🐧 Implementation of clear command

Clear command is one of the most used command. The description of Clear command from the man page,

clear clears your terminal's screen and its scrollback buffer, if any. clear
retrieves the terminal type from the environment variable TERM, then consults
the terminfo terminal capability database entry for that type to determine how
to perform these actions.

This command can be implemented in two ways!

Using newlines escape sequence!

This method uses newline escape sequence to clear the screen. This behavior is similar to CTRL + L keybinding.

But how many newline characters? The answer depends on the terminal window size. If the total rows are 20, then 20 + 20 newline characters. The terminal size can be obtained easily using libraries such as ncurses. But here let’s use winsize structure from unistd.h as explained in this stackoverflow page.

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

int main (int argc, char **argv)
{
    struct winsize w;
    ioctl(STDOUT_FILENO, TIOCGWINSZ, &w);

    for (int i = 0; i < w.ws_row + 10; i++) {
        printf("\n");
    }

    return 0;
}

Here winsize has two members ws_row and ws_col. After the ioctl call this structure is populated with row and column values.

Once the number of rows are available just loop over and print newline characters.

Using ANSI escape sequence

Yes, \n was a escape sequence but there are other escape sequences which are available for the terminal manipulation. One such escape sequence that could be used to clear the screen is ESC[2J. Here ESC is the escape sequence code which could be \033 or \0x1B`.

#include <stdio.h>

int main(int argc, char **argv) {
    printf("\033[2J");
    return 0;
}

Although this works in Gnome Terminal it might not work in other terminals. Also the cursor doesn’t move back to home location that is 0, 0 to achieve this one more escape sequence could be used that is \033[H.

Conclusion

Although these two approaches work but these are not portable to all the terminals. But this gives an idea about the implementation of the clear command.