CS 213, Fall 2001Lab Assignment L5: Writing Your Own Unix ShellAssigned: Oct. 25, Due: Fri., Nov. 2, 11:59PMDave O’Hallaron ([email protected]) is the lead person for this assignment.IntroductionThe purpose of this assignment 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.LogisticsAs always, you may work in a group of up to two people in solving the problems for this assignment. Theonly “hand-in” will be electronic. Any clarifications and revisions to the assignment will be posted on thecourse Web page.Hand Out InstructionsAll files you need are in the directory/afs/cs.cmu.edu/academic/class/15213-f01/L5Start by copying the file L5.tar in that directory to the protected directory (the lab directory) in whichyou plan to do your work. Then do the following:Type the command tar xvf L5.tar to expand the tarfile.Type the command make to compile and link some test routines.Type your team member names and Andrew IDs in the header comment at the top of tsh.c.1Looking at the tsh.c (tiny shell) file, you will see that it contains a functional skeleton of a simple Unixshell. To help you get started, we have already implemented the less interesting functions. Your assignmentis to complete the remaining empty functions. As a sanity check, we’ve listed the number of lines of codefor each of these functions in our reference solution, including comments.eval: Main routine that parses and interprets the command line. [40 lines]builtin cmd: Recognizes and interprets the built-in commands: quit, fg, bg, and jobs. [25lines]do bgfg: Implements the bg and fg built-in commands. [50 lines]waitfg: Waits for a foreground job to complete. [46 lines]sigchld handler: Catches SIGCHILD signals. [38 lines]sigint handler: Catches SIGINT (ctrl-c) signals. [14 lines]sigtstp handler: Catches SIGTSTP (ctrl-z) signals. [16 lines]Each time you modify your tsh.c file, type make to recompile it. To run your shell, type tsh to thecommand line:unix> tshtsh> [type commands to your shell here]General Overview of Unix ShellsA 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 bythe contents of the command line.The command line is a sequence of ASCII text words delimited by whitespace. The first word in thecommand line is either the name of a built-in command or the pathname of an executable file. The remainingwords are command-line arguments. If the first word is a built-in command, the shell immediately executesthe command in the current process. Otherwise, the word is assumed to be the pathname of an executableprogram. In this case, the shell forks a child process, then loads and runs the program in the context of thechild. The child processes created as a result of interpreting a single command line are known collectivelyas 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 thatthe shell does not wait for the job to terminate before printing the prompt and awaiting the next commandline. Otherwise, the job runs in the foreground, which means that the shell waits for the job to terminatebefore awaiting the next command line. Thus, at any point in time, at most one job can be running in theforeground. However, an arbitrary number of jobs can run in the background.For example, typing the command line2tsh> jobscauses the shell to execute the built-in jobs command. Typing the command linetsh> /bin/ls -l -druns the ls program in the foreground. By convention, the shell ensures that when ls begins executing itsmain 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 linetsh> /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 processesin a job. Typing ctrl-c causes a SIGINT signal to be delivered to each process in the foreground job. Thedefault action for SIGINT is to terminate the process. Similarly, typing ctrl-z causes a SIGTSTP signalto be delivered to each process in the foreground job. The default action for SIGTSTP is to place a processin the stopped state, where it remains until it is awakened by the receipt of a SIGCONT signal. Unix shellsalso provide various built-in commands that support job control. For example:jobs: List the running and stopped background jobs.bg <job>: Change a stopped background job to a running background job.fg <job>: Change a stopped or running background job to a running in the foreground.kill <job>: Terminate a job.The tsh SpecificationYour tsh shell should have the following features:3The 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 immediatelyand wait for the next command line. Otherwise, tsh should assume that name is the path of anexecutable file, which it loads and runs in the context of an initial child process (In this context, theterm 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 be sent to the current fore-ground job, as well as any descendents of that job (e.g., any child processes that it forked). If there isno foreground job, then the signal should have no effect.If the command line ends with an ampersand, then tsh should run the job in the background. Other-wise, it should run the job in the foreground.Each job can be identified by either a process ID (PID) or a job ID (JID), which is a small positiveinteger assigned by tsh. JIDs should be denoted on the command line by the prefix ’%’. For example,“%5” denotes JID 5, and “5” denotes PID 5. (We have provided you with all of the routines you needfor manipulating the job list.)tsh
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