如何让Popen等待命令完成？

Question

The objective is that a command is launched in a bash shell with Popen, and then the output of this command is passed to a variable, but Popen should wait until the command finishes before moving on, which doesn't happen.

I tried using call instead of Popen, but call doesn't output anything but the exit code, which isn't ideal. I tried using wait() too, but I get an error.

var1 = Popen(["command | grep 'stuff' | cut -d 'delimiter' -f3"], 
             shell=True, stdout=PIPE,
             universal_newlines=True).communicate()[0].rstrip()
var2 = Popen(["command | grep 'stuff' | cut -d 'delimiter' -f1"], 
             shell=True, stdout=PIPE,
             universal_newlines=True).communicate()[0].rstrip()
print(var1 + var2)

Python prints var1 correctly, but not var2, which is None because it was not assigned by anything since the command didn't run or wasn't allowed time to finish. Running the var2 command directly on the terminal, exactly as in the python program, yields the desired results, but when python tries to run it, it simply skips it and prints var1 and var2, but only the var1 string is displayed, var2 is completely ignored.

Answer 1

The AttributeError: 'str' object has no attribute 'wait' error occurs because the communicate() method returns a tuple of (stdout, stderr) strings. wait() is a method on the Popen object itself. You don't need to call wait() if you use communicate() , since it will do the wait for you. And you can't call wait() on the Popen object if you throw away the only reference to it.

That said, do you really need to use grep and cut to accomplish the job? Something like this is likely going to be faster and less likely to contain a security bug:

p = Popen(['command'], stdout=PIPE, universal_newlines=True)
result = []
for line in p.stdout:
    if 'stuff' in line:
        value = line.split('delimiter')[4]
        result.append(value)

Answer 2

You have to pass the command as a string, or parse it into tokens yourself. You'll want to use subprocess.run instead of the low-level Popen . Trivially

var = subprocess.run("command | grep 'stuff' | cut -d 'delimiter' -f3", 
         shell=True, stdout=PIPE,
         universal_newlines=True).stdout

A much better solution is to replace grep and cut with Python code, and get rid of the shell=True as a useful bonus.

var = list()
output = subprocess.run(["command"], stdout=PIPE, universal_newlines=True)
for line in output.stdout.split('\n'):
    if 'stuff' in line:
        var.append(line.split('delimiter')[2])