I want to do ssh to a remote machine and run tcpdump command in the background.Is it possible in the lua script and to provide password automatically.

You want the Lua script to provide the SSH password?

You could probably drum up something like that, but it would be easier to use the mechanisms that SSH already has: SSH keys.

Create a key on your local machine:

Then copy that key to the remote computer:

Now you can login to the remote without a password.

but i want to do it using password method only without using any sshpass utility and sshkey mechanisms

It's doable but can you first say why? Keys are very easy. Using SSH keys is considered a well-established best practice and password authentication is something to avoid for the most part.

As the remote servers may not allow to do that.

Ok...so since you'd like a script, can you show us what you have done/tried/written so far?? Read the "Question Guidelines" link in my posting signature. We are happy to help you, but we will NOT write scripts for you. You have to show your own efforts

This is what i have tried so far,but its wrong i should'nt have used sshpass and there is problem that i can't see tcpdump process running in remote machine :

function tcpdump_file(pwd,user,ip,filename,cmd)

os.execute('sshpass -p '..pwd..' ssh '..user..'@'..ip..'<<exit')
if cmd=='start' then
os.execute('tcpdump -i 2 -w '..filename..' &')

elseif cmd=='stop' then
os.execute("kill -9 `ps -ef | grep "..filename.." | grep -v grep | awk '{print $2}'`")
end
os.execute('exit')
end
tcpdump_file("lee123","guest","192.168.14.68","test123.pcap",'start')

Use CODE tags when posting code. And since you acknowledge you shouldn't have used sshpass, is there a reason you can just do an SSH keyswap, and do it without a password?? You say:
...but do you *KNOW* they won't?? If you have an SSH login, you almost certainly can do this. Further, if you're the administrator, and you're doing something authorized, you should easily be able to get any access you need to accomplish your task.

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Yes i know that i have used sshpass its just part for trial and error method because i don't know the method to provide the password automatically.Can you give some hint.

The hint is to use SSH keys. There are thousands of guides and tutorials. You'll want to ignore any leftovers recommending DSA keys. Use Ed25519 keys with SSH to look forward or fall back to RSA if backward compatibility with specific dongles is needed.

The programs to look at are ssh-keygen and ssh-copy-id. For the former, be sure to remember the -f and -C options. See "man ssh-keygen" and "man ssh_copy-id" The file to look at on the destination machine is ~/.ssh/authorized_keys

As for more of a hint, that was given back there in post #2 with specifics.

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@OP Kindly explain why would you want to run tcpdump on someone else's computer.

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You were given 'hints'...they are to do an SSH keyswap. There are *THOUSANDS* of easily found guides that tell you how to do this, including the ssh-copy-id command, which you could find with a brief search. This has also been covered many, MANY times on this site as well.

why this command is terminating the process s.execute('sshpass -p lee123 ssh user@192.168.14.81 "nohup sudo tcpdump -i wlan0 -w t5.pcap &"')

The ssh client quits after it has launched the remote process under nohup into the background using the ampersand & there. The file t5.pcap should continue to grow. Log into the remote machine and verify that t5.pcap is there and growing. You should use a full path for that file though so you can put it somewhere useful.

Again I will nag you to set up SSH keys, that will allow you to eliminate several unnecessary steps. If you have root (sudo) then there is no reason not to be able to use keys.

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Because you are ignoring the advice you were given about the correct way to do this.