using tail to follow a log and execute a command instantly? Only seems to work by starting a new line - bash

Question

I am trying to figure out a command which will enable me to read a log file in real time and execute a command when the string matches? I am using logkeys and trying to make it when I type a word it immediately triggers a command. This script works, but only when I press enter (start a newline) does it execute, and it seems anything I have found online also requires the press of the enter key to work. Is there a way to get around this somehow?
#/bin/bash
echo Waiting...
string='test'
tail /path/to/logfile -n0 -f | while read line; do
if [[ $line =~ $string ]]; then
echo "hello"
fi
done

Answer 1

I've played with buffering settings to no avail, so my conclusion is that read waits for a newline before it finishes. If you instead did read -n1, read would read exactly one character, which isn't quite what we want either, because then $line would always be just that one char.
Unfortunately, grep appears to have the same behavior (even with buffering options changed), even with grep -o:
$ tail logfile -f -n0 | grep -o test &
[1] 25524
$ echo -n test >> logfile
$ echo -n test >> logfile
$ echo test >> logfile
test
test
test
I think the general solution would be to roll our own "ring buffer grep" search tool that reads character per character into a ring buffer.
Here's my perl version of that, hope it helps. (Save as: ringgrep.pl)
#!/usr/bin/perl -w
use strict;
if (!$ARGV[0]) {
print "Usage: $0 needle\n";
exit 1;
}
my $needle = $ARGV[0];
my $buffer_len = length($needle);
my #buffer = (0) x $buffer_len;
my $i = 0;
my $input;
while(sysread(STDIN, $input, 1)) {
$buffer[$i] = $input;
my $string = join("", #buffer);
$string = (($i+1)%$buffer_len == 0 ? "" : substr($string, $i-$buffer_len+1)) . substr($string, 0, $i+1);
# print "string is: $string\n";
if ($string =~ /$needle/) {
print "got test!\n";
#buffer = (0) x $buffer_len;
$i = 0
} else {
$i = (++$i) % $buffer_len
}
}
Usage:
$ chmod +x ringgrep.pl
$ tail logfile -n0 -f | ./ringgrep.pl "this is a test" &
[1] 25756
$ echo -n "test" >> logfile
$ echo -n "test" >> logfile
$ echo -n "test" >> logfile
$ echo -n "test" >> logfile
$ echo -n "this is a test" >> logfile
got test!
$ echo -n "this is a test" >> logfile
got test!
$ echo -n "this is a test" >> logfile
got test!
$ (echo -n t; echo -n h; echo -n i; echo -n s; echo -n ' '; echo -n i; echo -n s; echo -n ' '; echo -n a; echo -n ' '; echo -n t; echo -n e; echo -n s; echo -n t) >> logfile
got test!