How to pass data between shells without reading/writing to intermediate files

Question

Let's say I have a process that is echoing the user input, running in a shell. Then how can you pass data to/from that shell from another terminal? Is it possible without using an intermediate file?

E.g., if I have the code

fgets(string, LEN, stdin);
printf("%s", string);

Then, is it possible to pass data to stdin, and to get data from stdout via a different shell ?

Answer 1

I'm not exactly sure if I got what you mean, but.

Within a single shell session (terminal), you'd use a pipeline to pass data from one command to another, like so:

$ ls -l | grep something

If you need to do that between two different shells, you can use a named pipe:

tty1$ mkfifo /tmp/mypipe
tty1$ ls -l > /tmp/mypipe
tty2$ grep something < /tmp/mypipe

It would be safer to use mktemp to create a directory to place the named pipe in:

tty1$ dir=$(mktemp -d)
tty1$ mkfifo "$dir/mypipe"
tty1$ ls -l > "$dir/mypipe"
tty1$ rm -r "$dir"

though that requires copying the path to the other window, possibly by hand.

Of course, a named pipe acts a bit like an intermediate file, in that it needs a path name. But it acts more like a pipe in that the data doesn't get written to permanent storage, and the reader waits for the writer if the writer is slow, instead of possibly encountering a premature end-of-file.

_{(You'd usually use ls -l *something* instead of ls | grep, but it works as an example.)}

Answer 2

I think @ilkkachu's answer is helpful, and provides what you need. I will try to explain with some details, and at the same time I learn how to use a fifo.

• Prepare two command line windows, w1 and w2 in the same computer

• Create the listening program, I made a shellscript in w1

  #!/bin/bash
  
  while true
  do
   read string
   if [ "${string:0:4}" == "Stop" ]
   then
    printf "Gotcha\n"
    break
   elif [ "$string" != "" ]
   then
    printf "$string "
   else
    sleep 3
   fi
  done
  

• Prepare the fifo in w1

  dir=$(mktemp -d)
  mkfifo "$dir/mypipe"
  

• Start the program and let it wait for input from to the fifo in w1

  < "$dir/mypipe" ./program
  

• Look for the fifo and echo some strings to it in w2

  $ find /tmp -name mypipe 2>/dev/null
  /tmp/tmp.dRhpqajJqz/mypipe
  
  $ > '/tmp/tmp.dRhpqajJqz/mypipe' echo qwerty
  $ > '/tmp/tmp.dRhpqajJqz/mypipe' echo asdf
  $ > '/tmp/tmp.dRhpqajJqz/mypipe' echo Stopp
  

• Look at the output in w1

  qwerty asdf Gotcha
  $ 
  

You can also make this more automatic, for example like the following, which assumes that there is only one temporary file with the name mypipe,

• start the program again in w1

  < "$dir/mypipe" ./program
  

• in w2

  > $(find /tmp -name mypipe 2>/dev/null) echo 'Hello World'
  > $(find /tmp -name mypipe 2>/dev/null) echo 'Stop the World'
  

• Look at the output in w1

  Hello World Gotcha
  $
  

---

Demo C program,

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

int main () {

 char string[21];
 while(1){
    fgets(string, 20, stdin);
    string[strlen(string)-1] = 0;
    if(strcmp("Stop", string) == 0){
        printf("Gotcha");
        return 1;
    }
 }
}

This C program writes only, when it has read 'Stop'.