Question

I connected to a unix server through ssh and tried to execute a “ls” command and obtain it’s output. The code is like this

SessionChannelClient session = client.openSessionChannel();
       session.startShell();
       String cmd = "ls -l";
       session.executeCommand(cmd);           
       ChannelInputStream in = session.getInputStream();
       ChannelOutputStream out = session.getOutputStream();
       IOStreamConnector input = new IOStreamConnector(System.in, session.getOutputStream());
       IOStreamConnector output = new IOStreamConnector(session.getInputStream(), System.out);

After running I was not getting any output in log file. What I found is that the channel request is failing as shown

1019 [main] INFO com.sshtools.j2ssh.connection.ConnectionProtocol – Channel request succeeded
1020 [main] INFO com.sshtools.j2ssh.session.SessionChannelClient – Requesting command execution
1021 [main] INFO com.sshtools.j2ssh.session.SessionChannelClient – Command is ls -l
1021 [main] INFO com.sshtools.j2ssh.connection.ConnectionProtocol – Sending exec request for the session channel
1021 [main] INFO com.sshtools.j2ssh.transport.TransportProtocolCommon – Sending SSH_MSG_CHANNEL_REQUEST
1021 [main] INFO com.sshtools.j2ssh.connection.ConnectionProtocol – Waiting for channel request reply
1032 [Transport protocol 1] INFO com.sshtools.j2ssh.transport.TransportProtocolCommon – Received SSH_MSG_CHANNEL_EXTENDED_DATA
1033 [ssh-connection 1] DEBUG com.sshtools.j2ssh.transport.Service – Routing SSH_MSG_CHANNEL_EXTENDED_DATA
1033 [ssh-connection 1] DEBUG com.sshtools.j2ssh.transport.Service – Finished processing SSH_MSG_CHANNEL_EXTENDED_DATA
1075 [Transport protocol 1] INFO com.sshtools.j2ssh.transport.TransportProtocolCommon – Received SSH_MSG_CHANNEL_FAILURE
1075 [main] INFO com.sshtools.j2ssh.connection.ConnectionProtocol – Channel request failed

Why is this happening ?

Answer 1

You can use startShell or executeCommand, but not both.

executeCommand is only intended to invoke a particular shell of your choosing, e.g. /bin/bash. In most cases you can just use startShell to start the default shell.

This is all you need to execute the command you provided:

final String cmd = "ls -l" + '\n';       
session.startShell();
session.getOutputStream().write(cmd.getBytes());

Or, alternatively, using executeCommand:

final String cmd = "ls -l";    
session.executeCommand(String.format("sh -c \"%s\"", cmd));

You can hook up to the stdout & stderr InputStreams in the same manner to see the result of your command. Further, note that a SessionChannelClient can only process one command. You’ll need to instantiate another one to send a second command.

I suggest you download the J2SSH documentation. Inside you’ll find j2ssh-getting-started.htm which explains the basic operation of the API.