When you log in to a Juniper device, you normally land on the Juniper CLI. This is the command shell that most engineers are familiar with. But not all commands are available on the Juniper CLI. Sometimes you will need to interact with the shell of the underlying OS, which can be Linux or FreeBSD. One example reason for you to go to this shell mode, is when you need to log in to a specific line card. Using start shell drops you to shell mode. But how can you do this from a script?
Executing shell commands from a script
Junos PyEZ, the Python library to automate Juniper devices, also has methods available to issue shell commands. The code can be found here.
This code offers you the StartShell class that comes with a set of methods to log in to the Juniper device and land directly into shell mode. The SSH session that enables all this is created with Paramiko. This part of the code can be found here.
To use these methods, you first create a Device object. This can then be used to instantiate a StartShell object that will allow you to open up an SSH connection to the device and issue shell commands.
To play around with the StartShell class, you can paste the following into a Python interactive shell (or run it as a script):
from jnpr.junos.utils.start_shell import StartShell from jnpr.junos import Device dev=Device(host='10.175.74.101', user='salt', password='salt123') ss = StartShell(dev) ss.open() # checks files in /var/tmp cmd_1_result = ss.run('ls -ltr /var/tmp/') # check FPC software version cmd_2_result = ss.run('cprod -A fpc0 -c "show version"') # check FPC syslog messages cmd_3_result = ss.run('cprod -A fpc0 -c "show syslog messages"') ss.close() # print the tuple that ss.run returns: print(cmd_result) # printing the result of the shell commands: print(cmd_1_result) print(cmd_2_result) print(cmd_3_result)
The ss.open and ss.close are used to open and close the connection to the device. The ss.run method will send a command to the device and return a tuple that contains two items. The first item will be True in case the shell command was executed successfully, and False otherwise. The second item contains the command output that the device returns.
In case you are dealing with a command that takes a long time to complete, you can pass a timeout value along with the run method. Let try this out by running a command that takes some time to complete. The log messages that are accessible via show log messages are stored in /var/log/messages. Unless they recently rolled over, chances are this file contains quite some lines. Let’s look at the content of this log file using the following script:
from jnpr.junos.utils.start_shell import StartShell from jnpr.junos import Device dev=Device(host='10.175.74.101', user='salt', password='salt123') ss = StartShell(dev) ss.open() cli_1 = ss.run('cat /var/log/messages"', timeout=2) cli_2 = ss.run('cat /var/log/messages', timeout=90) ss.close() print(cli_1) print(cli_2)
When we run the script, we can see the following:
sh-4.4# python3 ex_3.py False True
The first command did not complete because the timeout was too short. The second command, which had an increased timeout, did complete.
Another thing worth mentioning is that StartShell can be used with the context manager. This means you can use with to open the connection. As soon as the block of code in the with statement is executed, the device connection is automatically closed:
from jnpr.junos.utils.start_shell import StartShell from jnpr.junos import Device dev=Device(host='10.175.74.101', user='salt', password='salt123') ss = StartShell(dev) with StartShell(dev) as ss: # check older log messages cmd_1 = ss.run('zcat /var/log/messages.0.gz', timeout=90) # have the device send a syslog message cmd_2 = ss.run('logger -e testing123') # run a Junos CLI command from shell cmd_3 = ss.run('cli -c "show version | no-more"') print(cmd_1) print(cmd_2) print(cmd_3)
Hope this helps!