python subprocess output without \n

Question

Here is a simple script running subprocess that retrieves IP from the ifconfig command output from the terminal. I have noticed that subprocess.check_output() always returns a value with \n.

I desire to get a return value without \n. How can this be done?

$ python
>>> import subprocess
>>> subprocess.check_output("ifconfig en0 | awk '{ print $2}' | grep -E -o '([0-9]{1,3}[\.]){3}[0-9]{1,3}'", shell=True)
'129.194.246.245\n'

Answer 1

For a generic way :

subprocess.check_output("echo hello world", shell=True).strip()

Answer 2

subprocess.check_output() does not add a newline. echo does. You can use the -n switch to suppress the newline, but you have to avoid using the shell built-in implementation (so use /bin/echo):

>>> import subprocess
>>> subprocess.check_output('/bin/echo -n hello world', shell=True)
'hello world'

If you use echo -n instead, you could get the string '-n hello world\n', as not all sh implementations support the -n switch support echo (OS X for example).

You could always use str.rstrip() or str.strip() to remove whitespace, of course, but don't blame subprocess here:

>>> subprocess.check_output('echo hello world', shell=True).rstrip('\n')
'hello world'

Your question update added a more complex example using awk and grep:

subprocess.check_output("ifconfig en0 | awk '{ print $2}' | grep -E -o '([0-9]{1,3}[\.]){3}[0-9]{1,3}'", shell=True)

Here grep adds the (final) newline. grep -o may print just the matching text, but still adds a newline to separate matches. See the grep manual:

-o
--only-matching

Print only the matched (non-empty) parts of matching lines, with each such part on a separate output line.

Emphasis mine.

You can add a tr -d '\n' at the end to remove any newlines from the output of your pipe:

>>> subprocess.check_output(
...     "ifconfig en0 | awk '{ print $2}' | "
...     "grep -E -o '([0-9]{1,3}[\.]){3}[0-9]{1,3}' | "
...     "tr -d '\n'", shell=True)
'172.17.174.160'