How to fire and forget a subprocess? - ruby

I have a long running process and I need it to launch another process (that will run for a good while too). I need to only start it, and then completely forget about it.
I managed to do what I needed by scooping some code from the Programming Ruby book, but I'd like to find the best/right way, and understand what is going on. Here's what I got initially:
exec("whatever --take-very-long") if fork.nil?
Process.detach($$)
So, is this the way, or how else should I do it?
After checking the answers below I ended up with this code, which seems to make more sense:
(pid = fork) ? Process.detach(pid) : exec("foo")
I'd appreciate some explanation on how fork works. [got that already]
Was detaching $$ right? I don't know why this works, and I'd really love to have a better grasp of the situation.

Alnitak is right. Here's a more explicit way to write it, without $$
pid = Process.fork
if pid.nil? then
# In child
exec "whatever --take-very-long"
else
# In parent
Process.detach(pid)
end
The purpose of detach is just to say, "I don't care when the child terminates" to avoid zombie processes.

The fork function separates your process in two.
Both processes then receive the result of the function. The child receives a value of zero/nil (and hence knows that it's the child) and the parent receives the PID of the child.
Hence:
exec("something") if fork.nil?
will make the child process start "something", and the parent process will carry on with where it was.
Note that exec() replaces the current process with "something", so the child process will never execute any subsequent Ruby code.
The call to Process.detach() looks like it might be incorrect. I would have expected it to have the child's PID in it, but if I read your code right it's actually detaching the parent process.

Detaching $$ wasn't right. From p. 348 of the Pickaxe (2nd Ed):
$$ Fixnum The process number of the program being executed. [r/o]
This section, "Variables and Constants" in the "Ruby Language" chapter, is very handy for decoding various ruby short $ constants - however the online edition (the first
So what you were actually doing was detaching the program from itself, not from its child.
Like others have said, the proper way to detach from the child is to use the child's pid returned from fork().

The other answers are good if you're sure you want to detach the child process. However, if you either don't mind, or would prefer to keep the child process attached (e.g. you are launching sub-servers/services for a web app), then you can take advantage of the following shorthand
fork do
exec('whatever --option-flag')
end
Providing a block tells fork to execute that block (and only that block) in the child process, while continuing on in the parent.

i found the answers above broke my terminal and messed up the output. this is the solution i found.
system("nohup ./test.sh &")
just in case anyone has the same issue, my goal was to log into a ssh server and then keep that process running indefinitely. so test.sh is this
#!/usr/bin/expect -f
spawn ssh host -l admin -i cloudkey
expect "pass"
send "superpass\r"
sleep 1000000

Related

Is it ok to use check PID for rare exceptions?

I read this interesting question, that basically says that I should always avoid reaching PID of processes that aren't child processes. It's well explained and makes perfect sense.
BUT, while OP was trying to do something that cron isn't meant to, I'm in a very different situation :
I want to run a process say every 5 minutes, but once in a hundred times it takes a little more than 5 minutes to run (and I can't have two instances running at once).
I don't want to kill or manipulate other processes, I just want to end my process without doing anything if another instance of the process is running.
Is it ok to fetch PID of "not-child processes" in that case ? If so, how would I do it ?
I've tried doing if pgrep "myscript"; then ... or stuff like that, but the process finds its own PID. I need to detect if it finds more than one.
(Initially before being redirected I read this question, but the solution given doesn't work: it can give pid of the process using it)
EDIT: I should have mentioned it before, but if the script is already in use I still need to write something in a log file, at least : date>>script.log; echo "Script already in use">>script.log", I may be wrong but I think flock doesn't allow to do that.
Use lckdo or flock to avoid duplicated running.
DESCRIPTION
lckdo runs a program with a lock held, in order to prevent multiple
processes from running in parallel. Use just like nice or nohup.
Now that util-linux contains a similar command named flock, lckdo is
deprecated, and will be removed from some future version of moreutils.
Of course you can implement this primitive lockfile feature by yourself.
if [ ! -f /tmp/my.lock ];then
touch /tmp/my.lock
run prog
rm -f /tmp/my.lock
fi

Provide full access over terminal for child, while it alive

I use fork() in order to exec some another program, for example "vim" or "man bash". But, at that moment I lost control, I can't print anything in vim or scroll or quite.
So, my question: how give child full access over terminal(input/output) and get it back for parent after child death?
So, my question: how give child full access over terminal(input/output) and get it back for parent after child death?
waitpid() and SIGCHLD are mechanisms for knowing when a child process has finished. The functions tcgetpgrp(fd) and tcsetpgrp(fd,pgrp) and ctermid() can be used to place a program in the foreground of a terminal.
Otherwise, the information in tcsetpgrp() in 'C' is probably instructive.

How to execute and manage ruby script from ruby?

I have a script named program.rb and would like to write a script named main.rb that would do the following:
system("ruby", "program.rb")
constantly check if program.rb is running until it is done
if program.rb has reached completion
exit main.rb
end
otherwise keep doing this until program.rb reaches completion{
if program.rb is not running and stopped before completing
restart program.rb from where it left off
end}
I've looked into Pidify but could not find a way to apply it to fit this exactly the right way...
Any help in how to approach this script would be greatly appreciated!
Update:
I could figure out how to resume running the script from where it left off in program.rb if there's no way to do it in main.rb
It's impossible to "restart script from where it left off" without full cooperation from the program.rb. That is, it should be able to advertise its progress (by writing current state to a file, maybe?) and be able to start correctly from a step specified in ARGV. There's no external ruby magic that can replace this functionality.
Also, if a program terminated abnormally, it means one of two things:
the error is (semi-)permanent (disk is full, no appropriate access rights to a file, etc). In this case, simply restarting the program would cause it to fail again. And again. Infinite fail loop.
the error is temporary (shaky internet connection). In this case, program should do better job with exception handling and retry on its own (instead of terminating).
In either case, there's no need for restarting, IMHO.
Well, here is one way.
Modify program.rb to take an optional flag argument --restart or something.
When program.rb starts up without this argument it will initialize a file to record its current state. Periodically, it will write whatever it needs into this file to record some kind of checkpoint.
When program.rb starts up with the restart flag, it will read its checkpoint file and start processing at that point. For this to work, it must either checkpoint all state changes or arrange for all processing between checkpoints to be idempotent so it can be repeated without ill effect.
There are lots of ways to monitor the health of program.rb. The best way is with some sort of ping, perhaps something like GET /health_check or a dummy message via a socket or pipe. You could just have a locked file to detect if the lock is still held, or you could record the PID on startup and check that it still exists.

Why might ruby fail to recognize that a child process (opened using popen) has terminated?

The child process is launched using the following code:
IO.popen("/path/to/process/in/question") do |command|
command.each do |line|
puts line
end
end
puts "Child Process Complete"
The individual lines output by the child process are correctly shown on the console up to and including one immediately before the process exits. However, the message "Child Process Complete" is not shown until I hit ctrl-c.
A similar process, triggered using the same mechanism, is correctly recognized as having terminated, so the problem is probably a result of something that the child process is doing. Unfortunately I have no idea what that something may be.
[Edit] I should probably also mention that the child process can also be triggered directly from the command prompt, and that no issues are observed when doing so.
Benoit Garret put me onto the right track when he suggested that something was waiting for user input. In attempting to sanitize the child process script (and the things it called in turn) I stumbled upon what appeared to be an error in a script lower down (which has been sitting there unnoticed for several years).
The Ruby code, posted above made a call to an initial shell script. That in turn was calling a second shell script which contained the following abomination:
(some_application) &
>/dev/null 2>&1
I'm not sure if this is what happened, but the way I read this is that some_application gets started and run as a background process. Standard output gets redirected to /dev/null and standard error gets redirected to standard output... but which process does this redirection relate to?
Changing the call to
some_application >/dev/null 2>&1 &
(I.e. run some_application throwing away standard out and capturing standard error to the console) made all my problems go away.

How are PIDs generated on Ubuntu?

I've just wrote a program that forks one process. The child process just displays "HI" 200 times. The father process just says he's the father.
I've printed out both pids.
When I run my program multiple times, I see that the parent's pid stays the same, which is normal. What I don't understand is why the child's pid keeps getting incremented by 2, and exactly 2.
My question: Is this the standard method of pid generation in Ubuntu? Incrementing by 2?
PIDs happen to be handed out monotonically increasing in Linux 2.6, but why does it matter which you get? Don't rely on any specific behavior. If there is a skip of +2 it might simply be because another process happened to spawn a child. Or because +1 would have reached a PID that is already in use.
Found a reference here saying that vfork() consumes a pid as a byproduct of its operation. As well, in some cases, if you're forking from a shell script, the fork might spawn a new shell before your actual script gets involved, which would also consume a pid.
I'd suggest suspending your program between a couple forks, and see if there's another process occupying those "missing" pids.

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