FileNotFoundError in subprocess: Command Not Found

Why This Happens

subprocess.run() looks for the executable on the system PATH. If not installed or misspelled, it raises FileNotFoundError. On Linux and macOS the same error appears when a command with arguments is passed as one string without shell=True: subprocess.run('ls -l') makes Python look for an executable literally named 'ls -l'.

The Problem

import subprocess
result = subprocess.run(['my-script', '--flag'])

The Fix

import subprocess, shutil
if shutil.which('my-script'):
    result = subprocess.run(['my-script', '--flag'])
else:
    print('my-script is not installed')

Step-by-Step Fix

1. 1

   Verify command exists

   Use shutil.which('command') to check.

2. 2

   Use full path

   Specify the complete path to the executable.

3. 3

   Pass a list, not a string

   Split the command into a list such as ['ls', '-l'] (shlex.split() helps), or the whole string is treated as the program name.

4. 4

   Check shell features

   Use shell=True for pipes/redirects, but avoid with untrusted input.