OSError: [Errno 8] Exec format error

Question

I am having hard time parsing the arguments to subprocess.Popen. I am trying to execute a script on my Unix server. The script syntax when running on shell prompt is as follows: /usr/local/bin/script hostname = <hostname> -p LONGLIST. No matter how I try, the script is not running inside subprocess.Popen

The space before and after "=" is mandatory.

import subprocess Out = subprocess.Popen(['/usr/local/bin/script', 'hostname = ', 'actual server name', '-p', 'LONGLIST'],shell=True,stdout=subprocess.PIPE,stderr=subprocess.PIPE) 

The above does not work.

And when I use shell=False, I get OSError: [Errno 8] Exec format error

Answer 1

OSError: [Errno 8] Exec format error can happen if there is no shebang line at the top of the shell script and you are trying to execute the script directly. Here's an example that reproduces the issue:

>>> with open('a','w') as f: f.write('exit 0') # create the script ...  >>> import os >>> os.chmod('a', 0b111101101) # rwxr-xr-x make it executable                        >>> os.execl('./a', './a')     # execute it                                             Traceback (most recent call last):   File "<stdin>", line 1, in <module>   File "/usr/lib/python2.7/os.py", line 312, in execl     execv(file, args) OSError: [Errno 8] Exec format error 

To fix it, just add the shebang e.g., if it is a shell script; prepend #!/bin/sh at the top of your script:

>>> with open('a','w') as f: f.write('#!/bin/sh\nexit 0') ...  >>> os.execl('./a', './a') 

It executes exit 0 without any errors.

---

On POSIX systems, shell parses the command line i.e., your script won't see spaces around = e.g., if script is:

#!/usr/bin/env python import sys print(sys.argv) 

then running it in the shell:

$ /usr/local/bin/script hostname = '<hostname>' -p LONGLIST 

produces:

['/usr/local/bin/script', 'hostname', '=', '<hostname>', '-p', 'LONGLIST'] 

Note: no spaces around '='. I've added quotes around <hostname> to escape the redirection metacharacters <>.

To emulate the shell command in Python, run:

from subprocess import check_call  cmd = ['/usr/local/bin/script', 'hostname', '=', '<hostname>', '-p', 'LONGLIST'] check_call(cmd) 

Note: no shell=True. And you don't need to escape <> because no shell is run.

"Exec format error" might indicate that your script has invalid format, run:

$ file /usr/local/bin/script 

to find out what it is. Compare the architecture with the output of:

$ uname -m 

Answer 2

I will hijack this thread to point out that this error may also happen when target of Popen is not executable. Learnt it hard way when by accident I have had override a perfectly executable binary file with zip file.