# CPU Task Scheduler Simulator with Shell Integration
Github Link : https://github.com/dennis15984/CPU-Task-Scheduler-Simulator-with-Shell-Integration
## Overview

A Unix-like shell integrated with a user-level task scheduler supporting three scheduling algorithms: Round Robin (RR), First-Come-First-Served (FCFS), and Preemptive Priority (PP). The system features command history, pipeline processing, I/O redirection, background job execution, resource management with 8 shared resources, task lifecycle tracking, and comprehensive timing statistics.

## Core Components

### 1. Shell Implementation (`shell.c`, `command.c`, `builtin.c`)

#### Shell Features
- **Command Parsing**: Tokenizes input and handles special characters
- **Pipeline Support**: Multi-stage command execution with pipes (`|`)
- **I/O Redirection**: Input (`<`) and output (`>`) redirection
- **Background Execution**: Asynchronous process execution (`&`)
- **Command History**: Record and replay previous commands
- **Built-in Commands**: System-level operations

#### Built-in Commands
| Command | Syntax | Description |
|---------|--------|-------------|
| `help` | `help` | Display available commands |
| `cd` | `cd [directory]` | Change working directory |
| `echo` | `echo [-n] [text]` | Print text to stdout |
| `exit` | `exit` | Terminate shell |
| `record` | `record` | Show command history |
| `replay` | `replay [number]` | Re-execute command from history |
| `mypid` | `mypid -i\|-p [pid]\|-c [ppid]` | Process information utilities |
| `add` | `add [name] [function] [priority]` | Create new task |
| `del` | `del [name]` | Delete task |
| `ps` | `ps` | Display task status |
| `start` | `start` | Begin task execution |

#### Command Processing Flow

User Input → Tokenization → Command Structure → Execution
     ↓
┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐
│  read_line()    │ →  │  split_line()   │ →  │ fork_pipes() or │
│                 │    │                 │    │   execute()     │
│ - Input buffer  │    │ - Parse tokens  │    │ - Process mgmt  │
│ - History mgmt  │    │ - Handle pipes  │    │ - I/O redirect  │
│ - Replay logic  │    │ - I/O redirect  │    │ - Background    │
└─────────────────┘    └─────────────────┘    └─────────────────┘

### 2. Task Scheduler (`task.c`)

#### Scheduling Algorithms

**First-Come-First-Served (FCFS)**
- Non-preemptive execution
- Tasks run to completion unless voluntarily yielding
- Simple FIFO queue processing

void fcfs_algo() {
    while(ready_idx != 0 && ready_queue->next != NULL) {
        working_struct = ready_queue->next;
        working_struct->status = RUNNING;
        swapcontext(&main_task, &working_struct->task);
    }
}

**Round Robin (RR)**
RR uses signals for time-based preemption. A virtual timer (SIGVTALRM) fires every 10 microseconds, tracking each task's CPU time. When a task reaches its 30ms time quantum, the signal handler forcibly interrupts it, saves its execution state, and returns control to the scheduler for the next task in the circular queue.

// Timer interrupt every 10μs
if(time_quantum == 30 && ready_idx > 1 && strcmp(algo_type, rr_word) == 0) {
    getcontext(&store_task);
    working_struct->task = store_task;
    time_quantum = 0;
    setcontext(&main_task);  // Return to scheduler
}

**Preemptive Priority (PP)**
Preemptive Priority (PP) uses the same signal mechanism but for priority-based preemption. The timer continuously monitors the ready queue - if a higher priority task becomes available (e.g., wakes up from sleep or gets added), the signal handler immediately preempts the current lower-priority task and switches to the higher-priority one.

void* prior_select() {
    task_struct* higher = ready_queue->next;
    task_struct* temp = ready_queue->next;
    for(int i = 0; i < ready_idx; i++) {
        if(temp->priority < higher->priority) {
            higher = temp;
        }
        temp = temp->next;
    }
    return higher;
}

#### Task State Management

┌─────────┐    create     ┌─────────┐    schedule   ┌─────────┐
│ CREATED │ ────────────→ │  READY  │ ────────────→ │ RUNNING │
└─────────┘               └─────────┘               └─────────┘
                               ↑                         │
                               │                         │ sleep/block
                               │ wakeup                  ↓
                          ┌─────────┐               ┌─────────┐
                          │ WAITING │               │TERMINATED│
                          └─────────┘               └─────────┘

#### Task Structure

typedef struct tasks {
    ucontext_t task;           // CPU execution context
    task_state status;         // Current state
    char* task_name;           // Identifier
    int priority;              // Scheduling priority
    int TID;                   // Task ID
    int running_time;          // CPU time consumed
    int waiting_time;          // Time spent waiting
    int turnaround_time;       // Total lifetime
    int sleep_time;            // Sleep duration remaining
    int resource_count;        // Number of resources held
    int* resources;            // Resource ownership bitmap
    int* resource_check;       // Resource allocation tracking
    int resource_flag;         // Resource state indicator
    struct tasks* next;        // Queue linkage
    struct tasks* previous;    // Bidirectional linking
} task_struct;

### 3. Context Switching Mechanism

#### User-Level Context Management
The system implements cooperative multitasking using POSIX ucontext functions:

**Context Capture and Restoration**

// Save current execution state
getcontext(&working_struct->task);

// Restore previous execution state  
setcontext(&working_struct->task);

// Atomic save and switch
swapcontext(&main_task, &working_struct->task);

// Create new execution context
makecontext(&new_task->task, function_pointer, 0);

**Context Switch Flow**

Scheduler Context
       │
       ├─ setcontext() ─────────→ Task Context
       │                              │
       │                         (task execution)
       │                              │
       │                         getcontext() 
       │                         (voluntary yield)
       │                              │
       ←─ setcontext() ─────────────────┘
       │
   (next task selection)

### 4. Resource Management (`resource.c`)

#### Resource Allocation System
- **8 Shared Resources**: Numbered 0-7, each binary available/in-use

int resource_flag[8] = {1,1,1,1,1,1,1,1};  // 1=available, 0=in-use

void get_resources(int count, int *resources) {
    // Check availability
    if(check_resource(count, working_struct->resources, working_struct->resource_check)) {
        // Grant all requested resources atomically
        for(int i = 0; i < count; i++) {
            resource_flag[resources[i]]--;
            working_struct->resource_check[resources[i]] = 1;
        }
        working_struct->resource_flag = 1;
    } else {
        // Block and wait
        wait_resource();
    }
}

#### Resource State Transitions

┌──────────────┐    request    ┌──────────────┐
│   AVAILABLE  │ ────────────→ │   IN USE     │
└──────────────┘               └──────────────┘
       ↑                              │
       │ release                      │ (task holds resource)
       └──────────────────────────────┘

### 5. Task Functions (`function.c`)

#### Computational Tasks
- **task1**: Bubble sort algorithm (CPU-intensive)
- **task2**: Matrix multiplication (memory-intensive)
- **task3**: Linear search operation (I/O simulation)

#### Resource Management Tasks
- **task4-9**: Various resource usage patterns
- **test_resource**: Resource allocation testing

#### Control Flow Tasks
- **test_exit**: Immediate termination
- **test_sleep**: Sleep/wake cycle testing
- **idle**: Infinite loop for system testing

## Compilation and Usage

### Build System

TARGET 	= scheduler_simulator
CC     	= gcc
FLAGS  	= -Wall -pthread
OBJ    	= builtin.o command.o shell.o function.o resource.o task.o
INCLUDE = ./include/
SRC		= ./src/

all: $(TARGET) 

$(TARGET): main.c $(OBJ) 
	$(CC) $(FLAGS) -o $(TARGET) $(OBJ) $<

%.o: ${SRC}%.c ${INCLUDE}%.h
	$(CC) $(FLAGS) -c $<

.PHONY: clean
clean:
	rm -f ${TARGET} *.o out*
clean_obj:
	rm -f *.o

### Execution

./scheduler_simulator [ALGORITHM]

Where `[ALGORITHM]` is one of: `FCFS`, `RR`, `PP`

## Usage Examples

### Basic Task Management

# Start shell with Round Robin scheduling
./scheduler_simulator RR

# Create tasks with different priorities
>>> $ add sorting_task task1 5
>>> $ add matrix_task task2 3  
>>> $ add search_task task3 7

# View current task queue
>>> $ ps

# Begin execution
>>> $ start

# Monitor execution and return to shell
>>> $ ps
>>> $ exit

### Shell Operations

# Command history
>>> $ echo "test command"
>>> $ record
>>> $ replay 1

# Pipeline operations
>>> $ cat input.txt | grep "pattern" | sort > output.txt

# Background execution
>>> $ long_running_command &

# Process information
>>> $ mypid -i
>>> $ mypid -p 1234
>>> $ mypid -c 5678

### Resource Management Demo

>>> $ add resource_user task4 1
>>> $ add resource_competitor task5 2
>>> $ start
# Observe resource contention and blocking
>>> $ ps

## Testing Framework

### Automated Scheduler Testing

# Test single algorithm
python3 auto_run.py FCFS test_case1.txt

# Test all algorithms
python3 auto_run.py all general.txt

### Shell Functionality Testing

# Comprehensive shell testing
python3 judge_shell.py all

# Specific feature testing
python3 judge_shell.py 1.1 2.3  # Test external commands and record/replay

## Performance Metrics

### Timing Statistics
- **Running Time**: CPU cycles consumed by task
- **Waiting Time**: Time spent in ready queue
- **Turnaround Time**: Total time from creation to completion
- **Response Time**: Time to first execution

### Task Status Display

TID|        name|       state|     running|     waiting|  turnaround|   resources|    priority
----------------------------------------------------------------------------------------------
  1   task1_sort      RUNNING           45          12           57        1 3 5            5
  2   task2_matrix        READY            0          30           30         none            3
  3   task3_search   TERMINATED          120          25          145         none            7