DEV Community: Suraj Kareppagol

Building Linux Commands From Scratch 🐧 clear Command

Suraj Kareppagol — Sat, 30 Mar 2024 13:03:25 +0000

Here i am building clear command from scratch in C/C++. The clear command is used to clear the terminal screen.

This command can be implemented in many ways. Here are some of the methods.

#include <stdio.h>
#include <sys/ioctl.h>
#include <unistd.h>

int main()
{
  // Get Terminal Size

  struct winsize w;
  ioctl(STDOUT_FILENO, TIOCGWINSZ, &w);

  // printf("%d %d\n", w.ws_row, w.ws_col);

  for (int i = 0; i < w.ws_row + 40; i++)
    printf("\n");

  printf("\033[0;0H");

  return 0;
}

struct winsize w;

Here winsize is a structure that has ws_row, ws_col as members. This can be populated using ioctl().

ioctl(STDOUT_FILENO, TIOCGWINSZ, &w);

int ioctl(int __fd, unsigned long __request, …);

This is the function prototype of the ioctl().

Here we will print “\n” for Terminal Rows + 40 times. This will although works but the prompt will be moved to the bottom. However ASCII sequence “[m;hH” can be used to move the cursor to the top left of the terminal.

printf("\033[0;0H");

This will move the cursor back to the top left of the terminal.

This is will work exactly like clear command, but there is another ASCII sequence that will directly clear the terminal screen.

printf("\033c");

This is another method for writing the clear command and an easy one (just one line to clear the screen).

#include <stdio.h>

int main()
{
  printf("\033c");

  return 0;
}

Also instead of loop and ASCII sequence, the ASCII sequence also works just fine.

#include <stdio.h>

int main()
{
  printf("\033[0;0H");

  return 0;
}

The code can be found at GitHub.

Building Linux Commands From Scratch 🐧 cat Command — Adding New Features

Suraj Kareppagol — Sun, 17 Mar 2024 09:59:08 +0000

In the last post i wrote the basic cat command in C that could prints the content of the files to the standard output. Now it is time to update it, adding two features.

Using the program as cat without any command line arguments
Using -n option to print line numbers

For the first part, these are the changes made to original program (Please read the previous blog).

#define MAX 1000

int main(int argc, char *argv[])
{
  char input[MAX];

  else
  {
    while (1)
    {
      scanf("%s", input);
      printf("%s\n", input);
    }
  }

Here the input is an array of type char. If the argc is 0 then it comes under else block and then to the while loop, where for each iteration the input is read from standard input and prints it to standard output.

For the second part, a new function is created printFile().

#define TRUE 1
#define FALSE 0

void printFile(char *argv[], int argc, int index, int numbers)
{
  char character;
  int count = 0;

  for (int i = index; i < argc; i++)
  {
    FILE *file = fopen(argv[i], "r");

    if (numbers)
      printf("\t%d ", ++count);
    while ((character = fgetc(file)) != EOF)
    {
      printf("%c", character);
      if (numbers && character == '\n')
        printf("\t%d ", ++count);
    }

    fclose(file);
  }
}

Here the signature of the function is,

void printFile(char *argv[], int argc, int index, int numbers);

Here the index is from where the loop starts, and numbers takes value of 1 and 0, if it is 1 it prints the numbers or else it doesn’t.

if (argc > 1)
{
   if (strcmp(argv[1], "-n") == 0)
   {
      printFile(argv, argc, 2, TRUE);
   }
   else
   {
      printFile(argv, argc, 1, FALSE);
   }
}

Here checking if first argument is “-n”, if it is then calling printFile() with argv, argc, index = 2, and numbers = TRUE or else it is calling printFile() with argv, argc, index = 1, and numbers = FALSE.

#include <stdio.h>
#include <string.h>

#define MAX 1000
#define TRUE 1
#define FALSE 0

void printFile(char *argv[], int argc, int index, int numbers)
{
  char character;
  int count = 0;

  for (int i = index; i < argc; i++)
  {
    FILE *file = fopen(argv[i], "r");

    if (numbers)
      printf("\t%d ", ++count);
    while ((character = fgetc(file)) != EOF)
    {
      printf("%c", character);
      if (numbers && character == '\n')
        printf("\t%d ", ++count);
    }

    fclose(file);
  }
}

int main(int argc, char *argv[])
{
  char input[MAX];

  if (argc > 1)
  {
    if (strcmp(argv[1], "-n") == 0)
    {
      printFile(argv, argc, 2, TRUE);
    }
    else
    {
      printFile(argv, argc, 1, FALSE);
    }
  }
  else
  {
    while (1)
    {
      scanf("%s", input);
      printf("%s\n", input);
    }
  }

  return 0;
}

Checkout the repository at GitHub.

Building Linux Commands From Scratch 🐧 cat Command

Suraj Kareppagol — Thu, 14 Mar 2024 14:25:00 +0000

I am “Building Linux Commands From Scratch With C/C++”. The main reason behind this is to get some experience around C/C++, also i am a fan of Linux and it’s ecosystem.

Command ➡️ cat

cat commands allows us to concatenate files and print on the standard output.

cat README.md

This command prints the content of README.md on the standard output.

#include <stdio.h>

int main(int argc, char *argv[])
{
  char character;

  if (argc > 1)
  {
    for (int i = 1; i < argc; i++)
    {
      FILE *file = fopen(argv[i], "r");

      while ((character = fgetc(file)) != EOF)
        printf("%c", character);

      fclose(file);
    }
  }

  return 0;
}

int main(int argc, char *argv[])

Here int argc is the number of command line arguments. char *argv[] is the array holding the command line arguments.

Since the file name of the program is the first argument passed it starts reading files from index 1, a for loop is used from 1 to argc.

Then a FILE pointer is created and file is opened in read mode. Then a while loop is used to read each single character from the file.

while ((character = fgetc(file)) != EOF)

The loop continues until EOF (End Of File) is not encountered. Once EOF is encountered loop stops and the file is closed. This will be repeated for other files.

gcc main.c -o main

After compiling the program it can be executed as,

./main README.md main.c

It will print the content of both README.md and main.c to the standard output (i.e Terminal Screen).