Start a remote process using java

Question

Machine A connects to a remote machine B using sshpass and that remote machine B doesn't give ssh access to root directly so I connect to it through user and use sudo and I also have disabled password for the user when he does sudo su. When the commands are simple like ls -lrt /some/dir I get correct output for the command but when I start a server/shell script on remote machine (server takes no time while the script takes 1 minute to run) the output is shown but the bash doesn't return back to normal(i.e. it looks like it is doing some work but I know that the server has started/script has finished).

Command used :

sshpass -p 'password' ssh -o StrictHostKeyChecking=no user@192.168.0.3 "sudo ls /dev/ttyUSB*"

above works fine, I get the output and the bash is returned so I can write more commands and do stuff.

sshpass -p 'password' ssh -o StrictHostKeyChecking=no user@192.168.0.3 "sudo /root/path/server &"

sshpass -p 'password' ssh -o StrictHostKeyChecking=no user@192.168.0.3 "sudo /root/path/to/script/scriptTakes1Minute param1 param2"

But, the above doesn't work as expected. It looks like it is doing work but it isn't the bash is not returned and so I can't write commands further.

I can't share the script or the server program as it is company's code. But the server code uses UNIX datagram sockets to communicate.

/* Creating a UNIX datagram socket for Server Application */
        if ((sock = socket(AF_UNIX, SOCK_DGRAM, 0)) < 0)
        {
                perror("server: socket");
                exit(1);
        }

EDIT 1:

When I tried

sshpass -p 'password' ssh -o StrictHostKeyChecking=no user@192.168.0.3 "sudo /root/path/server >/dev/null &2>1 </dev/null &"

the server program started correctly but the script didn't work as expected and didn't return the bash but as soon as I pressed ENTER the bash returned, I know it means something but don't know what.

EDIT 2:

I can't redirect output to a file as I don't know how much time will the command take to run in total it may take a minute or few.

EDIT 3:

I found something that will help a lot(I guess). The script that is kept on that machine B creates few processes in background and so the sshpass doesn't return until all those background processes are killed manually from Machine B. As soon as those programs are killed we get that output as well as the bash back.

EDIT 4:

The script creates 8 ppp sessions that run in background.

Try calling `process.getOutputStream().close()` and see whether it makes a difference. — Dakshinamurthy Karra, Mar 08 '18 at 09:59
I dont know how much time will it take so I can't close the output stream but the sshpass command is at fault here not java. Please help me with that or some alternative. — Shashank Singh, Mar 08 '18 at 10:48
Closing output stream doesn’t stop the process. The process gets a EOF. — Dakshinamurthy Karra, Mar 08 '18 at 10:51
You can put this just after the exec() call. Since a Control+C seems to make your app work - it looks like the app is waiting for some input. Sending a EOF might solve the issue. — Dakshinamurthy Karra, Mar 08 '18 at 11:08
But the process takes 1 minute to run so wont it stop it immediately without giving me the complete response that I want. — Shashank Singh, Mar 08 '18 at 11:12
Hmm... how long does it take to try it? And if it works you can always try to figure out how it works :) — Dakshinamurthy Karra, Mar 08 '18 at 11:19
@KDM It takes 1-2 minutes for the process to run, it keeps on printing the output but I want the complete output. — Shashank Singh, Mar 09 '18 at 04:21
Sending a EOF (closing getOutputStream()) doesn't stop the process. You should be receiving full output. — Dakshinamurthy Karra, Mar 09 '18 at 12:54
So basically you want to execute a command and get a bash once the command is finished? — Tarun Lalwani, Mar 12 '18 at 07:35
Yes...it comes back for few commands but not for those I mentioned. — Shashank Singh, Mar 12 '18 at 07:52
Can you please try this `sshpass -p 'password' ssh -o StrictHostKeyChecking=no user@192.168.0.3 "sudo /root/path/server >/dev/null &2>1 — Tarun Lalwani, Mar 12 '18 at 21:31
The `server` started as expected but I want the output of that script as well. I have edited the question, please have a look. — Shashank Singh, Mar 13 '18 at 04:41
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/166731/discussion-between-tarun-lalwani-and-shashank-singh). — Tarun Lalwani, Mar 13 '18 at 07:24

Abhijit Pritam Dutta · Answer 1 · 2018-03-08T09:44:33.847

4

Simply need to use one more line of code process.waitFor(); which will fix your issue:-

 process = Runtime.getRuntime().exec(SHELL_COMMAND);
 process.waitFor();// add this line to complete execution of your shell command
 System.out.println("LLLLLLLOOOOOOOOOKKKKKKKK started process");

edited Mar 08 '18 at 09:44

answered Mar 08 '18 at 07:55

Abhijit Pritam Dutta

5,521
2
11
17

`IllegalMonitorStateException` occured, I am trying to solve it, was just letting you know. – Shashank Singh Mar 08 '18 at 09:07
@ShashankSingh, I have modified my answer with a different method. Try with this and let me know. – Abhijit Pritam Dutta Mar 08 '18 at 09:45
Still the issue is there. Thank you for trying. One more observation ,when I pressed `CTRL+C` then the output can be seen and it is the correct output which is expected. I don't know if it is something that is `sshpass` problem or `java` problem. – Shashank Singh Mar 08 '18 at 09:55
Just try to execute a `ls` command from java and check whether java issue or any other other issue. Also verify permission as well. – Abhijit Pritam Dutta Mar 08 '18 at 10:03
also comment out //process.destroy(); and check. – Abhijit Pritam Dutta Mar 08 '18 at 10:05
If you are executing a shell script check whether you have added `exit` in the shell script or not. – Abhijit Pritam Dutta Mar 08 '18 at 10:12
I am really sorry if I wasted your time its not `java`'s fault let me edit the question again. – Shashank Singh Mar 08 '18 at 10:42
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/166455/discussion-between-shashank-singh-and-abhijit-pritam). – Shashank Singh Mar 08 '18 at 10:46
I am in office now. My office don't allow me to chat sorry. – Abhijit Pritam Dutta Mar 08 '18 at 10:49
You may delete your answer before it gets downvotes. I changed the question and it has nothing to do with java. – Shashank Singh Mar 12 '18 at 05:53
I have edited the question with a very important details in EDIT 3. – Shashank Singh Mar 15 '18 at 07:00
@ShashankSingh, Butter post it https://unix.stackexchange.com/ here. You will get the solution very quickly. And never change the whole question in future. Your original question is totally different from the current one. This question is nothing to do with Java anymore. Always post a different question if that is totally different from the current one. – Abhijit Pritam Dutta Mar 16 '18 at 10:53
Sorry. Won't happen again. – Shashank Singh Mar 19 '18 at 05:44

score 3 · Answer 2 · answered Mar 12 '18 at 23:47

3

If you want ssh to execute a command and return before the command completes, use the -f option, such as ssh -f user@host myServer. If it may produce output, you probably want to redirect stdout and/or stderr.

See the man page for details: https://man.openbsd.org/ssh#f

answered Mar 12 '18 at 23:47

Chris

4,734
2
19
26

I need to capture the output in a `java` program. Will it be a best practice to first output each command in a file then read the file and delete it? The programs has many shell commands that I have to execute like this. – Shashank Singh Mar 13 '18 at 04:44
Yeah, you could redirect to files and read them in Java. Or you could use ProcessBuilder. It lets you launch external programs and get their output as Java InputStreams to read from (and in that case you wouldn't use `-f` with ssh). – Chris Mar 13 '18 at 22:10
I have edited the question with a very important details in EDIT 3. – Shashank Singh Mar 15 '18 at 07:00
From what you've posted, it looks to me like ProcessBuilder is what you want. For examples of how to run a program from Java and collect its output, see: https://stackoverflow.com/questions/16714127/how-to-redirect-process-builders-output-to-a-string – Chris Mar 16 '18 at 02:56

Tarun Lalwani · Answer 3 · 2018-03-19T07:59:00.240

So I created a simple server using express

express = require('express');

app = express();

app.all('/', (req, res) => {
   res.send('anc\n');
})

app.listen(9099, () => {
   console.log('server started');
});

and then executed using

sshpass -p 'vagrant' ssh -o StrictHostKeyChecking=no vagrant@192.168.33.100 "node /tmp/server/index.js 1>/tmp/log 2>&1"

It logs the output to /tmp/log. You can see the execution on below screenshot

As you can see I can capture the logs as well. If you have a list of commands to run then I would use a shell script and instead that script individual scripts can be logged to their respective files or spit output on main stdout and be captured in the /tmp/log we used in the command

Working with sudo

As you can see in below image it even works with sudo. But a word of advise, you shouldn't use sudo su because that would require a user shell and you want to use sudo -u <user> in your command which you pass to SSH

Edit: 19-Mar-18

Based on your edit you provided a script

echo "First Line" 
ping 10.100.208.102 >/dev/null & 
echo "Last Line"

Now I assume you want the output Last Line as well and then you don't want to hang the terminal also. Below test command worked for me

$ sshpass -p 'vagrant' ssh -o StrictHostKeyChecking=no vagrant@192.168.33.100 "sudo sh /tmp/server/ping.sh 1>/tmp/log 2>&1 && cat /tmp/log"
First Line
Last Line

$

I need to start a script or server kept at `/root/some/path` so I need a `sudo` before it and I have removed the password when we do `sudo su`. I can output the commands output using `redirection` to a file but I was hoping to get this way working as I can't figure out why is it that what I am trying is not working. — Shashank Singh, Mar 13 '18 at 08:44
reply the actual command you used and I will tel you how to use it. I have used password less sudo while testing this and it works well — Tarun Lalwani, Mar 13 '18 at 08:46
Please join this [chat](https://chat.stackoverflow.com/rooms/166731/discussion-between-tarun-lalwani-and-shashank-singh) — Tarun Lalwani, Mar 13 '18 at 09:01
I have edited the question with a very important details in EDIT 3. — Shashank Singh, Mar 15 '18 at 07:00

Start a remote process using java

3 Answers3