I have a bit of an interesting situation that I can't figure out.
I need the ability to issue a command remotely over SSH, which then pipes the results into an ncat tunnel back to the original server.
My commands are a little more complex than this (involving innobackupex, a MySQL backup utility), but this minimal example shows the same issue:
ncat -l 9970 &
ssh db-202 "echo 'testing' | ncat $BACKUPSERVER 9970"
... other commands
The issue is that after the echo command completes (or whatever is run), the script just hangs and the later commands don't run. I need to send a ctrl-c (SIGINT) to continue. Obviously this isn't ideal in the context of a bash script, where lots of things may need to happen after this command completes automatically.
Not sure if it's relevant, but sending a sigint doesn't always properly terminate the ncat tunnel on both sides. Sometimes I need to kill -9 it, probably due to the sigint messing something up.
Can anybody explain this behavior and how I can get around it? Or a better way to do what I want?
Thanks!
Finally figured it out.
I added the --recv-only and --send-only arguments to each side of the tunnel.
According to the manpage:
--send-only Only send data, ignoring received; quit on EOF
This may explain it, but I though that any EOF would terminate the tunnel. Maybe without these arguments, both sides need to send an EOF?
Anyway, here are the commands that work.
ncat --recv-only -l 9970 &
ssh db-202 "echo 'testing' | ncat --send-only $BACKUPSERVER 9970"
Hopefully this helps someone in the future.
Related
I'm running a Bukkit (Minecraft) server on a Linux machine and I want to have the server gracefully shut down using the server's stop command and the computer suspend at a certain time using pm-suspend from the command line. Here's what I've got:
me#comp~/dir$ perl -e 'sleep [time]; print "stop\\n";' | ./server && sudo pm-suspend
(I've edited by /etc/sudoers so I don't have to enter my password when I suspend.)
The thing is, while the perl -e is sleeping, the server is expecting a constant stream of bytes, (That's my guess. I could be misunderstanding something.) so it prints out all of the nothings it receives, taking up precious resources:
me#comp~/dir$ ...
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>...
Is there any such thing as a buffered pipe? If not, are there any ways to send delayed input to a script?
You may want to have a look at Bukkit's wiki, which recommends an init script for permanently running servers.
This init script uses rather unconventional approach to communicate with running server. The server is started in screen session, then all commands are send to the server console via screen, e.g.
screen -p 0 -S $SCREEN -X eval 'stuff \"stop\"\015'
See https://github.com/Ahtenus/minecraft-init/blob/master/minecraft
This approach suggest that bukkit may be expecting standard input to be attached to a terminal, thus requiring screen wrapper (which is itself terminal emulator) for unattended runs.
I'm trying to write a shell script that automates certain startup tasks based on my location (home/campusA/campusB). I go to University and take classes in two different campuses (hence campusA/campusB). My location is determined by which wireless network I'm connected to. For the purposes of this script, we can assume that I will be connected to one of these networks when the script is called and my script knows which one I'm connected to based on a call to iwconfig.
This is what I want it to do:
cat file1 > file2 # always do this, regardless of where I am
if Im at home:
start tweetdeck, thunderbird, skype
else if Im at campusA:
activate the login script # I need to login on a webform before I get internet access.
# I have written a script to automate this.
# Wait for this script to finish before doing anything else
myProg2 & # I want myProg2 running in the background until I shutdown my computer.
else if Im at campusB:
ssh username#domain # this is the problematic line
myProg2 & # I want myProg2 running in the background until I shutdown my computer.
start tweetdeck, thunderbird
close the terminal with the "exit" command
The problem is that campusB's wireless network is behind a firewall, which grants me internet access ONLY after I successfully ssh by username#domain. After a successful ssh, I need to keep the terminal window active in order to hold keep the internet access. If I close the terminal window, I lose internet access (this is bad).
When I try doing just ssh username#domain, the script stops because I don't exit the ssh command. I can't ^C out of it, which means that the rest of the script is never executed. I also have the same problem if I just close the terminal window in an attempt to kill the ssh session.
Some googling brought me to subshell, which I'm either using wrong or can't use to solve my problem. So how should I go about solving this problem? I'd appreciate any help - I've been at this for a while now and am unable to find anything helpful. If it makes a difference, I'd rather not store my ssh password in the script
Further, ampersanding the ssh call (ssh username#domain &) doesn't seem to do any good (can anyone explain why?)
Thank you in advance
EDIT
I must clarify, that the ssh connection has to be active in order for me to have internet access. Thus, when I close the terminal window, I need the ssh connection to still be active.
I had a script that looped on 6 servers, calling via ssh in the background. In 1 part of the script, there was a mis-behaving vendor application; the application didn't 'let go' of the connection properly. (other parts of the script using ssh in background worked fine).
I found that using ssh -t -t cured the problem. Maybe this can help you too.
(a teammate found this on the web, and we had spent so much time, I never went back to read the article that suggested this. The man page on our system gave no hint that such a thing was possible)
Hope this helps.
You may want to try to double background myProg2 to detach it from the tty:
# cf. "Wizard Boot Camp, Part Six: Daemons & Subshells",
# http://www.linux-mag.com/id/5981
(myProg2 &) &
Another option may be to use the daemon tool from the libslack package:
http://ingvar.blog.linpro.no/2009/05/18/todays-sysadmin-tip-using-libslack-daemon-to-daemonize-a-script/
Having a ssh with pseudy tty on background shell
In addition to #shellter's answer, I would like make some precision:
where #shelter said:
The man page on our system gave no hint that such a thing was possible
On my system (Debian 7 GNU/Linux), if I hit:
man -Pcol\ -b ssh| grep -A3 '^ *-t '
I could read:
-t Force pseudo-tty allocation. This can be used to execute arbi‐
trary screen-based programs on a remote machine, which can be
very useful, e.g. when implementing menu services. Multiple -t
options force tty allocation, even if ssh has no local tty.
Yes: Multiple -t options force tty allocation, even if ssh has no local tty.
This mean: If you remotely run a tool that require access to pseudo terminal ( pty like /dev/pts/0), you could run them by using -t switch.
But this would work only if ssh is run from a shell console (aka having his own pty). If you plan to run them is shell session without console, like background scripts, you may use Multiple -t to enforce pseudo tty allocation from ssh.
Multiple ssh shell on one ssh connection
In addition to answers from #tommy and #geekosaur, I would make some precision:
#tommy point to a very intersting feature of ssh. Not sure this have a lot to do with answer, but speaking around long time connection, this feature has to be clearly understood.
Once a connection is established, ssh could (and know how to) use them to drive a lot of thing in this one connection:
-L let you drive remote TCP connections to local machines/network. (full syntax is: -L localip:localport:distip:distport) where localip could be specified to permit other hosts from same local domain to access same tcp bind, and distip could by any host from distant network ( not only localhost ) sample: -L192.168.1.31:8443:google.com:443 permit any host from local domain to reach google through your host: http://192.168.1.31:8443
-R Same remarks in reverse way!
-M Tell ssh to open a local unix socket for bindind next ssh consoles. Simply open two terminal window. First in both window, hit: ssh somewhere than hit netstat -tan | grep :22 or netstat -tan | grep 192.168.1.31:22 (assuming 192.168.1.31 is your onw host's ip)
Than compare close all your ssh session and in first terminal, hit: ssh -M somewhere and in second, simply ssh somewhere. you may see in second terminal:
$ ssh somewhere
+ ssh somewhere
Last login: Mon Feb 3 08:58:01 2014 from elsewhere
If now you hit netstat -tan | grep 192.168.1.31:22 (on any of two oppened ssh session;) you must see that there is only one tcp connection.
This kind of features could be used in combination with -L and maybe some sleep 86399...
To work around a tcp killer router that close every inactive TCP connection from more than 120 seconds, I run:
ssh -M somewhere 'while :;do uptime;sleep 60;done'
This ensure connection stay up even if I dont hit a key for more than two minutes.
Here's a few thoughts that might help.
Sub-shells
Sub-shells fork new processes, but don't return control to the calling shell. If you want to fork a sub-shell to do the work for you, then you'll need to append a & to the line.
(ssh username#domain) &
But this doesn't look like a compelling reason to use a sub-shell. If you had a number commands you wanted to execute in order from each other, yet in parallel from the calling shell, then maybe it would be worth it. For example...
(dothis.sh; thenthis.sh; andthislastthingtoo.sh) &
Forking
I'm not sure why & isn't working for you, but it may be worth looking into nohup as well. This makes the command "immune" to hang up signals.
nohup ssh username#domain (try with and without the & at the end)
Passwords
Not storing passwords in the script is essential for any ssh automation. You can accomplish that using public key cryptography which is an inherent feature of ssh. I wont go into the details here because there are a number of great resources all across the interwebs on setting this up. I strongly suggest investigating this further.
HOWTO: set up ssh keys - Paul Keck, 2001
SSH Keys - archlinux.org
SSH with authentication key instead of password - Debian Administration
Secure Shell - Wikipedia, the free encyclopedia
If you do go this route, I also suggest running ssh in "batch mode" which will disable password querying and will automatically disconnect from the server if it becomes unresponsive after 5 minutes.
ssh -o 'BatchMode=yes' username#domain
Persistence
Then if you want to persist the connection, run some silly loop in bash! :)
ssh -o 'BatchMode=yes' username#domain "while (( 1 == 1 )); do sleep 60; done"
The problem with & is that ssh loses access to its standard input (the terminal), so when it goes to read something to send to the other side it either gets an error and exits, or is killed by the system with SIGTTIN which will implicitly suspend it. The -n and -f options are used to deal with this: -n tells it not to use standard input, -f tells it to set up any necessary tunnels etc., then close the terminal stream.
So the best way to do this is probably to do
ssh -L 9999:localhost:9999 -f host & # for some random unused port
and then manually kill the ssh before logout. Alternately,
ssh -L 9999:localhost:9999 -n host 'while :; do sleep 86400; done' </dev/null &
(The redirection is to make sure the SIGTTIN doesn't happen anyway.)
While you're at it, you may want to save the process ID and shut it down from your .logout/.bash_logout:
ssh -L 9999:localhost:9999 -n host 'while :; do sleep 86400; done' < /dev/null & echo $! >~.ssh_pid; chmod 0600 ~/.ssh_pid
and in .bash_logout:
if test -f ~/.ssh_pid; then
set -- $(sed -n 's/^\([0-9][0-9]*\)$/\1/p' ~/.ssh_pid)
if [ $# = 1 ]; then
kill $1 >/dev/null 2>&1
fi
rm ~/.ssh_pid
fi
The extra code there attempts to avoid someone sabotaging your ~/.ssh_pid, because I'm a professional paranoid.
(Code untested and may have typoes)
It's been a while since I've used ssh, and I can't test it right now, but have you tried the -f switch?
ssh -f username#domain
The man page says it backgrounds ssh. Not sure why & wouldn't work, but I guess it's interpreting it as a command to be run on the remote machine.
Maybe screen + ssh would fit the bill as well?
Something like:
screen -d -m -S sessionName cmd
screen -d -m -S sessionName cmd &
# reconnect with
screen -r sessionName
I ssh to another server and run a shell script like this nohup ./script.sh 1>/dev/null 2>&1 &
Then type exit to exit from the server. However it just hangs. The server is Solaris.
How can I exit properly without hanging??
Thanks.
I assume that this script is a long running one. In this case you need to detach the process from the terminal that you wish to close when you terminate your ssh session.
Actually you already done most of the work by reassigning both stdout and stderr to /dev/null, however you didn't do that for stdin.
I used the test case of:
ssh localhost
nohup sleep 10m &> /dev/null &
^D
# hangs
While
ssh localhost
nohup sleep 10m &> /dev/null < /dev/null &
^D
# exits
I second the recommendation to use the excellent gnu screen, that will do this service for you, among others.
Oh, and have you considered running the script directly and not within a shell? I.e.:
ssh user#host script.sh
If you're trying to leave a command running remotely after you close your SSH link, I strongly recommend you use screen and learn to detach the screen. That's much better than leaving background processes around; it also lets you reconnect and see what the process is up to.
Since you haven't provided us with script.sh, I don't think we can know for sure why the command is hanging.
You can use the command :
~.
This command close the ssh session.
sh -c ./script.sh &
I would like to execute a ssh command and pipe the output to a file.
In general I would do:
ssh user#ip "command" >> /myfile
the problem is that ssh close the connection once the command is executed, however - my command sends the output to the ssh channel via another programm in the background, therefore I am not receiving the output.
How can I treat ssh to leave my shell open?
cheers
sven
My understanding is that command starts some background process that perhaps will write some output to the terminal later. If command terminates before that the ssh session will be terminated and there will be no terminal for the background program to write to.
One simple and naive solution is to just sleep long enough
ssh user#ip "command; sleep 30m" >> /myfile
A better solution than sleep would be to wait for the background process(es) to finish in some more intelligent way, but that is impossible to say without further details.
Something more powerful than bash would be Python with Paramiko and PyExpect.
In a bash script I execute a command on a remote machine through ssh. If user breaks the script by pressing Ctrl+C it only stops the script - not even ssh client. Moreover even if I kill ssh client the remote command is still running...
How can make bash to kill local ssh client and remote command invocation on Crtl+c?
A simple script:
#/bin/bash
ssh -n -x root#db-host 'mysqldump db' -r file.sql
Eventual I found a solution like that:
#/bin/bash
ssh -t -x root#db-host 'mysqldump db' -r file.sql
So - I use '-t' instead of '-n'.
Removing '-n', or using different user than root does not help.
When your ssh session ends, your shell will get a SIGHUP. (hang-up signal). You need to make sure it sends that on to all processes started from it. For bash, try shopt -s huponexit; your_command. That may not work, because the man page says huponexit only works for interactive shells.
I remember running into this with users running jobs on my cluster, and whether they had to use nohup or not (to get the opposite behaviour of what you want) but I can't find anything in the bash man page about whether child processes ignore SIGHUP by default. Hopefully huponexit will do the trick. (You could put that shopt in your .bashrc, instead of on the command line, I think.)
Your ssh -t should work, though, since when the connection closes, reads from the terminal will get EOF or an error, and that makes most programs exit.
Do you know what the options you're passing to ssh do? I'm guessing not. The -n option redirects input from /dev/null, so the process you're running on the remote host probably isn't seeing SIGINT from Ctrl-C.
Now, let's talk about how bad an idea it is to allow remote root logins:
It's a really, really bad idea. Have a look at HOWTO: set up ssh keys for some suggestions how to securely manage remote process execution over ssh. If you need to run something with privileges remotely you'll probably want a solution that involves a ssh public key with embedded command and a script that runs as root courtesy of sudo.
trap "some_command" SIGINT
will execute some_command locally when you press Ctrl+C . help trap will tell you about its other options.
Regarding the ssh issue, i don't know much about ssh. Maybe you can make it call ssh -n -x root#db-host 'killall mysqldump' instead of some_command to kill the remote command?
What if you don't want to require using "ssh -t" (for those as forgetful as I am)?
I stumbled upon looking at the parent PID, because CTRL/C from the initiating session results in the ssh-launched process on the remote process exiting, although its child process continues. By way of example, here's my script that is on the remote server.
#!/bin/bash
Answer=(Alive Dead)
Index=0
while [ ${Index} -eq 0 ]; do
if ! kill -0 ${PPID} 2> /dev/null ; then Index=1; fi
echo "Parent PID ${PPID} is ${Answer[$Index]} at $(date +%Y%m%d%H%M%S%Z)" > ~/NowTime.txt
sleep 1
done
I then invoke it with "ssh remote_server ./test_script.sh"
"watch cat ~/NowTime.txt" on the remote server shows the timestamp in the file increasing and declaring that the parent process is alive; once I hit CTRL/C in the launching process, the script on the remote server notes that its parent process has died, and the script exits.