COMP3000 Operating Systems W22: Tutorial 3
In this tutorial, you will be experimenting with and extending 3000shell.c, a proof-of-concept program to show you how a Linux shell works. Also, you will be learning to read and modify C code, which prepares you for subsequent tutorials and assignments.
Make sure you use the original code from 3000shell for each question/task.
Tutorials are graded based on participation and effort (so no need to try to have the “correct” answers — what matters is the process), but you should still turn in your work. Submit your answers on Brightspace as a single text file named "<username>-comp3000-t3.txt" (where username is your MyCarletonOne username). The first four lines of this file should be "COMP 3000 Tutorial 3", your name, student number, and the date of submission.
The deadline is usually four days after the tutorial date (see the actual deadline on the submission entry). Note that the submission entry is enforced by the system, so you may fail to get the effort marks even if it is one minute past the deadline.
You should also check in with your assigned TA online (by responding to the poll in the Teams channel tutorials-public or the private channel). Your TA will be your first point of contact when you have questions or encounter any issues during the tutorial session.
You get 1.5 marks for submitting answers that shows your effort and 0.5 for checking in, making this tutorial worth 2 points total.
Background
Processes
Each application running on a system is assigned a unique process identifier (PID). The ps command shows the process identifiers for running processes. Each process running on the system is kept separated from other processes by the operating system.
When you enter a command at a shell prompt, most of the time you are creating a new process which runs the program you specified.
Controlling Processes
On Linux, once the processes have been created, you can control them by sending them signals. You send signals when you type certain key sequences in most shells: Control-C sends INT (interrupt), Control-Z sends STOP.
You can send a signal to a process using the kill command:
kill -<signal> <process ID>
So, to stop process 4542, type:
kill -STOP 4542
By default, kill sends the TERM signal.
Each signal has both a string name (e.g., STOP) and a numeric form (e.g., 19 correspondingly). See the full list of signals with kill -l.
Standard input/output, Shell and Terminal
Recall that the shell is a command interpreter (program). In comparison, a terminal is a device used to enter data into and display/printing data from a computer. It used to be physical - teletypes, or "TTY"'s, hence we call the virtual ones today pseudoterminals. In this context, we refer to it as pts -- pseudo teletypes, e.g., /dev/pts/1 (they are just the slave side. Those who are curious can refer to man ptmx, but not needed in this tutorial).
The standard input (e.g., /dev/stdin), output (e.g., /dev/stdout) and error (/dev/stderr) are just symbolic endpoints in any programs (of which the shell is an example), that can be redirected anywhere, where output goes, input comes and error is reported. By default, they refer to file descriptors 0, 1, and 2 for the current process.
So, in summary, you connect to a terminal to interact with a computer, by running a shell to receive your commands, which redirects standard in/out/err to the terminal.
Tasks/Questions
The purpose of the following questions and tasks is to help you understand how 3000shell (a simple implementation of the shell, but still more complex than the previous csimpleshell) works. Eventually, you should have an understanding of every function and every line of the code. If you understand 3000shell, then you understand the basics of all UNIX shells. Your understanding of the code will be tested later, so use this opportunity to dive deep into the code.
- Compile and run 3000shell.c.
- Compared to csimpleshell from Tutorial 1, what functionality improvements has 3000shell introduced? List at least two improvements (functional differences). You can mention whether you found them by reading the source code or trying both shells.