CSCI 380 -- Lab 1a

Writing Your Own Unix Shell -- Core

The purpose of this assignment is to become more familiar with the concepts of process control and signaling. You’ll do this by writing a simple Unix shell program.

Start by copying lab1a-handout.tar to your Linux Lab home directory.

• tar xvf lab1a-handout.tar to expand the tarfile
• make to compile and link some test routines
• Remember to add documentation to tsh.c

Tasks to Perform

You are tasked with implementing a unix shell! This sounds intimidating but it's really not so bad.

Your shell must be able to:

• execute arbitrary commands typed into the shell prompt
• successfully wait for a foreground job to complete before displaying the shell prompt again
• able to run two built-in commands: quit and fg
  • quit -- quits the shell program
  • fg -- brings a job that was started in the background (&) to the foreground
• Ctrl-C interrupt -- this should kill the foreground job (if running)
• Ctrl-Z stop -- this should stop the foreground job

Remember, a shell is an interactive program that runs programs on behalf of the user. It follows a REPL interface: Read Evaluate Print Loop

The command line is a sequence of ASCII test words delimited by whitespace. The first word of this line will be either (1) the name of a built-in command or (2) an absolute pathname of an executable file. The remaining words on the line are command line arguments

• If the command entered is a built-in command, it is immediately invoked within the current process
• If the command entered is a pathname to an executable file, then the shell will fork a child process, then loads and runs the executable command. The child process(es) created as a result of interpreting a single command line are collectively known as a job.
• If the command line ends with an ampersand &, then the job runs in the background, which means 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 the shell waits for the job to terminate before awaiting the next command line.

ONLY ONE JOB CAN RUN IN THE FOREGROUND AT A TIME

Example

tsh> /bin/ls -l -d

Runs the ls command which will do a long listing on directory files

• runs the program in the foreground
• has three total "arguments" (argc == 3)
  • argv[0] == "/bin/ls"
  • argv[1] == "-l"
  • argv[2] == "-d"

tsh> /bin/sleep 10 &

Runs the sleep command which will wait at least 10 seconds before ending

• runs the program in the background
• has two total "arguments" (argc == 2)
  • argv[0] == "/bin/sleep"
  • argv[1] == "10"

Basic Job Control

You are only expected to be able to

1. start a job in the background (&)
2. switch the stopped job to the foreground (fg)
3. kill a currently running job with ctrl+c
4. stop a currently running job with ctrl+z

Shell Specification

• Prompt should be the string tsh>
• Typing ctrl-c should cause a SIGINT signal to be sent to the current foreground job as well as any descendents of the job. If there is no foreground job, then the signal should have no effect
• Typing ctrl-z should cause a SIGTSTP signal to be sent to the current foreground job as well as any descendents of the job. If there is no foreground job, then the signal should have no effect
• If the command line ends with an ampersand &, then the new job should be run in the background. Otherwise, it should run the job in the foreground
• Support the following built-in commands:
  • quit -- terminates the shell
  • fg -- restarts the stopped job by sending SIGCONT and run it in the foreground
• Reap all of its zombie children. If any job terminates because it recevies a signal that it didn't catch, then tsh should recognize this event and print a message with the job's PID and a description of the offending signal

Structure

You are welcome to organize this in any way, but you may find it helpful to structure your code into the following methods:

• eval -- main routine that parses and interprets the command line
• builtin_cmd -- recognize and interprets the built-in commands
• waitfg -- waits for a foreground job to complete
• sigchld_handler -- catches SIGCHILD signals
• sigint_handler -- catches SIGINT (ctrl-c) signals
• sigstp_handler -- catches SIGTSTP (ctrl-z) signals

Submission

Upload/submit tsh.c to autolab.

Grading Criteria

This lab will be graded out of 100 points.

• 80pt -- autograded
• 20pt -- discussion / comments of your implementation