How to supply a command to 'stdin' of a running process from a second shell?

If you have a process that is waiting for user-input from the stdin scope, then how can you supply that user-input from a second terminal ? Specifically, if I run the c-program while(1){ fgets(string, len, stdin); string[strlen(string)-1] = 0; if(strcmp("Stop", string) == 0){ printf("Gotcha"); return 1; } } then how can I supply the string "Stop" to stdin of that process from another process, such that the first process will stop (and print "Gotcha") ? I've tried to run the c-program in terminal 'pts/0' and open a new terminal ('pts/1') with commands: $ echo "Stop" > /proc//fd/0 $ echo "Stop" > /dev/pts/0 where pid is the process id. The "Stop"-command is "repeated" in the first shell, but the process does not receive it.