Bash script with non-blocking read

Question

I want to send some data to a root process with a named pipe. Here is the script and it works great:

#!/bin/sh
pipe=/tmp/ntp

if [[ ! -p $pipe ]]; then
    mknod -m 666 $pipe p
fi

while true
do
    if read line <$pipe; then
         /root/netman/extra/bin/ntpclient -s -h $line > $pipe 2>&1
    fi
done

I actually have several script like this one. I would like to enclose all of them in a single script. The problem is that execution blocks on the first "read" and I cannot execute multiple "reads" in a single process. Isn't there anything I can do? Is it possible to have a "non-blocking" bash read?

Answer 1

Just put the reading cycle into background (add & after done)?

Answer 2

Bash's read embedded command has a -t parameter to set a timeout:

-t timeout
    Cause read to time out and return failure if a complete line of input is not
    read within timeout seconds. This option has no effect if read is not reading
    input from the terminal or a pipe.

This should help you solve this issue.

Edit:

There are some restrictions for this solution to work as the man page indicates: This option has no effect if read is not reading input from the terminal or a pipe.

So if I create a pipe in /tmp:

mknod /tmp/pipe p

Reading directly from the pipe is not working:

$ read -t 1 </tmp/pipe  ; echo $?

Hangs forever.

$ cat /tmp/pipe | ( read -t 1 ; echo $? )
1

It is working but cat is not exiting.

A solution is to assign the pipe to a file descriptor:

$ exec 7<>/tmp/pipe

And then read from this file descriptor either using redirection:

$ read -t 1 <&7  ; echo $?
1

Or the -u option of read:

$ read -t 1 -u 7  ; echo $?
1

Answer 3

You can use stty to set a timeout. IIRC its something like

stty -F $pipe -icanon time 0