Deleting Specific Lines from a Remote File using SSH in Python

Step 1: Understanding the Goal

Before we start coding, it's essential to understand our goal. We want to:

1. Connect to a remote server using SSH.
2. Read a file from the server.
3. Delete lines containing a specific string.
4. Save the modified file back to the server.

Full Code is available at the end of this article.

Step 2: Setting Up

To achieve our goal, we'll use the paramiko library in Python. If you haven't installed it yet, you can do so using pip:

pip install paramiko

Step 3: Starting with the Basics - The SSH Client

First, let's learn how to establish an SSH connection using paramiko

import paramiko

# Create an SSH client instance
ssh = paramiko.SSHClient()

Here, we've imported the paramiko library and created an SSH client instance.

Step 4: Trusting the Server's Host Key

Before connecting, we need to decide how to handle the server's host key. For simplicity, we'll automatically trust it (though this is insecure for real-world applications).

# Automatically add untrusted hosts (make sure okay for security policy in your environment)
ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())

Step 5: Establishing the Connection

Now, let's connect to the SSH server using our credentials.

# Connect to the server
host = "your_ubuntu_ip"
port = 22  # Default SSH port
username = "your_username"
password = "your_password"

ssh.connect(host, port, username, password)

Replace the placeholders with your server's details.

Step 6: Reading the File

Once connected, we'll use SFTP (Secure File Transfer Protocol) to read our target file.

sftp = ssh.open_sftp()
with sftp.file("/path/to/your/file.txt", 'r') as file:
    lines = file.readlines()

Replace "/path/to/your/file.txt" with the path to your target file on the server.

Step 7: Modifying the File Content

Now, let's filter out lines containing a specific string. We'll use a list comprehension for this.

target_string = "CASB"
new_lines = [line for line in lines if target_string not in line]

another version of the same code:

        # Remove lines containing the target string (case-insensitive) using a for loop
        new_lines = []
        for line in lines:
            if target_string.lower() not in line.lower():
                new_lines.append(line)

Step 8: Writing the Modified Content Back

With our modified content ready, let's write it back to the file on the server.

with sftp.file("/path/to/your/file.txt", 'w') as file:
    file.writelines(new_lines)

Step 9: Cleaning Up After our operations, it's a good practice to close the connections.

sftp.close()
ssh.close()

Step 10: Handling Exceptions

In real-world scenarios, things might not always go as planned. It's crucial to handle exceptions to understand any issues that arise.

Wrap the entire code inside a try...except block to catch and print any errors:

try:
    # [All the above code]
except Exception as e:
    print(f"An error occurred: {e}")

Full Code

ssh_file_line_deletion.py

import paramiko

def ssh_delete_line_with_string(host, port, username, password, file_path, target_string):
    try:
        # Create an SSH client instance
        ssh = paramiko.SSHClient()

        # Automatically add the server's host key (this is insecure and used for demonstration; in a real-world scenario, you'd verify the host key)
        ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())

        # Connect to the SSH server
        ssh.connect(host, port, username, password)

        # Fetch the file content
        sftp = ssh.open_sftp()
        with sftp.file(file_path, 'r') as file:
            lines = file.readlines()

        # Remove lines containing the target string (case-insensitive) using a for loop
        new_lines = []
        for line in lines:
            if target_string.lower() not in line.lower():
                new_lines.append(line)
                
        # Write the updated content back to the file
        with sftp.file(file_path, 'w') as file:
            file.writelines(new_lines)

        print(f"Lines containing '{target_string}' (case-insensitive) have been deleted from {file_path}!")

        # Close the SFTP and SSH connections
        sftp.close()
        ssh.close()

    except Exception as e:
        print(f"An error occurred: {e}")


# Usage
host = "your_target_server_ip"
port = 22  # Default SSH port
username = "your_username"
password = "your_password"
file_path = "/home/nishanth/Desktop/tobedeleted/delete.txt"
target_string = "casb"

ssh_delete_line_with_string(host, port, username, password, file_path, target_string)