Tiny Shell

Tiny Shell is a simple Unix shell program that supports job control, making use of the interface provided by Unix operating systems.

How does it work?

Tiny Shell performs a sequence of (read/evaluate) steps and then terminates.

It has the following detailed behaviour:

• The read step reads a command-line from the user. The evaluate step parses the command-line and runs programs on behalf of the user.

• tiny-shell file contains eval() function that contains the code which evaluates the command-line. Its first task is to call the parseline() function, which parse the space-separated command-line arguments and builds the argv vector that will eventually be passed to execve. The first argument is assumed to be either the name of a built-in shell command that is interpreted immediately, or an executable object file that will be loaded and run in the context of a new child process.

• If the last argument is an & character,then parseline() returns 1, indicating that the program should be executed in the background (the shell doesn't wait for it to complete). Otherwise, it returns 0, indicating that the program should be run in the foreground (the shell waits for it to complete).

• After parsing the command-line, the eval() function calls the builtin_command() function, which checks whether the first command-line argument is a built-in shell command. If so, it interprets the command immediately and returns 1. Otherwise, it returns 0.

• If builtint_command() returns 0, then the shell creates a child process and excutes the requested program inside the child and adds it to the jobs list. If the user has asked for the program to run in the background, then the shell returns to the top of the loop and waits for the next command-line. Otherwise the shell uses the waitfg() function to wait for the job to terminate. when the job terminates the shell reaps it, delete it from jobs list, and goes on to the next iteration.

• If any of the background jobs terminated, it sends SIGCHLD signal to the shell, and the shell then reaps it through the sigchld_handler() function.

what commands does Tiny Shell support?

Tiny Shell supports the following built-in commands:

• The quit command terminates the shell.
• The jobs command lists all background jobs.
• The bg <job> command restarts <job> by sending it a SIGCONT signal, and then runs it in the background. The <job> argument can be either a PID or a JID.
• The fg <job> command restarts <job> by sending it a SIGCONT signal, and then runs it in the foreground. The <job> argument can be either a PID or a JID.

Requirements

• Git
• GCC

How to test it?

Run the following commands in a terminal:

• git clone https://github.com/ahmed-salah-ucf/tiny-shell.git
• make
• .\tsh

Thoughts

• The Project helped me become more familiar with the concepts of process control and signalling.