SSHLibrary vs Paramiko: 5 Reasons for Output Bugs (2025)

Introduction: The Mystery of Garbled SSH Output

You've been there. You write a seemingly simple Python script or Robot Framework test case to execute a command on a remote server. You expect clean, predictable output, but instead, you get a garbled mess, truncated text, or worse—nothing at all. Your command works perfectly when you SSH manually, so what gives? This frustrating experience is a rite of passage for many developers and automation engineers working with SSH in Python. The culprits are often the two most popular libraries for the job: Paramiko and SSHLibrary.

While SSHLibrary is built on top of Paramiko, their different levels of abstraction create unique pitfalls. In this deep dive, we'll uncover the five most common reasons you're encountering SSH output bugs in 2025 and provide clear, actionable solutions for both libraries.

Understanding the Tools: SSHLibrary vs. Paramiko

Before diagnosing the problems, it's crucial to understand the relationship between these two tools. They are not direct competitors but rather a foundation and a framework built upon it.

Paramiko: The Low-Level Foundation

Paramiko is a pure Python implementation of the SSHv2 protocol. It provides the fundamental building blocks for creating SSH clients and servers. When you use Paramiko directly, you have granular control over every aspect of the connection: channel types, authentication methods, PTY allocation, and raw data streams. This power comes with responsibility—you are responsible for handling buffers, decoding output, and managing separate stdout and stderr streams.

SSHLibrary: The High-Level Abstraction

SSHLibrary is a test library for Robot Framework that simplifies remote operations via SSH. Its primary goal is to provide easy-to-use keywords like Execute Command or File Should Exist. It uses Paramiko under the hood, abstracting away much of its complexity. This convenience is fantastic for rapid test development, but the abstraction can obscure what's really happening, making it difficult to debug when output doesn't behave as expected.

5 Common Causes for SSH Output Bugs (2025 Update)

Most output-related bugs stem from a mismatch between how your script *expects* the remote command to behave and how the SSH channel *actually* delivers the output. Let's break down the top five causes.

Reason 1: Pseudo-Terminal (PTY) Allocation Mismatches

A pseudo-terminal (PTY) emulates a real, interactive terminal for the command you're executing. Some programs change their behavior dramatically depending on whether a PTY is present.

• Without a PTY: Output is typically raw, uncolored, and designed for piping to other commands.
• With a PTY: Output may include ANSI escape codes (for colors), progress bars, and different line buffering, designed for human interaction.

The Bug: Your script expects raw text, but a PTY is allocated (or vice-versa), resulting in output polluted with control characters (e.g., [?25l) or missing data because the command is waiting for interactive input that never comes.

In Paramiko: You have direct control with the get_pty=True/False flag in exec_command. The default is False.

In SSHLibrary: The Open Connection keyword has a terminal argument (defaulting to False). Setting terminal=True requests a PTY. A common mistake is running a command that requires a PTY (like one with a password prompt) without enabling this setting.

Reason 2: Buffer Sizing and Read Timeouts

Data over SSH is sent in chunks and stored in a buffer on the client side. If you don't read this buffer correctly, you'll get incomplete output.

The Bug: Your script executes a command that produces a large amount of output. Your code reads only the first chunk from the buffer and then exits, leading you to believe the command produced less output than it did. This is especially common with commands that run for a long time, like watching a log file.

In Paramiko: The stdout.read() or stdout.recv() methods are your responsibility. A naive stdout.read(1024) will only read up to 1024 bytes. For long-running or large-output commands, you must loop and continue reading from the channel until it's closed.

In SSHLibrary: The Execute Command keyword tries to handle this by reading until the command finishes and the exit status is received. However, if the command hangs or produces output slowly, SSHLibrary's internal timeout might trigger, cutting off the output collection prematurely.

Reason 3: Mishandling of stdout vs. stderr Streams

In Linux/Unix environments, processes have two primary output streams: standard output (stdout) for normal output and standard error (stderr) for errors and diagnostic messages. SSH channels maintain this separation.

The Bug: Your command fails, printing a crucial error message to stderr. Your script, however, is only reading from stdout. The result is an empty output string and a potentially misleading successful exit code (if the error was non-fatal), leaving you completely in the dark about the failure.

In Paramiko: exec_command returns three objects: stdin, stdout, stderr. It is your job to read from both stdout.read() and stderr.read() to get a complete picture.

In SSHLibrary: By default, Execute Command only returns stdout. This is a massive pitfall. To avoid this, you must use the return_stderr=True argument. This will make the keyword return both stdout and stderr as a tuple, allowing you to inspect errors properly.

Reason 4: The Perils of Character Encoding

Computers store text as bytes. Encoding is the rulebook for converting those bytes back into human-readable characters. A mismatch in rulebooks leads to gibberish.

The Bug: The remote server sends output encoded in latin-1, but your script tries to decode it as UTF-8 (the de-facto standard). This results in a UnicodeDecodeError crash or, worse, silently corrupts characters, turning accented letters or special symbols into garbage like ``.

In Paramiko: You receive raw bytes. You are responsible for decoding them correctly, e.g., output = stdout.read().decode('utf-8'). You must know the encoding of the remote system.

In SSHLibrary: The Open Connection keyword accepts an encoding argument, which defaults to UTF-8. If your remote systems use a different default encoding, you must set this argument correctly during the connection setup to prevent mangled output across all subsequent commands.

Reason 5: The 'Shell' vs. 'Exec' Channel Dilemma

SSH has two primary modes for running commands: 'exec' and 'shell'.

• 'exec' channel: Executes a single, non-interactive command. The environment is clean and minimal. It does not load shell profiles like .bashrc or .profile.
• 'shell' channel: Invokes an interactive login shell. This loads all the user's environment variables, aliases, and functions from their profile scripts. It's stateful.

The Bug: Your command relies on an alias or an environment variable set in .bashrc. You run it using a standard 'exec' channel, which doesn't load the profile. The command fails with a "command not found" or similar error because its required environment is missing.

In Paramiko: You choose explicitly between client.exec_command() and client.invoke_shell(). The difference is clear.

In SSHLibrary: This is more subtle and a common source of confusion. Execute Command uses an 'exec' channel. If you need a stateful shell that loads a profile, you must use keywords like Login or Start Command, which utilize a 'shell' channel. Trying to run `cd /tmp` with one `Execute Command` and then `ls` with another will not work as you expect, because each 'exec' channel is a separate, stateless session.

At a Glance: SSHLibrary vs. Paramiko Output Handling

Practical Solutions and Best Practices

For Paramiko Users

• Always Check stderr: Even if you don't expect errors, read from the stderr stream. It's your primary diagnostic tool.
• Decode Explicitly: Know your target system's encoding and decode the byte streams from stdout and stderr with a try...except UnicodeDecodeError block.
• Loop for Large Output: Don't assume .read() will get everything. Use a loop with .recv() and check .exit_status_ready() to know when the command is truly finished.
• Choose Your Channel Wisely: Use exec_command() for simple, stateless commands. Use invoke_shell() only when you truly need an interactive, stateful session.

For SSHLibrary Users

• Always Use `return_stderr=True`: Make it a habit. Add return_stderr=True to every Execute Command call and assert that the returned stderr is empty.
• Set Encoding and Terminal on Connect: Don't rely on defaults. Explicitly set `encoding` and `terminal` in your Open Connection keyword based on the target system and command requirements.
• Understand Your Keywords: Know that Execute Command is for single, stateless commands. If your tests require a persistent environment (like setting variables or changing directories), switch to a stateful pattern using Login, Write, and Read.
• Increase Timeouts for Long Jobs: If a command is expected to run for a long time, increase the timeout in `Open Connection` or on the `Execute Command` keyword itself to prevent SSHLibrary from giving up too early.

Conclusion: Control vs. Convenience

The frustrating world of SSH output bugs almost always boils down to the abstractions of the SSH protocol. Paramiko gives you transparent, direct control, forcing you to confront these complexities head-on. SSHLibrary gives you wonderful convenience, but its abstractions can hide the root cause of a problem.

By understanding these five core issues—PTY allocation, buffering, stream separation, encoding, and channel types—you are equipped to diagnose and solve problems in both libraries. You can now make an informed choice: use Paramiko for fine-grained control or configure SSHLibrary defensively to get the best of both worlds—simplicity and robustness.