shell-lab

The shell is a implementation of lab assignments from Computer Systems: A Programmer's Perspective. In this lab you would be required to implement an the shell program. By this lab, you would gain much more experiences on process controlling and signaling. The shell program is an terminal based user interface program which allows users to perform their task on the computer.

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 example, typing the command line

Usage

We have provided some tools to help you check your work. Reference solution. The Linux executable tshref is the reference solution for the shell. Run this program to resolve any questions you have about how your shell should behave. Your shell should emit output that is identical to the reference solution (except for PIDs, of course, which change from run to run). Shell driver. The sdriver.pl program executes a shell as a child process, sends it commands and signals as directed by a trace file, and captures and displays the output from the shell. Use the -h argument to find out the usage of sdriver.pl:

unix> ./sdriver.pl -h
Usage: sdriver.pl [-hv] -t <trace> -s <shellprog> -a <args>
Options:

  -h Print this message
  -v Be more verbose
  -t <trace> Trace file
  -s <shell> Shell program to test
  -a <args> Shell arguments
  -g Generate output for autograder
  

We have also provided 16 trace files trace{01-16}.txt that you will use in conjunction with the shell driver to test the correctness of your shell. The lower-numbered trace files do very simple tests, and the higher-numbered tests do more complicated tests. You can run the shell driver on your shell using trace file trace01.txt (for instance) by typing:

unix> ./sdriver.pl -t trace01.txt -s ./tsh -a "-p"

(the -a "-p" argument tells your shell not to emit a prompt), or

unix> make test01

Similarly, to compare your result with the reference shell, you can run the trace driver on the reference shell by typing:

unix> ./sdriver.pl -t trace01.txt -s ./tshref -a "-p"

or

unix> make rtest01

For your reference, tshref.out gives the output of the reference solution on all races. This might be more convenient for you than manually running the shell driver on all trace files. The neat thing about the trace files is that they generate the same output you would have gotten had you run your shell interactively (except for an initial comment that identifies the trace). For example:

bass> make test15
./sdriver.pl -t trace15.txt -s ./tsh -a "-p"
#
# trace15.txt - Putting it all together
#
tsh> ./bogus
./bogus: Command not found.
tsh> ./myspin 10
Job (9721) terminated by signal 2
tsh> ./myspin 3 &
[1] (9723) ./myspin 3 &
tsh> ./myspin 4 &
[2] (9725) ./myspin 4 &
tsh> jobs
[1] (9723) Running ./myspin 3 &
[2] (9725) Running ./myspin 4 &
tsh> fg %1
Job [1] (9723) stopped by signal 20
tsh> jobs
[1] (9723) Stopped ./myspin 3 &
[2] (9725) Running ./myspin 4 &
tsh> bg %3
%3: No such job
tsh> bg %1
[1] (9723) ./myspin 3 &
tsh> jobs
[1] (9723) Running ./myspin 3 &
[2] (9725) Running ./myspin 4 &
tsh> fg %1
tsh> quit
bass>