running a command line containing Pipes and displaying result to STDOUT

Another way similar to Popen would be:

command=r"""cat file.log | tail -1 """
output=subprocess.check_output(command, shell=True)

This:

import subprocess
p = subprocess.Popen("cat file.log | tail -1", shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)
#for shell=False use absolute paths
p_stdout = p.stdout.read()
p_stderr = p.stderr.read()
print p_stdout

---

Or this should work:

import os
result = os.system("cat file.log | tail -1")

import subprocess
task = subprocess.Popen("cat file.log | tail -1", shell=True, stdout=subprocess.PIPE)
data = task.stdout.read()
assert task.wait() == 0

Note that this does not capture stderr. And if you want to capture stderr as well, you'll need to use task.communicate(); calling task.stdout.read() and then task.stderr.read() can deadlock if the buffer for stderr fills. If you want them combined, you should be able to use 2>&1 as part of the shell command.

But given your exact case,

task = subprocess.Popen(['tail', '-1', 'file.log'], stdout=subprocess.PIPE)
data = task.stdout.read()
assert task.wait() == 0

avoids the need for the pipe at all.

Use a subprocess.PIPE, as explained in the subprocess docs section "Replacing shell pipeline":

import subprocess
p1 = subprocess.Popen(["cat", "file.log"], stdout=subprocess.PIPE)
p2 = subprocess.Popen(["tail", "-1"], stdin=p1.stdout, stdout=subprocess.PIPE)
p1.stdout.close()  # Allow p1 to receive a SIGPIPE if p2 exits.
output,err = p2.communicate()

Or, using the sh module, piping becomes composition of functions:

import sh
output = sh.tail(sh.cat('file.log'), '-1')