Reading the output of an executed command

In the previous recipe, we saw how to first connect to a device and then execute a command. So far, we have ignored the output though.

In this recipe, you will see how to programmatically open an SSH connection, send a command, and then write the output of that command back to a file. We will use this to back up a running configuration.

Getting ready

Open your code editor and start by creating a file called read_out.py. Next, navigate your terminal to the same directory that you just created the read_out.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. Then execute a single command of your choice on this device and save the output:

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 running-config" 
   stdin, stdout, stderr = client.exec_command(CMD)

6. We will use the stdout object to retrieve what the command has returned:

   output = stdout.readlines()

7. Next, we write the output back to a file:

   with open("backup.txt", "w") as out_file
       for line in output:
           out_file.write(line)

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

   python3 read_out.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. We then used the returned file-like object for the standard out to read everything that our remote device printed into the standard out and write it into a file.

A file-like object in the context of Python is an object that offers the same functions as a file object. When using the open() function to create a new file object to read/write from locally, we are offered some functions such as write() or readlines(). These functions allow us to read or write from or to a file. A file-like object offers the same functions and can thus be used as if it was a remote file. We use the readlines() function on the stdout object to read everything returned – in this example, the running configuration of our device – and write it line by line to a local file.

There's more...

If you prefer to just see the output of your command instead of writing it to a file, you can also use the following construct, instead of the code provided in step 7 of the recipe, to print out your entire configuration:

for line in output:
    print(line.strip())  

The strip() method used in the preceding example will delete any unnecessary leading or trailing whitespaces.