Shell piping with subprocess in Python

Question

I read every thread I found on StackOverflow on invoking shell commands from Python using subprocess, but I couldn't find an answer that applies to my situation below:

I would like to do the following from Python:

Run shell command command_1. Collect the output in variable result_1
Shell pipe result_1 into command_2 and collect the output on result_2. In other words, run command_1 | command_2 using the result that I obtained when running command_1 in the step before
Do the same piping result_1 into a third command command_3 and collecting the result in result_3.

So far I have tried:

p = subprocess.Popen(command_1, stdout=subprocess.PIPE, shell=True)

result_1 = p.stdout.read();

p = subprocess.Popen("echo " + result_1 + ' | ' + 
command_2, stdout=subprocess.PIPE, shell=True)

result_2 = p.stdout.read();

the reason seems to be that "echo " + result_1 does not simulate the process of obtaining the output of a command for piping.

Is this at all possible using subprocess? If so, how?

See [this example in the docs](http://docs.python.org/library/subprocess.html#replacing-shell-pipeline) for the right way to do it. — Sven Marnach, Mar 03 '12 at 01:00
Thanks @SvenMarnach, would that still allow me to collect the output of the first command in a Python variable? — Amelio Vazquez-Reina, Mar 03 '12 at 01:03

score 8 · Accepted Answer · answered Mar 03 '12 at 01:31

8

You can do:

pipe = Popen(command_2, shell=True, stdin=PIPE, stdout=PIPE)
pipe.stdin.write(result_1)
pipe.communicate()

instead of the line with the pipe.

answered Mar 03 '12 at 01:31

Daniel

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This looks great. How would the above change if I want to pipe `result_1` *again* to another command? – Amelio Vazquez-Reina Mar 03 '12 at 01:46
1

At that point `result_1` is a string. You should be able to repeat the same 3 lines with the new command. – Daniel Mar 04 '12 at 06:21

Shell piping with subprocess in Python

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