Executing a command via SSH

With our connection now open, we can go ahead and execute a command on our remote device. Similar to the way we deal with issuing commands on a remote device by hand, we have three different streams that come back to us: the standard out (or stdout), which is the normal output, the standard error (or stderr), which is the default stream for the system to return errors on, and the standard in (or stdin), which is the stream used to send text back into the executed command. This can be useful if, in your workflow, you would normally interact with the command line.

In this recipe, you will see how to programmatically open an SSH connection and then send a command of your choice to the device.

Getting ready

Open your code editor and start by creating a file called command.py. Next, navigate your terminal to the same directory that you just created the command.py file in.

How to do it...

Let's start by importing the Paramiko library and create a client object as seen in the last recipe. We'll then execute a single command of your choice on this device:

1. Import the Paramiko library:

   from paramiko.client import SSHClient

2. Specify the host, username, and password. You can name these variables however you like. In the Python community, it has become a standard to uppercase these global variables:

   SSH_USER = "<Insert your ssh user here>"
   SSH_PASSWORD = "<Insert your ssh password here>"
   SSH_HOST = "<Insert the IP/host of your device/server here>"
   SSH_PORT = 22 # Change this if your SSH port is different

3. Create an SSHClient object, which we just imported from Paramiko:

   client = SSHClient()

4. While we have created our client object, we have not yet connected to the device. We will use the connect method of the client object to do so. Before actually connecting, we will need to make sure that our client knows the host keys:

   client.load_system_host_keys()
   client.connect(SSH_HOST, port=SSH_PORT,
                            username=SSH_USER,
                            password=SSH_PASSWORD)

5. Finally, we can use the client to execute a command. Executing a command will return three different file-like objects to us representing stdin, stdout, and stderr:

   CMD = "show ip interface brief" # You can issue any command you want
   stdin, stdout, stderr = client.exec_command(CMD)
   client.close()

6. To run this script, go to your terminal and execute it with this:

   python3 command.py

How it works...

In this example, we first created a new client as seen in the previous example. We then used the exec_command() method to execute a command of our choice.

The function returns three different file-like objects for the three different streams: stdin, stdout, and stderr. In the next recipe, Reading the output of an executed command, we will use this to read back the output that was provided when executing a command.