如何使两个程序使用std io通信？

Question

I have two different programs A and B. Both the programs take user input from stdin and output to stdout. How can I have the programs talk to each other? ie i want program A to use B as it's stdout and stdin and program B to take A's output as stdin and give it's own output to A.

I can't seem to figure out how i can do it using Linux pipelines.

I'm using bash. I want a solution which doesn't involve creating another program or modifying any of the existing ones.

Answer 1

socat lets you relay I/O from anything to anything else and back, including between two programs:

$ cat a
#!/bin/bash -e
var="World"
echo "$var"
echo "a wrote '$var'" >&2
read -r var
echo "a read '$var'" >&2

$ cat b
#!/bin/bash -e
read -r var
echo "b read '$var'" >&2
echo "Hello $var"
echo "b replied" >&2

$ chmod +x a b

$ socat exec:./a exec:./b
b read 'World'
b replied
a wrote 'World'
a read 'Hello World'

Note that it is your responsibility to avoid deadlocks by ensuring that both programs flush their buffers appropriately.

If you eg replace exec:./b with exec:rev , you will cause a deadlock because most standard tools will buffer output to pipes by default.

Answer 2

i want program A to use B as it's stdout and stdin and program B to take A's output as stdin and give it's own output to A.

Create two fifos and run A | B A | B in background, but connect input to A from a fifo and output from B to a fifo. Then connect fifos together with cat:

fifoAin=$(mktemp -u)
fifoBout=$(mktemp -u)
mkfifo "$fifoAin" "$fifoBout"
A < "$fifoAin" | B > "$fifoBout" &
cat < "$fifoBout" > "$fifoAin"

The following, although looks cool, will not(!) work, because cat will block on file descriptor:

exec 3<> <(A | B)
cat <&3 >&3

Example with comments:

# output 1
# then read number and if a number is greater then 20
# output the number on stderr and exit
# otherwise add to number 3 and output
A() {
  echo 1
  while read n; do
    if ((n > 20)); then
      echo "Answer: $n" >&2
      break;
    fi
    echo "$((n + 3))"
  done
}


# read a number and output the number multiplied by 2
B() {
  while read n; do
    echo "$((n * 2))"
  done
}

# the code
fifoAin=$(mktemp -u)
fifoBout=$(mktemp -u)
mkfifo "$fifoAin" "$fifoBout"
A < "$fifoAin" | B > "$fifoBout" &
cat < "$fifoBout" > "$fifoAin"

Will output on stderr:

Answer: 26

Answer 3

It may be worth thinking about what you need to achieve and the best way to do that.

You have a problem with the current software. The solution (pseudo code) (A || B) && (B || A) is difficult to achieve properly, even with bash . There may well be easier, cleaner solutions. Such as removing dependencies between A and B, or modifying the software so that A depends on B, or vice versa.

For example if B depended solely on the output of A, all that is needed is connect A and B with a pipe, and the data streams from A to B:

A || B