Python3 subprocess output

Solution 1:

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])

Solution 2:

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.

Solution 3:

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)