How to Resolve Python Error "OSError: [Errno 8] Exec format error"

When attempting to execute an external file (often a shell script .sh or another executable) from within Python using modules like subprocess, you might encounter OSError: [Errno 8] Exec format error. Similar to the Docker error with the same message, this indicates that the operating system kernel cannot recognize the format of the file you're trying to run as a valid executable for the current system.

This guide covers the common causes of this error in a general Python context – missing shebangs, lack of execute permissions, and file corruption/type issues – and provides solutions.

Understanding the Error: Kernel Executable Recognition

The OSError: [Errno 8] Exec format error originates from the operating system's kernel. When Python's subprocess.run(), subprocess.Popen(), subprocess.call(), or os.system() attempts to execute an external file directly (e.g., ./my-script.sh), it asks the kernel to run that file. The kernel examines the first few bytes of the file to determine how to execute it:

• If it starts with specific "magic bytes" identifying a native binary executable format (like ELF on Linux), it loads and runs it.
• If it starts with the shebang sequence (#!), it uses the interpreter specified after the shebang (e.g., /bin/bash, /bin/sh, /usr/bin/env python) to run the script.
• If neither of the above matches, the kernel doesn't know how to execute the file and reports the "Exec format error".

Cause 1: Missing or Incorrect Shebang (#!) in Script

For script files (like .sh, .py, .pl, etc.) that you intend to execute directly, the absence of a valid shebang line at the very beginning is a primary cause of this error. Without it, the kernel treats the script as an unknown binary format.

# my-script.sh (Error Scenario - Missing Shebang)
echo "Hello from script!"
exit 0

Example of Error Scenario:

import subprocess
import os

script_path = './my-script.sh'

# Assume my-script.sh has execute permissions (chmod +x) but NO shebang
# Make executable (Linux/macOS)
# try:
#     os.chmod(script_path, 0o755)
# except Exception: pass # Ignore if already executable or on Windows

try:
    # ⛔️ OSError: [Errno 8] Exec format error: './my-script.sh'
    # Trying to execute the script directly without a shebang fails.
    print(f"Attempting to run {script_path} directly...")
    result = subprocess.run([script_path], capture_output=True, text=True, check=True)
    print("Script output:", result.stdout)
except OSError as e:
    if e.errno == 8: # Check if it's specifically Exec format error
        print(f"Caught expected error: {e}")
    else:
        print(f"Caught other OSError: {e}")
except Exception as e:
    print(f"Caught unexpected error: {e}")

Solution 1: Add/Correct the Shebang in the Script

Edit the script file (e.g., my-script.sh) and add the appropriate shebang as the very first line.

• For standard shell scripts (Bash/Sh):

  #!/bin/sh
  # OR
  #!/bin/bash
  # OR (often more portable)
  #!/usr/bin/env sh
  # OR
  #!/usr/bin/env bash
  
  echo "Hello from script!"
  exit 0

• For Python scripts intended to be executable:

  #!/usr/bin/env python3
  # OR
  #!/usr/bin/env python
  
  print("Hello from Python script!")

Important:

• The #! must be the absolute first two bytes. No preceding spaces or lines.
• Ensure the interpreter path following #! (e.g., /bin/sh, /usr/bin/env) is valid on the system where the script will run.
• After adding the shebang, you might still need to ensure the file has execute permissions (Solution 2).

Cause 2: Missing Execute Permissions (Linux/macOS)

On Unix-like systems (Linux, macOS), files need explicit "execute" permission to be run directly as programs or scripts. Even with a correct shebang, if the permission isn't set, you might get a "Permission denied" error, but sometimes it can also manifest as or be related to an "Exec format error" under certain circumstances or with specific execution methods.

Solution 2: Grant Execute Permissions (chmod +x)

Use the chmod command in your terminal to add execute permission for the user (and optionally group/others).

# Give the owner execute permission
chmod u+x my-script.sh

# Or give owner, group, and others execute permission (common)
chmod +x my-script.sh

This step is necessary on Linux and macOS in addition to having a correct shebang if you intend to run the script directly like ./my-script.sh or pass just the script path to subprocess.run([script_path]).

Cause 3: Incorrect File Type or Corruption

The file you are trying to execute might not actually be the script you think it is, or it might be corrupted.

• Wrong File Type: You might be trying to execute a data file, an image, a library (.so, .dll), or some other non-executable file type.
• Corruption: The file might have been damaged during download, transfer, or saving, making its initial bytes unrecognizable to the kernel.
• Architecture Mismatch (Less Common outside Docker): Trying to run a binary compiled for a different CPU architecture (e.g., ARM binary on an x86 machine) can also cause this error.

Solution 3: Verify File Type and Integrity

• Check File Type (Linux/macOS): Use the file command:

  file my-script.sh
  The output should indicate it's a shell script (e.g., my-script.sh: Bourne-Again shell script, ASCII text executable) or Python script, etc. If it shows something else (data, ELF 64-bit LSB executable, image data, etc.), you're pointing to the wrong file or it's corrupted.
• Inspect Content: Open the file in a text editor. Does it start with a valid shebang and contain the expected script code? Are there unusual characters, especially near the beginning?
• Re-download/Re-create: If corruption is suspected, obtain a fresh copy of the file.

Solution 4: Explicitly Specify Interpreter in subprocess Call

You can often bypass the need for a shebang and execute permissions within the script file itself by telling subprocess which interpreter to use to run the script. Pass the interpreter as the first argument in the command list.

# main_explicit_interpreter.py
import subprocess

script_path = './my-script.sh' # Assume this script has NO shebang

try:
    # ✅ Explicitly tell subprocess to use 'sh' to run the script
    print(f"Attempting to run {script_path} using 'sh'...")
    result = subprocess.run(['sh', script_path], capture_output=True, text=True, check=True)
    # Or ['bash', script_path] if it's a bash script
    # Or ['python3', script_path] for a python script

    print("Script ran successfully via explicit interpreter.")
    print("Script output:", result.stdout)
except FileNotFoundError:
    print(f"Error: Interpreter ('sh', 'bash', 'python3') not found or script '{script_path}' not found.")
except subprocess.CalledProcessError as e:
    print(f"Script executed with an error: {e}")
    print(f"Stderr:\n{e.stderr}")
except Exception as e:
    print(f"Caught unexpected error: {e}")

This approach directly invokes the interpreter (e.g., sh) and passes the script file as an argument to it. The kernel only needs to execute the interpreter (sh), which it usually knows how to do. The interpreter then reads and executes the script file. This often avoids the Exec format error even if the script lacks a shebang or execute permission (though the interpreter still needs read permission on the script).

Conclusion

The OSError: [Errno 8] Exec format error in Python signals that the operating system cannot execute a specified file directly.

The primary solutions are:

1. Add/Fix Shebang: Ensure the target script file begins with the correct #! line pointing to a valid interpreter (e.g., #!/bin/sh, #!/usr/bin/env python3).
2. Set Execute Permissions: On Linux/macOS, use chmod +x your_script.sh to make the file executable.
3. Verify File Integrity: Ensure the file is not corrupted and is the correct type (a script, not binary data).
4. Explicit Interpreter: Call the interpreter directly within your subprocess command and pass the script path as an argument (e.g., subprocess.run(['sh', './my-script.sh'])). This often bypasses shebang/permission issues for direct script execution.

By checking these points, you can usually resolve the exec format error and successfully execute your external files from Python.