Write a program to implement the shell. It should display the command prompt “myshell$”. Tokenize the command line and execute the given command by creating the child process. Additionally it should interpret the following commands.
myshell$ search f filename  pattern : To display first occurrence of pattern in the file.
myshell$ search c filename pattern :To count the number of occurrence of pattern in the file.


#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

// Function to find and display the first occurrence of a pattern in a file
void search_first_occurrence(const char *filename, const char *pattern) {
    FILE *file = fopen(filename, "r");
    if (file == NULL) {
        perror("fopen failed");
        return;
    }

    char line[1024];
    while (fgets(line, sizeof(line), file)) {
        char *pos = strstr(line, pattern);
        if (pos != NULL) {
            printf("First occurrence: %s", line);
            fclose(file);
            return;
        }
    }

    printf("Pattern \"%s\" not found in file \"%s\".\n", pattern, filename);
    fclose(file);
}

// Function to count the number of occurrences of a pattern in a file
void count_occurrences(const char *filename, const char *pattern) {
    FILE *file = fopen(filename, "r");
    if (file == NULL) {
        perror("fopen failed");
        return;
    }

    char line[1024];
    int count = 0;
    while (fgets(line, sizeof(line), file)) {
        char *pos = line;
        while ((pos = strstr(pos, pattern)) != NULL) {
            count++;
            pos += strlen(pattern);
        }
    }

    printf("Number of occurrences of pattern \"%s\" in file \"%s\": %d\n", pattern, filename, count);
    fclose(file);
}

// Function to execute other commands
void execute_command(char **tokens) {
    pid_t pid = fork();
    if (pid < 0) {
        perror("fork failed");
        exit(EXIT_FAILURE);
    } else if (pid == 0) {
        // Child process
        execvp(tokens[0], tokens);
        perror("execvp failed");
        exit(EXIT_FAILURE);
    } else {
        // Parent process
        int status;
        waitpid(pid, &status, 0);
    }
}

int main() {
    char input[1024];
    char *tokens[100];
    
    while (1) {
        printf("myshell$ ");
        if (fgets(input, sizeof(input), stdin) == NULL) {
            perror("fgets failed");
            exit(EXIT_FAILURE);
        }

        // Remove trailing newline
        input[strcspn(input, "\n")] = '\0';

        // Tokenize the input
        int token_count = 0;
        char *token = strtok(input, " ");
        while (token != NULL && token_count < 99) {
            tokens[token_count++] = token;
            token = strtok(NULL, " ");
        }
        tokens[token_count] = NULL;

        // Check if it's a special search command
        if (token_count >= 3 && strcmp(tokens[0], "search") == 0) {
            if (strcmp(tokens[1], "f") == 0 && token_count == 4) {
                // search f filename pattern
                search_first_occurrence(tokens[2], tokens[3]);
            } else if (strcmp(tokens[1], "c") == 0 && token_count == 4) {
                // search c filename pattern
                count_occurrences(tokens[2], tokens[3]);
            } else {
                printf("Usage: search [f|c] filename pattern\n");
            }
        } else {
            // Execute other commands
            execute_command(tokens);
        }
    }

    return 0;
}