Python3 subprocess output

I suggest that you use subprocess.getoutput() as it does exactly what you want—run a command in a shell and get its string output (as opposed to byte string output). Then you can split on whitespace and grab the first element from the returned list of strings.

Try this:

import subprocess
stdoutdata = subprocess.getoutput("wc --lines /var/log/syslog")
print("stdoutdata: " + stdoutdata.split()[0])

Since Python 3.6 you can make check_output() return a str instead of bytes by giving it an encoding parameter:

check_output('wc --lines /var/log/syslog', encoding='UTF-8')

But since you just want the count, and both split() and int() are usable with bytes, you don't need to bother with the encoding:

linecount = int(check_output('wc -l /var/log/syslog').split()[0])

While some things might be easier with an external program (e.g., counting log line entries printed by journalctl), in this particular case you don't need to use an external program. The simplest Python-only solution is:

with open('/var/log/syslog', 'rt') as f:
    linecount = len(f.readlines())

This does have the disadvantage that it reads the entire file into memory; if it's a huge file instead initialize linecount = 0 before you open the file and use a for line in f: linecount += 1 loop instead of readlines() to have only a small part of the file in memory as you count.

Equivalent to Curt J. Sampson's answer is also this one (it's returning a string):

subprocess.check_output('wc -l /path/to/your/file | cut -d " " -f1', universal_newlines=True, shell=True)

from docs:

If encoding or errors are specified, or text is true, file objects for stdin, stdout and stderr are opened in text mode using the specified encoding and errors or the io.TextIOWrapper default. The universal_newlines argument is equivalent to text and is provided for backwards compatibility. By default, file objects are opened in binary mode.

Something similar, but a bit more complex using subprocess.run():

subprocess.run(command, shell=True, check=True, universal_newlines=True, stdout=subprocess.PIPE).stdout

as subprocess.check_output() could be equivalent to subprocess.run().

To avoid invoking a shell and decoding filenames that might be an arbitrary byte sequence (except '\0') on *nix, you could pass the file as stdin:

import subprocess

with open(b'/var/log/syslog', 'rb') as file:
    nlines = int(subprocess.check_output(['wc', '-l'], stdin=file))
print(nlines)

Or you could ignore any decoding errors:

import subprocess

stdoutdata = subprocess.check_output(['wc', '-l', '/var/log/syslog'])
nlines = int(stdoutdata.decode('ascii', 'ignore').partition(' ')[0])
print(nlines)