I start ncat (on Windows 10) with
ncat -vvlp 1234 -e code.exe
and then connect with a second instance of ncat to the first instance
(ncat 127.0.0.1 1234).
code.exe is a C program written by me that can be controlled over stdin.
Everything I send via the second ncat gets forwarded to the stdin of code.exe. I know this because I can see code.exe create a folder after sending the command to do so. But the output is not send back until code.exe closes itself.
Why is that — and how can I fix it?
Ok I found a solution to my problem. I disabled buffering of stdout by using
setbuf(stdout, NULL);
at the start of my C program.
From myhost.mydomain.com, I start a nc listener. Then login to another host to start a netcat push to my host:
nc -l 9999 > data.gz &
ssh repo.mydomain.com "cat /path/to/file.gz | nc myhost.mydomain.com 9999"
These two commands are part of a script. Only 32K bytes are sent to the host and the ssh command terminates, the nc listener gets an EOF and it terminates as well.
When I run the ssh command on the command line (i.e. not as part of the script) on myhost.mydomain.com the complete file is downloaded. What's going on?
I think there is something else that happens in your script which causes this effect. For example, if you run the second command in the background as well and terminate the script, your OS might kill the background commands during script cleanup.
Also look for set -o pipebreak which terminates all the commands in a pipeline when one of them returns with != 0.
On a second note, the approach looks overly complex to me. Try to reduce it to
ssh repo.mydomain.com "cat /path/to/file.gz" > data.gz
(ssh connects stdout of the remote with the local). It's more clear when you write it like this:
ssh > data.gz repo.mydomain.com "cat /path/to/file.gz"
That way, you can get rid of nc. As far as I know, nc is synchronous, so the second invocation (which sends the data) should only return after all the data has been sent and flushed.
I am trying to issue commands to telnet. When I initially issue a simple connection command such as:
telnet localhost 9300
I am immediately connected which is fantastic but there are messages that instantly start printing in the shell every 1 second. These are expected responses from the program connected to that port. The issue is, how do I issue a second command when the shell won't stop logging data? I can't type in the shell when it is moving. Thanks for any help and sorry for the newbie type question.
You can type blind, just ignore that your line disappears in the stdout of telnet.
With copy-paste from a buffer, you can see your line, but it will still move out of the screen.
Another way is redirecting the output of telnet to a file.
When you want to see the telnet output, open a second window.
Window 1:
telnet localhost 9300 > telnet9300.out 2>&1
enter your commands here
Window 2:
# wait for telnet9300.out being created
tail -f telnet9300.out
# Press ^C when you have seen enough
I work with a protocol that's easy to use simply with netcat. The protocol starts with a login message, so I thought I could bang out a little script which pipes the login message before stdin to netcat for me.
I was able to get close, but there's one problem I can't figure out. The following script works, in that it sends the login message and allows me to interact with netcat. But if netcat exits (because the server side closed the connection), the script just hangs there (presumably because cat is still reading stdin even though no one is reading stdout any more).
( echo "${LOGIN}"; cat ) | nc ${HOST} ${PORT}
It's a tricky problem, and you're right about the cause. Processes don't get a NOPIPE error and SIGPIPE until they actually try to write to the pipe.
If nothing else, you can use the interaction scripting tool expect:
expect <(echo '
spawn nc google.com 80
send "GET / HTTP/1.0\n"
send "Host: www.google.com\n"
interact
')
This will run nc, send some HTTP headers, and then gives control to you. When nc exits, so does the command.
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