Read named pipe once every time input is written

Question

I need to write a bash program that runs commands echoed to a named pipe it reads, but I cannot get it work only when a command is sent. It keeps repeating the last command until a new one is written.

That is:

Execute ./read_pipe.sh
It waits until a command is echoed to pipe and reads it.
It executes the command once. <- What doesn't work. It keeps executing it forever.
Repeat from step 2.

My read_pipe.sh

#!/bin/bash
pipe="mypipe"
if [ ! -p $pipe ]; then
  echo 'Creating pipe'
  mkfifo $pipe
fi

while true
do
  if read line <$pipe; then
    COMMAND=$(cat $pipe)
    echo "Running $COMMAND ..."
    # sh -c $COMMAND
  fi
done

If I cat "echo 'Hello World'" > mypipe the output is this forever:

Running "echo 'Hello World'" ...
Running "echo 'Hello World'" ...
Running "echo 'Hello World'" ...
Running "echo 'Hello World'" ...
...

How can I run the command once and wait for another echoed command?

score 8 · Answer 1 · answered Jun 06 '17 at 09:38

8

In your case, you can do simply:

tail -f $pipe | sh &

no need for loops.

answered Jun 06 '17 at 09:38

Gnudiff

975

Satō Katsura · Accepted Answer · 2017-06-06T09:10:20.560

7

One way to do it:

#! /usr/bin/env bash
pipe=/path/to/pipe
[ -p "$pipe" ] || mkfifo -m 0600 "$pipe" || exit 1
while :; do
    while read -r cmd; do
        if [ "$cmd" ]; then
            printf 'Running %s ...\n' "$cmd"
            # sh -c "$cmd" sh
        fi
    done <"$pipe"
done

It's probably a really bad idea to uncomment the sh line though.

edited Jun 06 '17 at 09:10

answered Jun 06 '17 at 08:00

Satō Katsura

13,368
2
31
50

I tried your solution, but It doesn't work properly. It executes the command once, but then your program exits. The intended way is keep executing the script, and when a new command is echoed to pipe, execute it. – Pedro Adame Vergara Jun 06 '17 at 08:05
Ok, I added a loop around it. – Satō Katsura Jun 06 '17 at 08:10
That's exactly what I've done. It executes the command sent to pipe forever until a new one is sent. I want it to be executed only once, and then wait for another command. – Pedro Adame Vergara Jun 06 '17 at 08:11
Which is what happens after my latest edit. – Satō Katsura Jun 06 '17 at 08:13
Just tested it: It executes the same command forever. http://imgur.com/a/cmrLS I need to execute it once, and only once each time a command is written to the pipe. – Pedro Adame Vergara Jun 06 '17 at 08:19
1

This is not what I'm seeing here. shrug – Satō Katsura Jun 06 '17 at 08:29
1

Sorry @SatoKatsura I've just found the solution: rm -f pipe && mkfifo pipe. Now it works correctly. For some reason it was a simple text file. -.-' – Pedro Adame Vergara Jun 06 '17 at 08:58
Why is it a bad idea to uncomment the sh line? – rottweiler Feb 27 '20 at 08:14

Read named pipe once every time input is written

2 Answers2