make clean · unix from scratch

Rebuilding Unix Tools from Scratch in C

Every episode takes a standard Unix tool: cat, head, tail, grep, ls, and rebuilds it from a blank file. The only tools allowed are the C language and POSIX system calls.

No stdio.h. No printf. No frameworks. Just read(), write(), open(), lseek(), and the operating system. Every decision gets explained. Nothing is glossed over.

// the work

episode 01: cat.c

/*
** cat.c: concatenate and print files
** episode 01: file I/O, read(), write(), POSIX loops
*/

#include <unistd.h>
#include <fcntl.h>

#define BUFFER_SIZE  4096

int     main(int argc, char **argv)
{
    char    buf[BUFFER_SIZE];
    int     fd;
    ssize_t n;
    int     i;

    i = 1;
    if (argc < 2)
    {
        while ((n = read(0, buf, BUFFER_SIZE)) > 0)
            write(1, buf, n);
        return (0);
    }
    while (i < argc)
    {
        fd = open(argv[i], O_RDONLY);
        while ((n = read(fd, buf, BUFFER_SIZE)) > 0)
            write(1, buf, n);
        close(fd);
        i++;
    }
    return (0);
}

// published

01

cat(1) file I/O · read() · write() · POSIX loop

done 02

head(1) read loops · line counting · -n flag

done

// also published

03

tail(1) circular buffer · lseek() · reading from the end

done ★A

lines arbitrary line access · range syntax · early exit

done