$ git checkout hw0
$ make
$ bin/rshell
You should ssh into hammer.cs.ucr.edu and run the above commands to verify that you've submitted your code successfully.
If you forget how to use git, two students from previous cs100 courses (Rashid Goshtasbi and Kyler Rynear) made video tutorials on the git commands needed to submit your assignments via github.
Do not wait to upload your assignment to github until the project due date. You should be committing and uploading your assignment continuously. If you wait until the last day and can't figure out how to use git properly, then you will get a zero on the assignment. NO EXCEPTIONS.
You must have a directory called src which contains all the source code files for the project.
You must have a Makefile in the root directory.
In the Makefile you will have two targets.
The first target is called all and the second target is called rshell.
Both of these targets will compile your program using g++ with the flags: -Wall -Werror -ansi -pedantic.
You must NOT have a directory called bin in the project;
however, when the project is built, this directory must be created and all executable files placed here.
You must have a LICENSE file in your project.
You may select any open source license.
I recommend either GPL or BSD3.
You must have a README.md file.
This file should briefly summarize your project.
In particular, it must include a list of known bugs.
If you do not have any known bugs, then you probably have not sufficiently tested your code!
For more details on how to write a good README file, see this tutorial by former cs100 student Alexander Ortiz.
You must use the Markdown formatting language when writing your README.
You must have a directory called tests.
The directory will contain a file called exec.script that contains all of the test cases you tried.
You will generate the file using the script command, and it must be succinct (i.e. it cannot have unnecessary commands in it).
You should use comments in your script to document what you are testing with each test case.
This video tutorial explains how to use the script command.
IMPORTANT: The file/directory names above are a standard convention. You must use the exact same names in your project, including capitalization.
Your code must not generate any warnings on compilation.
Your final executable must have no memory leaks.
Your code must be properly indented. I don't care if you indent with tabs or with spaces, but you must be consistent. Any submission that uses both spaces and tabs for indentation will receive an automatic -20 points.
Every time you run a syscall, you must check for an error condition.
If an error occurs, then call perror.
For every syscall you use that is not error checked, you will receive an automatic -5 points.
For examples on when, how, and why to use perror, see this video tutorial.
Proper testing is the most important part of developing software. It therefore will have a very large impact on your grade.
It is not enough to simply show that your program works in some cases. You must show that it works in every possible edge case. That means you must try to write test cases that will break your program!
IMPORTANT: If you do not include a particular test case that we think is important, then we will assume your program fails that test. It doesn't matter if your program actually passes the test or not. If you didn't test that case and document it, then your program fails!
If you are unsure if your test cases are sufficient, ask one of the instructors to review them before the deadline.
You MAY NOT look at the source code of any other student.
You MAY discuss with other students in general terms how to use the unix functions.
You are ENCOURAGED to talk with other students about test cases. You are allowed to freely share ideas in this regard.
You are ENCOURAGED to look at bash's source code for inspiration.
30 points for executing commands
30 points for executing multiple commands on a single line with ;, &&, and ||
20 points for the exit command
20 points for comments
IMPORTANT: Your project structure is not explicitly listed in the grading schedule above. But for each part of the project structure you don't follow, you will receive an automatic -20 points.
IMPORTANT: Your test cases are also not explicitly listed above, but they are included implicitly in each category. For example, if you get comments working correctly, but do not test and document that feature, then you will get zero credit for comments.
Many shells display extra information in the prompt besides just a simple $.
For example, it is common to display the currently logged in user, and the hostname of the machine the user is logged into.
My username is mizbi001, and if I'm logged into the machine alpha023, then my prompt would look like:
mizbi001@alpha023$
You can get up to 10 points of extra credit if your prompt prints this extra information.
You will need to lookup the man pages for the functions getlogin and gethostname.
You must not hard code the username or hostname!
REMINDER: If anything is unclear about this assignment, remember that you can get extra credit by submitting pull requests and asking good questions in the issue tracker.
Here is a complete list of resources created by previous cs100 students that might help with this assignment: