Automating the Boring Stuff with Pexpect: A Sysadmin's Guide
Learn how to use the Pexpect Python module to automate CLI interactions with FTP servers, MySQL databases, and network appliances.
Ever wanted to automate some boring tasks while managing a bench of remote machines? Today we will play with Pexpect, an “Expect-like” Python module which can be used to interact with almost any application that has a Command Line Interface (CLI).
Prerequisites
To follow along, you need:
- Python (Works with 2.7.x, though Python 3 is recommended for modern environments).
- Pexpect module: Install it via
pip install pexpect.
Pexpect has two primary interfaces:
pexpect.run(): Used to interact with the local machine to execute commands (similar toos.system()).pexpect.spawn(): Used to interact with remote services and external processes. This is the interface we will be using today.
1. Playing with FTP Servers
If you have a remote FTP server and want to programmatically download files, Pexpect handles the challenge-response pattern beautifully.
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import pexpect
child = pexpect.spawn('ftp ftp.freebsd.org')
child.expect('Name .*: ')
child.sendline('anonymous')
child.expect('Password:')
child.sendline('[email protected]')
child.expect('ftp> ')
print(child.before) # Print output before the prompt
child.sendline('cd /pub/FreeBSD')
child.expect('ftp> ')
child.sendline('get README.TXT')
child.expect('ftp> ')
child.sendline('bye')
2. Playing with MySQL Databases
Sometimes we need to generate reports from DB servers—checking hosted databases, active processes, or global variables. Pexpect can navigate the MySQL shell for you.
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import pexpect
# Replace with your actual credentials and host
child = pexpect.spawn('mysql -umyuser -pmysupersecret -hxxx.xxx.xxx.xxx')
child.expect('mysql> ')
child.sendline('show databases;')
child.expect('mysql> ')
print(child.before)
child.sendline('show processlist;')
child.expect('mysql> ')
print(child.before)
child.sendline('quit')
3. Playing with Network Appliances
Pexpect is incredibly useful for configuring network gear. In this example, we log into a Citrix NetScaler via SSH to update its hostname.
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import pexpect
# Bypass StrictHostKeyChecking for automated scripts
child = pexpect.spawn('ssh [email protected] -o StrictHostKeyChecking=no')
child.expect('Password:')
child.sendline('mysupersecret')
child.expect('.*>')
child.sendline('show Hostname')
child.expect('>')
print(child.before)
child.sendline('set HostName EDGE-Device')
child.expect('>')
child.sendline('save config')
child.expect('>')
child.sendline('quit')
Conclusion
While there are specific libraries for specific tasks (like ftplib for FTP or paramiko for SSH), Pexpect remains a powerful, generic tool in a sysadmin’s toolkit. If it has a CLI, you can automate it.
Hopefully, this helps my fellow sysadmins in automating those repetitive, boring tasks!