I feel somewhat dumb asking this, but I'm relatively new to linux (more in terms of experience than time), and one thing that i've always wondered is if I can 'rejoin' (my own term) a process while it's running.
For example, if I set a game server or eggdrop IRC bot to run in the background, is there a command I can use to view that process in action and view all the output it delivers to the console?
I'm not talking about just viewing the process using the 'top' command, but actually linking to it as if I just ran it from the command line.
Thanks.
Debuggers can "attach" to running processes, but you might be better running your program in screen (which lets you detach and reattach to terminal in a fairly natural way).
There might be some good stuff good stuff in :
Redirect STDERR / STDOUT of a process AFTER it’s been started, using command line?
Can you be more specific? Are you just talking about backgrounding a process in the current session, then putting it back in the foreground.
E.g.:
doLongTask &
# Later
fg %3
3 in this example is the job number of this instance of doLongTask. You can see all running jobs with:
jobs
But note this will still only let you see what's being outputted to the console. I.E. stdout and stderr, minus any redirections.
The simple answer is:
>> ./runmyserver
<press ctrl-z>
>> bg
>> ...do something else ...
>> fg
You can also start in the background with:
>> ./runmyserver &
For more complicated stuff like disconnecting the server from your console session (so it's still running when you log out) you really want screen. Maybe beg them for it, it isn't really a security risk and it's a useful program to have around.
Also note that ctrl-z will actually pause your server until bg so if people are playing on it might skip a beat, best to do it quickly.
Finally, many game servers have a remote login for this kind of thing which would solve many of these issues. Make sure your game and host don't support this before looking for alternatives.
EDIT: Re-read your question. It sounds like you could at least get the output using redirect to a file. This won't let you add more input though:
./runmyserver > log.txt
If you know ahead of time that you want to do this, use screen(1) and run your server in the foreground in a screen session. You will be able to detach from your screen session and have the process keep running. You can then later re-attach your screen session and view any output it has made since, up to the size of the scrollback buffer.
Related
What I'm trying to accomplish is to have a process running in background from a Linux terminal which takes user input and does things according to that input even if the terminal window is not focused, so I can work with other GUI applications, and then when I push some pre-defined buttons, something might alter the program's state without loosing the focus of my current window. Just as simple as that (not that simple for me though).
I don't ask for an specific kind of implementation. I'm fine with anything that may work: C, C++, Java, Linux Bash script... The only requisite is that it works under Linux.
Thank you very much
Well you can have your server read a FIFO or a unix domain socket (or even a message queue). Then write a client that takes command line input and writes it to the pipe/queue from some other terminal session. With FIFOs you can just echo input from the command line itself to the pipe but FIFOs come with their own headaches. The "push the button and magic happens" is a lot trickier but maybe that was badly phrased?
I'm currently running a process with the & sign.
$ example &
However, (please note i'm a newbie to Linux) I realised that pretty much a second after such command I'm getting a note that my process received a stopped signal. If I do
$ jobs
I'll get the list with my example process with a little note "Stopped". Is it really stopped and not working at all in the background? How does it exactly work? I'm getting mixed info from the Internet.
In Linux and other Unix systems, a job that is running in the background, but still has its stdin (or std::cin) associated with its controlling terminal (a.k.a. the window it was run in) will be sent a SIGTTIN signal, which by default causes the program to be completely stopped, pending the user bringing it to the foreground (fg %job or similar) to allow input to actually be given to the program. To avoid the program being paused in this way, you can either:
Make sure the programs stdin channel is no longer associated with the terminal, by either redirecting it to a file with appropriate contents for the program to input, or to /dev/null if it really doesn't need input - e.g. myprogram < /dev/null &.
Exit the terminal after starting the program, which will cause the association with the program's stdin to go away. But this will cause a SIGHUP to be delivered to the program (meaning the input/output channel experienced a "hangup") - this normally causes a program to be terminated, but this can be avoided by using nohup - e.g. nohup myprogram &.
If you are at all interested in capturing the output of the program, this is probably the best option, as it prevents both of the above signals (as well as a couple others), and saves the output for you to look at to determine if there are any issues with the programs execution:
nohup myprogram < /dev/null > ${HOME}/myprogram.log 2>&1 &
Yes it really is stopped and no longer working in the background. To bring it back to life type fg job_number
From what I can gather.
Background jobs are blocked from reading the user's terminal. When one tries to do so it will be suspended until the user brings it to the foreground and provides some input. "reading from the user's terminal" can mean either directly trying to read from the terminal or changing terminal settings.
Normally that is what you want, but sometimes programs read from the terminal and/or change terminal settings not because they need user input to continue but because they want to check if the user is trying to provide input.
http://curiousthing.org/sigttin-sigttou-deep-dive-linux has the gory technical details.
Just enter fg which will resolve the error when you then try to exit.
At location A I started a process that will be running on a server over the night. I never disowned this process nor did I use nohup. =\
I am now at location B and I want to keep this process running but I want to kill the shell and logout at location A.
Is it possible to do what I want from location B?
You can try reptyr. But since neercs is not what you want, which can also grab processes from another shell, maybe reptyr does not help you, too.
Of course, after moving the application to your new shell, you need to kill the old one to logout.
You can give neercs a shot.
Here's a video showing how it works.
It's supposed to very unstable, though, and isn't packaged in CentOS or Debian-based distributions.
Here is the problem:
i must move some files from one host to another, ok i use scp for
it.
But i need use it without blocking console, so should use scp &. But my job will killed after disconnecting (a heard it`s something called hup signal) so i found some tricks for fixing this.
But i wanna see progress bar after some time and all that tricks couldnt work because when i use jobs - it display jobs that only for these session.
So how to fix my problem ?
P.S. Sorry for my English.
I am not sure it is what you want but I'll suggest GNU Screen. It allows you to run a program so that when you log out, the program continues execution in the background. Later you can log back in and resume interaction with the program.
I guess you want ability to detach -and- attach terminal - The tool is called screen.
Here's the hypothetical scenario:
I'm running a test script on some hardware attached to box A, which I have root access to. This test script requires minimal user input (flip a switch every half hour or so). About an hour and a half into the test process, I realize that this script takes a long, long time to finish, to the tune of eight hours. Box A is located in a very cold, loud server room that is generally not that fun to physically occupy. Box B is located in my office, where I have a nice comfy chair and an endless supply of hot pockets. I want some way to monitor the output of the process running on box A from an ssh session in box B so I know when to go flip the switch, but I don't want to restart the testing process. Had I known to start with that the test would take so long to finish, I would have just piped its output to a log file and tail'd that file from my box B ssh session. If I know the PID of the process running on box A, is it possible to observe the stdout of that process from another session?
Of course, I could just run vnc on box A and log in from box B to take a look at the output, but that defeats the purpose of this hypothetical, which is to learn more about how process pipes, stdout, and output in general work in a Linux environment.
Thoughts?
http://ingvar.blog.linpro.no/2010/07/10/changing-a-process-file-descriptor-on-the-fly/
You might want to check out expect
It is useful for automating these kinds of interactions.
You could also redirect output of the script to file and monitor said file from another ssh session. I bet the brainy guys on stackoverflow can name about 6 other ways to do it too. :)