Python subprocess Examples
Syntax
Here is the syntax of important subprocess functions
<status> = subprocess.call (<command array>[, <options>])
<status> = subprocess.check_call (<command array>[, <options>])
<string> = subprocess.check_output (<command array>[, <options>])
<Popen> = subprocess.Popen (<command array>[, <options>])
Basic Example
import subprocess
subprocess.call(['find', '/etc', '-name', '*.conf'])
In the example a program named ‘find’ is executed without a shell. To launch the command in a shell pass ‘shell=True’ in the list of options
subprocess.call(['find', '/'], shell=True)
Catching STDOUT
To collect all output of a command use check_output()
output = subprocess.check_output(["ls"])
Redirecting STDERR
When you only want to redirect STDERR to STDIN and catch this output too, simply add stderr=subprocess.STDOUT as an option when calling check_output()
output = subprocess.check_output(["unknown_command"], stderr=subprocess.STDOUT)
Complex Pipe Use Cases
To also handle STDIN, STDOUT and STDERR you need use Popen() and Popen.communicate() to write and read from/to those pipes.
# Launch command with all pipes connected
p = subprocess.Popen(['rm', '-i', '*'], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
# Pass exactly one 'y' and read output
(out, err) = p.communicate('y')
# Check return code
if p.returncode != 0:
print("Error!")
Don’t forget to pass a stdxx=subprocess.PIPE option for each pipe you want to use
UTF-8 conversion
To avoid later conversion of byte array you can ask Popen to do the conversion for you with option encoding='utf8':
p = subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, encoding='utf8')
Passing Environment Variables
Popen(["ls"], env={"PATH": "/usr/local/bin"})