FAQ
overflow

QUESTION

I have a server log that outputs a specific line of text into its log file when the server is up. I want to execute a command once the server is up, and hence do something like the following:

tail -f /path/to/serverLog | grep "server is up" ...(now, e.g., wget on server)?

What is the best way to do this?

ANSWER

A simple way would be awk.

tail -f /path/to/serverLog | awk '/Printer is on fire!/ { system("shutdown -h now") }
                                  /new USB high speed/ { system("echo New USB" | mail admin")'

And yes, both of those are real message from a kernel log. Perl might be a little more elegant to use for this and can also replace the need for tail. If using perl, it will look something like this:

open(my $fd, "<", "/path/to/serverLog") or die "Can't open log";
while(1) {
    if(eof $dataFd) {
        sleep 1;
        $fd->clearerr;
        next;
    }
    my $line = <$fd>;
    chomp($line);
    if($line =~ /Printer is on fire!/) {
        system("shutdown -h now");
    } elsif($line =~ /new USB high speed/) {
        system("echo New USB" | mail admin");
    }
}

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