1. To connect to a remote host with the SSH server running, use the following command:

        $ ssh username@remote_host 

The options in this command are as follows:

Consider this example:

        $ ssh mec@192.168.0.1 
        The authenticity of host '192.168.0.1 (192.168.0.1)' can't be   
        established. 
        RSA key fingerprint is   
        2b:b4:90:79:49:0a:f1:b3:8a:db:9f:73:2d:75:d6:f9. 
        Are you sure you want to continue connecting (yes/no)? yes 
        Warning: Permanently added '192.168.0.1' (RSA) to the list of   
        known hosts. 
        Password: 
 
        Last login: Fri Sep  3 05:15:21 2010 from 192.168.0.82 
        mec@proxy-1:~$ 

SSH will ask for a password, and upon successful authentication it will connect to the login shell on the remote machine.

By default, the SSH server runs at port 22. However, certain servers run SSH service at different ports. In that case, use -p port_num with the ssh command to specify the port.

2. Connect to an SSH server running at port 422:

        $ ssh user@locahost -p 422 

When using ssh in shell scripts, we do not want an interactive shell, we simply want to execute commands on the remote system and process the command's output.

3. To run a command at the remote host and display its output on the local shell, use the following syntax:

        $ sshuser@host 'COMMANDS' 

Consider this example:

        $ ssh mec@192.168.0.1 'whoami' 
        mec

You can submit multiple commands by separating the commands with a semicolon:

        $ ssh user@host "command1 ; command2 ; command3" 

Consider the following example:

        $ ssh mec@192.168.0.1  "echo user: $(whoami);echo OS: $(uname)" 
        Password:
        user: mec
        OS: Linux 

In this example, the commands executed at the remote host are as follows:

        echo user: $(whoami); 
        echo OS: $(uname) 

We can pass a more complex subshell in the command sequence using the ( ) subshell operator.

3. The next example is an SSH-based shell script to collect the uptime of a list of remote hosts. Uptime is the length of time since the last power-on. It's returned by the uptime command.

It is assumed that all systems in IP_LIST have a common user test.

        #!/bin/bash 
        #Filename: uptime.sh 
        #Description: Uptime monitor 
 
        IP_LIST="192.168.0.1 192.168.0.5 192.168.0.9" 
        USER="test" 
 
        for IP in $IP_LIST; 
        do 
        utime=$(ssh ${USER}@${IP} uptime  |awk '{ print $3 }' ) 
          echo $IP uptime:  $utime 
        done 

Expected output:

        $ ./uptime.sh 
        192.168.0.1 uptime: 1:50, 
        192.168.0.5 uptime: 2:15, 
        192.168.0.9 uptime: 10:15,