Execute a command in another terminal via /dev/pts

I completely get what you are asking. You can achieve this by writing and executing a small piece of code in C yourself. This should give you some idea.

#include <stdio.h>#include <stdlib.h>#include <fcntl.h>#include <sys/stat.h>#include <sys/ioctl.h>#include <string.h>#include <unistd.h>void print_help(char *prog_name) {        printf("Usage: %s [-n] DEVNAME COMMAND\n", prog_name);        printf("Usage: '-n' is an optional argument if you want to push a new line at the end of the text\n");        printf("Usage: Will require 'sudo' to run if the executable is not setuid root\n");        exit(1);}int main (int argc, char *argv[]) {    char *cmd, *nl = "\n";    int i, fd;    int devno, commandno, newline;    int mem_len;    devno = 1; commandno = 2; newline = 0;    if (argc < 3) {        print_help(argv[0]);    }    if (argc > 3 && argv[1][0] == '-' && argv[1][1] == 'n') {        devno = 2; commandno = 3; newline=1;    } else if (argc > 3 && argv[1][0] == '-' && argv[1][1] != 'n') {        printf("Invalid Option\n");        print_help(argv[0]);    }    fd = open(argv[devno],O_RDWR);    if(fd == -1) {        perror("open DEVICE");        exit(1);    }    mem_len = 0;    for (i = commandno; i < argc; i++) {        mem_len += strlen(argv[i]) + 2;        if (i > commandno) {            cmd = (char *)realloc((void *)cmd, mem_len);        } else { // i == commandno            cmd = (char *)malloc(mem_len);        }        strcat(cmd, argv[i]);        strcat(cmd, " ");    }  if (newline == 0)        usleep(225000);    for (i = 0; cmd[i]; i++)        ioctl (fd, TIOCSTI, cmd+i);    if (newline == 1)        ioctl (fd, TIOCSTI, nl);    close(fd);    free((void *)cmd);    exit (0);}

Compile and execute it with sudo permissions. For example, if you want to execute a command on /dev/pts/3, then simply do a sudo ./a.out -n /dev/pts/3 whoami, runs a whoami on /dev/pts/3.

This code was completely taken from this page.

You seem to use the wrong quotes around the command.Either remove the quotes and the echo command, or use echo and back-ticks (`).

Try:

echo `date` > /dev/pts/3

or just

date > /dev/pts/3

Note that whatever runs on /dev/pts/3 wouldn't be able to read what pops up "from behind".