How to use `subprocess` command with pipes

See the documentation on setting up a pipeline using subprocess: http://docs.python.org/2/library/subprocess.html#replacing-shell-pipeline

I haven't tested the following code example but it should be roughly what you want:

query = "process_name"
ps_process = Popen(["ps", "-A"], stdout=PIPE)
grep_process = Popen(["grep", query], stdin=ps_process.stdout, stdout=PIPE)
ps_process.stdout.close()  # Allow ps_process to receive a SIGPIPE if grep_process exits.
output = grep_process.communicate()[0]

Or you can always use the communicate method on the subprocess objects.

cmd = "ps -A|grep 'process_name'"
ps = subprocess.Popen(cmd,shell=True,stdout=subprocess.PIPE,stderr=subprocess.STDOUT)
output = ps.communicate()[0]
print(output)

The communicate method returns a tuple of the standard output and the standard error.

To use a pipe with the subprocess module, you have to pass shell=True.

However, this isn't really advisable for various reasons, not least of which is security. Instead, create the ps and grep processes separately, and pipe the output from one into the other, like so:

ps = subprocess.Popen(('ps', '-A'), stdout=subprocess.PIPE)
output = subprocess.check_output(('grep', 'process_name'), stdin=ps.stdout)
ps.wait()

In your particular case, however, the simple solution is to call subprocess.check_output(('ps', '-A')) and then str.find on the output.

Using subprocess.run

import subprocess
    
ps = subprocess.run(['ps', '-A'], check=True, capture_output=True)
processNames = subprocess.run(['grep', 'process_name'],
                              input=ps.stdout, capture_output=True)
print(processNames.stdout)