CS456 - Systems Programming Spring 2026

Displaying ./code/h1/head_simple.c

/*

head_simple.c - a simple implementation of the head command in linux

this takes a file through the command line and prints a specified number of 
lines from the beginning of the file

if no number is specified, print 10 lines by default.

running below would print the first 10 lines of file (argc = 2)
>./head_simple file

but running below would print the first 18 lines of file (argc = 3)
>./head_simple file 18

*/
#include <stdio.h>
#include <stdlib.h>
#include <errno.h> //needed for perror

int main(int argc, char **argv){

  //check if file was provided , otherwise print usage statement and exit
  if(argc < 2){
    fprintf(stderr, "Usage: %s file <optional number>\n", argv[0]);
    exit(1);
  } 

  //read in a file from the command line using fopen
  FILE *fp = fopen(argv[1], "r"); 

  // checking return value of fopen
  // if fopen returns NULL, then it failed, so print error message and exit
  if(fp == NULL){
    perror("open");
    exit(-1);
  }


  //declaring a variable to store the number of lines specified in command line 
  int numlines;

  //if there was a number provided on the command line 
    //convert that string to an integer and assign it to numlines,
  // if not, set numlines to 10
  if(argc == 3)
     numlines = atoi(argv[2]);
  else
     numlines = 10;


  int c; //fgetc returns an int, hence declaring c as an int
  int lines = 0; //keep a count of newline characters encountered so far
  
  //loop through entire file
  while((c =fgetc(fp))!= EOF){
    //print the character one at a time

    //these two lines do the same thing (leave one commented out)
    //printf("%c", c); //print the character one at a time
    fputc(c, stdout);

    //check 'c' for a newline
      //if yes, increment count
      // if we reach the specified number of lines, break loop
    if(c == '\n')
	lines++;

    if(lines == numlines)
	break;


  } 

  
  fclose(fp); //closing the file as we're done with it

  return 0;

}