Fix SSHLibrary vs Paramiko Output Mismatch: 3 Steps 2025

Understanding the Frustration: When SSH Outputs Don't Match

You've been there. You write a simple Python script using the powerful Paramiko library to execute a command on a remote server. It works flawlessly. You then translate that same logic into a Robot Framework test case using SSHLibrary. You run the test, and it fails. The assertion screams that the output from your command doesn't match the expected string. You stare at the two outputs, and they look identical. This maddening scenario is a common pitfall for automation engineers, and it almost always stems from subtle, invisible differences in how these two tools handle SSH sessions.

SSHLibrary is, in fact, a high-level wrapper around Paramiko, designed to simplify SSH operations within the Robot Framework ecosystem. However, this abstraction comes with its own set of default behaviors that can differ significantly from a direct, bare-bones Paramiko implementation. This guide will demystify why this mismatch occurs and provide a clear, three-step process to diagnose and resolve it for good in 2025, ensuring your automated tests are robust and reliable.

The Root Cause: Why Do SSHLibrary and Paramiko Outputs Differ?

To fix the problem, we first need to understand its origins. The discrepancy isn't a bug; it's a feature of differing defaults and design philosophies between a low-level library and a high-level framework keyword.

The Wrapper Effect of SSHLibrary

Think of SSHLibrary as a helpful assistant that handles the tedious parts of SSH for you. It manages connections, handles timeouts, and formats output for easy use in test cases. This convenience layer, however, makes certain decisions on your behalf. It assumes you often want to simulate an interactive user session, which is fundamentally different from the non-interactive, programmatic execution that a direct Paramiko script typically performs.

The Prime Suspect: Pseudo-Terminal (PTY) Allocation

The single most common reason for output mismatch is Pseudo-Terminal (PTY) allocation. A PTY emulates a physical terminal device, like the one you use when you SSH into a server yourself. It's what allows for features like command history, line editing, and colored output.

• SSHLibrary's `Execute Command` keyword enables PTY by default (`pty=True`). This makes the remote server think it's talking to an interactive user. As a result, it might use different line endings (\r\n instead of just \n) and embed ANSI escape codes for color.
• Paramiko's `exec_command()` method disables PTY by default (`get_pty=False`). It assumes a non-interactive, script-based execution. The output is raw, uncolored, and typically uses standard Unix-style line endings (\n).

These invisible characters (\r and ANSI codes) are the reason your outputs look the same visually but fail a string comparison.

Shells and Environment Variables

An interactive session (with PTY) often sources different startup files (like .bashrc or .zshrc) than a non-interactive session (which might only source .bash_profile or nothing at all). This can lead to different $PATH variables, command aliases, or other environment settings that subtly alter a command's behavior and output.

The 3-Step Fix for Output Mismatch in 2025

Now that we understand the 'why', let's get to the 'how'. Follow these three steps to systematically find and fix any output discrepancy.

Step 1: Diagnose the Discrepancy with `repr()`

Your eyes can deceive you. The first step is to see exactly what your program sees. The built-in `repr()` function in Python is your best friend here, as it provides the unambiguous string representation of an object, revealing all hidden characters.

In Robot Framework (SSHLibrary):

Use the `Evaluate` keyword to wrap your output variable in `repr()` and log it.

*** Test Cases ***
Inspect SSH Output
    Open Connection    your_host
    Login    your_user    your_password
    ${output}=    Execute Command    ls -l /tmp
    ${output_repr}=    Evaluate    repr($output)
    Log    ${output_repr}
    [Teardown]    Close All Connections

The log will show you something like 'total 0\r\n-rw-r--r-- 1 user group 0 Jan 15 10:00 file1.txt\r\n', immediately exposing the carriage returns (\r).

In Python (Paramiko):

Do the same before printing or comparing the output.

import paramiko

ssh = paramiko.SSHClient()
ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
ssh.connect('your_host', username='your_user', password='your_password')

stdin, stdout, stderr = ssh.exec_command('ls -l /tmp')
output_bytes = stdout.read()
output_str = output_bytes.decode('utf-8')

print(repr(output_str))

ssh.close()

This will likely show 'total 0\n-rw-r--r-- 1 user group 0 Jan 15 10:00 file1.txt\n', confirming the difference in line endings.

Step 2: Align PTY Allocation

Once you've confirmed a PTY-related issue, the fix is to make both methods behave the same way. Since automated scripts are typically non-interactive, the best practice is to disable PTY in SSHLibrary.

To do this, simply set the pty argument to `False` in your `Execute Command` keyword.

*** Keywords ***
Execute Command Without PTY
    [Arguments]    ${command}
    ${output}=    Execute Command    ${command}    pty=${False}
    [Return]    ${output}

By adding pty=${False}, you are telling SSHLibrary to behave like the default Paramiko `exec_command`, eliminating the primary source of extra characters and different shell behaviors. In over 90% of cases, this single change will resolve the output mismatch.

If, for some reason, you need to emulate an interactive session in your Paramiko script, you can do the reverse by setting `get_pty=True` in `exec_command`, though this is less common for typical automation tasks.

Step 3: Normalize and Sanitize the Output

In the rare cases where aligning PTY isn't enough or isn't desirable, your final line of defense is to programmatically clean up the output before you perform any assertions. This involves creating a utility keyword or function to normalize the string.

A good normalization routine should:

• Normalize all line endings (e.g., convert \r\n to \n).
• Strip leading and trailing whitespace.
• Optionally, remove ANSI color codes using a regular expression.

Normalization Keyword in Robot Framework:

You can create a custom Python keyword and import it into your suite.

File: StringUtils.py

import re

def normalize_ssh_output(text):
    # Remove ANSI escape codes
    ansi_escape = re.compile(r'\x1B(?:[@-Z\\-_]|\[[0-?]*[ -/]*[@-~])')
    text = ansi_escape.sub('', text)
    # Normalize line endings
    text = text.replace('\r\n', '\n')
    # Strip whitespace
    return text.strip()

In your Robot file:

*** Settings ***
Library    StringUtils.py

*** Test Cases ***
Test With Normalized Output
    ${raw_output}=    Execute Command    some_colorful_command
    ${clean_output}=    Normalize SSH Output    ${raw_output}
    Should Be Equal    ${clean_output}    expected clean string

This three-step process—Diagnose, Align, Normalize—provides a robust framework for tackling any output mismatch you encounter.

Practical Case Study: Fixing a Failing `ls -l` Test

Let's see this in action. Imagine a test that verifies the permissions of a specific file.

The Failing Test

*** Test Cases ***
Verify File Permissions
    Open Connection    your_host
    Login    your_user    your_password
    ${output}=    Execute Command    ls -l /tmp/important.dat
    Should Be Equal    ${output}    -rw------- 1 automation_user dev 1024 Jan 15 11:30 /tmp/important.dat
    [Teardown]    Close All Connections

This test fails with a message: '\r\n-rw------- 1 automation_user dev 1024 Jan 15 11:30 /tmp/important.dat\r\n' != '-rw------- 1 automation_user dev 1024 Jan 15 11:30 /tmp/important.dat'

Applying the 3-Step Solution

1. Diagnose: The failure message itself, or logging `repr(${output})`, clearly shows the leading and trailing `\r\n`. This points directly to a PTY issue.
2. Align: We modify the `Execute Command` call to disable the PTY. This is the simplest and most direct fix.
3. Normalize: This step is unnecessary here because aligning the PTY solves the root problem.

The Corrected, Passing Test

*** Test Cases ***
Verify File Permissions
    Open Connection    your_host
    Login    your_user    your_password
    # The fix is adding pty=${False}
    ${output}=    Execute Command    ls -l /tmp/important.dat    pty=${False}
    # The output from the command might still have a trailing newline, so we strip it.
    ${stripped_output}=    Evaluate    $output.strip()
    Should Be Equal    ${stripped_output}    -rw------- 1 automation_user dev 1024 Jan 15 11:30 /tmp/important.dat
    [Teardown]    Close All Connections

By adding pty=${False} and stripping the single trailing newline that `ls` produces, the test now passes reliably. We have successfully aligned SSHLibrary's behavior with our programmatic expectations.

Conclusion: Achieving Consistent and Reliable SSH Automation

The perceived flakiness between SSHLibrary and Paramiko is not due to bugs, but to a fundamental difference in their default configurations, primarily concerning PTY allocation. By understanding that SSHLibrary aims for interactive-session simulation while Paramiko defaults to raw, non-interactive execution, you can anticipate and control their behavior.

By following the three-step method of Diagnosing with `repr()`, Aligning PTY settings, and Normalizing output as a fallback, you can eliminate these frustrating mismatches. This ensures your Robot Framework tests are not only accurate but also robust and maintainable, leading to more trustworthy automation pipelines and less time spent debugging invisible characters.