Greetings. This is my first post in this forum; I hope y'all find it useful. One caveat: "Concise" is my middle name. NOT!
I am almost done with a shell script that runs as a daemon. It monitors a message log that is frequently written to by a database server but it it works my client will have more such daemons.
After a few rough starts and things that just don't work, I am using dtksh (M-12/28/93d; the alternative on my client's old Solaris box was ksh 88). Here's how the loop works:
Now how do I send this daemon a message to clean up after itself and exit quietly? Well, before I started that loop, I set up a simple, one-signal trap command:
(I'm not even certain that break will do what I think.) All so very logical! So what's the problem?
Well, it is overwhelmingly likely that when I send it the "kill -2", my daemon will be sleeping on that "read newline" command. My output log shows that it did catch the signal but it goes right back to sleep on that read!
A Google search [ksh signal break read] showed me I am not alone; the first hit was a 5-year-old posting on this very forum. I was quite unhappy with the conclusion of that thread but even if ksh83v (the latest I've heard of) has fixed the issues discussed in that thread, I have no access to that. But I love to tinker so I added this to the trap handler:
What did I do up there?
I fed an extra empty line to the end of the file being tailed. (Remember, I'm not reading directly from the file but from a pipeline.) Hence, that read is immediately satisfied and the process awakens to see an empty line, continues back at the top of the loop, where it sees that $go_flag is $FALSE and breaks out of the loop.
This kluge worked for me, although it's a fair bet it won't work for everyone. And, of course, for all I know the SA_RESETHAND issue discussed in that post has been fixed.
I don't think the "resethand" issue is related. Your shell has no issue receiving the interrupt, and does exactly what you told it to do -- wait for input. What needs to be told that something is happening is read. How can you do that? Close whatever it's reading from. Killing tail would do it, for example. Or using a named pipe and forcing it to close.
Corona,
I like it better than my own solution because it would work where I cannot mess with the input file. I did call my solution a kluge, indicating some dissatisfaction with it.
I may use your technique in the future or I may even start tinkering with it now, although I have started to implement the script across my servers. Pragmatism says let it run as it, for now.
-- JS
This User Gave Thanks to jakesalomon For This Post:
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