Bash script sometimes works, sometimes doesn't... pipes issue

Question

I have a weird situation where Bash doesn't seem to be assigning one of my variables correctly, or it might be to do with pipes. I'm honestly not sure. Here's the code:

test=0

if [ "$#" -ne 4 ]; then
    echo "Error: You have passed an incorrect number of parameters" > client.pipe
    test=1 

if [ $test -eq 1 ]; then <-------THIS SOMETIMES DOESN'T EXECUTE
    echo "end_result" > client.pipe
    exit 1

What's going on here is this is a server script for a command: 'select'.

The select command takes in 4 parameters: an id (to tell it which pipe to output messages to), a database name, a table name, and then a string of column ids.

At the other end of the client pipe, the client script is listening for messages, and if it receives 'end_result' it stops listening. As you can see, no matter what happens 'end_result' should get sent back to the pipe and printed by the client script, but sometimes it doesn't happen, and I get stuck in an infinite while loop. Here's the client script so you can see where the 'listening' is happening.

     while true; do
        read message < client.pipe
           if [ "$message" == "end_result" ]; then
              echo $message 
              break
           else
              echo $message
           fi
       done

If I pass in the incorrect number of parameters to the first script, it prints out 'Error: you have passed an incorrect num of parameters' to the pipe, but then sometimes it doesn't assign 1 to test, and it doesn't then send 'end_result' to the pipe and exit. I can't really see what the issue is, and as I've said it works probably 7 times out of 10... I can get around the issue by having the parent script send 'end_result' to the client, but it's a bit of a hack.

I'd really appreciate it if anyone could see what the issue is, and I'm happy to provide more info about the code if required.

Many thanks, R

EDIT:

I'm almost certain the problem is to do with reading from the client pipe and that while loop, as though something were getting stuck in the pipe...

Is there any chance you can reduce this to a (much smaller) [mcve]? This is very much code to go through. — Benjamin W., Dec 03 '18 at 19:41
Also, without looking at it more closely, but seeing "variable assigning" and "pipe", might it be anything to do with https://mywiki.wooledge.org/BashFAQ/024? — Benjamin W., Dec 03 '18 at 19:43
Hi Ben, I've done some editing. I think what's going on is that the client is looping and listening but sometimes the loop and what's being passed into the pipe somehow miss one another? — Raph117, Dec 03 '18 at 19:47
You probably do not want to be re-opening the pipe on each iteration of the while loop. Replace `while true; do read message < client.pipe....` with `while true; do read message; ... done < client.pipe` — William Pursell, Dec 03 '18 at 19:49
@WilliamPursell that worked! I had to do one more bit of error checking to make sure nothing outputted if there was nothing in the pipe but otherwise great! How do I mark this as answered? — Raph117, Dec 03 '18 at 19:57

score 0 · Answer 1 · answered Dec 03 '18 at 19:58

0

This was the solution, provided by @WilliamPursell:

 while true; do
        read message 
           if [ "$message" == "end_result" ]; then
              echo $message 
              break
           else
              echo $message
           fi
       done < client.pipe

answered Dec 03 '18 at 19:58

Raph117

3,441
7
29
50

The idiomatic way to read lines from a file is described in [this Q&A](https://stackoverflow.com/questions/10929453) and in [BashFAQ/001](https://mywiki.wooledge.org/BashFAQ/001): `while IFS= read -r line; do ... done < client.pipe` – Benjamin W. Dec 03 '18 at 21:37

Bash script sometimes works, sometimes doesn't... pipes issue

1 Answers1