Introduction

The purpose of this project is to become more familiar with the concepts of process control and sig- nalling. You’ll do this by writing a simple Unix shell program that supports job control.


Hand Out Instructions

Start by copying the file shlab-handout.tar from Canvas to the protected directory (the lab directory) in which you plan to do your work. Then do the following:


• Type the command tar xvf shlab-handout.tar to expand the tarfile.

• Type the command make to compile and link some test routines.

• Type your name and PSU ID in the header comment at the top of tsh.c.


Looking at the tsh.c (tiny shell) file, you will see that it contains a functional skeleton of a simple Unix shell. To help you get started, we have already implemented the less interesting functions. Your project is to complete the remaining empty functions listed below. As a sanity check for you, we’ve listed the approximate number of lines of code for each of these functions in our reference solution (which includes lots of comments).


• eval: Main routine that parses and interprets the command line. [70 lines]

• builtin cmd: Recognizes and interprets the built-in commands: quit, fg, bg, and jobs. [25 lines]

• do bgfg: Implements the bg and fg built-in commands. [50 lines]



• waitfg: Waits for a foreground job to complete. [20 lines]

• sigchld handler: Catches SIGCHILD signals. 80 lines]

• sigint handler: Catches SIGINT (ctrl-c) signals. [15 lines]

• sigtstp handler: Catches SIGTSTP (ctrl-z) signals. [15 lines]


Each time you modify your tsh.c file, type make to recompile it. To run your shell, type tsh to the command line:


unix> ./tsh

tsh> [type commands to your shell here]



General Overview of Unix Shells

A shell is an interactive command-line interpreter that runs programs on behalf of the user. A shell repeat- edly prints a prompt, waits for a command line on stdin, and then carries out some action, as directed by the contents of the command line.

The command line is a sequence of ASCII text words delimited by whitespace. The first word in the command line is either the name of a built-in command or the pathname of an executable file. The remaining words are command-line arguments. If the first word is a built-in command, the shell immediately executes the command in the current process. Otherwise, the word is assumed to be the pathname of an executable program. In this case, the shell forks a child process, then loads and runs the program in the context of the child. The child processes created as a result of interpreting a single command line are known collectively as a job. In general, a job can consist of multiple child processes connected by Unix pipes.

If the command line ends with an ampersand ”&”, then the job runs in the background, which means that the shell does not wait for the job to terminate before printing the prompt and awaiting the next command line. Otherwise, the job runs in the foreground, which means that the shell waits for the job to terminate before awaiting the next command line. Thus, at any point in time, at most one job can be running in the foreground. However, an arbitrary number of jobs can run in the background.

For ple, typing the command line


tsh> jobs


causes the shell to execute the built-in jobs command. Typing the command line


tsh> /bin/ls -l -d


runs the ls program in the foreground. By convention, the shell ensures that when the program begins executing its main routine


int main(int argc, char *argv[])

the argc and argv arguments have the following values:



• argc == 3,

• argv[0] == ‘‘/bin/ls’’,

• argv[1]== ‘‘-l’’,

• argv[2]== ‘‘-d’’.


Alternatively, typing the command line


tsh> /bin/ls -l -d &


runs the ls program in the background.

Unix shells support the notion of job control, which allows users to move jobs back and forth between back- ground and foreground, and to change the process state (running, stopped, or terminated) of the processes in a job. Typing ctrl-c causes a SIGINT signal to be delivered to each process in the foreground job. The default action for SIGINT is to terminate the process. Similarly, typing ctrl-z causes a SIGTSTP signal to be delivered to each process in the foreground job. The default action for SIGTSTP is to place a process in the stopped state, where it remains until it is awakened by the receipt of a SIGCONT signal. Unix shells also provide various built-in commands that support job control. For ple:


• jobs: List the running and stopped background jobs.

• bg: Change a stopped background job to a running background job.

• fg: Change a stopped or running background job to a running in the foreground.

• kill: Terminate a job.


The tsh Specification
Your tsh shell should have the following features:


• The prompt should be the string “tsh> ”.

• The command line typed by the user should consist of a name and zero or more arguments, all sepa- rated by one or more spaces. If name is a built-in command, then tsh should handle it immediately and wait for the next command line. Otherwise, tsh should assume that name is the path of an executable file, which it loads and runs in the context of an initial child process (In this context, the term job refers to this initial child process).

• tsh need not support pipes (|) or I/O redirection (< and >).

• Typing ctrl-c (ctrl-z) should cause a SIGINT (SIGTSTP) signal to