Introduction

A process is the fundamental unit of execution in UNIX. While a program is a static set of instructions stored on disk, a process is a program in execution — it has its own memory space, resources, and state.

What is a Process?

• A process is an instance of a running program.
• Every process has a unique Process ID (PID) — a positive integer assigned by the kernel.
• Processes are created, executed, and eventually terminated.
• UNIX is a multi-processing system — many processes can exist simultaneously.

Key Attributes of a Process

Process Creation — fork() and exec()

1. fork() — Creating a Child Process:

• The fork() system call creates a new process (child) that is a copy of the calling process (parent).
• The child process gets a new PID but inherits:
• Copy of parent's memory (text, data, stack)
• Copy of parent's open file descriptors
• Parent's environment variables
• fork() returns:
• 0 to the child process.
• Child's PID to the parent process.
• -1 if the fork fails.

2. exec() — Replacing Process Image:

• The exec() family of system calls replaces the current process image with a new program.
• The PID remains the same, but the program code, data, and stack are replaced.
• Variants: execl(), execv(), execle(), execve(), execlp(), execvp().

3. wait() — Parent Waits for Child:

• The parent calls wait() to suspend itself until the child process terminates.
• Returns the child's exit status.

4. exit() — Process Termination:

• A process calls exit() to terminate itself.
• Resources are deallocated, but the process table entry remains until the parent calls wait().

Special Processes

• init is the parent of all orphan processes. If a parent terminates before its child, the child is adopted by init.
• A process whose parent has not yet called wait() after the child has exited is called a zombie process.

Process Hierarchy

               init (PID 1)
              /     |      \
         login    cron    sshd
          |                |
        bash             bash
       /    \
     ls     grep

• All processes form a tree rooted at init (PID 1).
• Each process (except init) has exactly one parent.

Summary

• A process is a program in execution with its own PID, memory, and resources.
• fork() creates a child process; exec() replaces the process image.
• wait() lets the parent collect the child's exit status.
• All processes descend from init (PID 1) in a tree hierarchy.