Write a bash script to print a particular line from a file in C

In C programming, we can read and print a specific line from a file using file handling functions. This involves opening the file, reading through it line by line, and printing the desired line number.

Syntax

FILE *fopen(const char *filename, const char *mode);
char *fgets(char *str, int n, FILE *stream);
int fclose(FILE *stream);

Method 1: Using Line Counter

This approach reads the file line by line and uses a counter to track the current line number −

Note: Create a text file named "text.txt" in the same directory with some content before running this program.

#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE *file;
    char line[256];
    int lineNumber = 5; /* Line number to print */
    int currentLine = 1;
    
    /* Open file for reading */
    file = fopen("text.txt", "r");
    if (file == NULL) {
        printf("Error: Could not open file text.txt<br>");
        return 1;
    }
    
    /* Read file line by line */
    while (fgets(line, sizeof(line), file) != NULL) {
        if (currentLine == lineNumber) {
            printf("Line %d: %s", lineNumber, line);
            break;
        }
        currentLine++;
    }
    
    if (currentLine < lineNumber) {
        printf("File has only %d lines<br>", currentLine - 1);
    }
    
    fclose(file);
    return 0;
}

Method 2: Using Dynamic Line Number Input

This approach allows the user to specify which line number to print −

#include <stdio.h>
#include <stdlib.h>

void printSpecificLine(const char *filename, int targetLine) {
    FILE *file;
    char buffer[1024];
    int currentLine = 1;
    int found = 0;
    
    file = fopen(filename, "r");
    if (file == NULL) {
        printf("Error: Cannot open file %s<br>", filename);
        return;
    }
    
    while (fgets(buffer, sizeof(buffer), file) != NULL) {
        if (currentLine == targetLine) {
            printf("Line %d: %s", targetLine, buffer);
            found = 1;
            break;
        }
        currentLine++;
    }
    
    if (!found) {
        printf("Line %d not found. File has %d lines.<br>", targetLine, currentLine - 1);
    }
    
    fclose(file);
}

int main() {
    char filename[] = "text.txt";
    int lineNumber = 3;
    
    printf("Reading line %d from file %s:<br>", lineNumber, filename);
    printSpecificLine(filename, lineNumber);
    
    return 0;
}

Key Points

• Always check if fopen() returns NULL to handle file opening errors.
• Use fgets() to read lines safely with buffer size limits.
• Remember to close the file using fclose() after reading.
• Line numbering typically starts from 1, not 0.

Conclusion

Reading specific lines from files in C requires careful file handling and line counting. The methods shown provide robust solutions for extracting particular lines while handling edge cases like missing files or insufficient line counts.