Python - Run process and wait for output

Question

I want to run a program, wait for it's output, send inputs to it and repeat until a condition.

All I could find was questions about waiting for a program to finish, which is NOT the case. The process will still be running, it just won't be giving any (new) outputs.

Program output is in stdout and in a log file, either can be used.
Using linux.

Code so far:

import subprocess

flag = True
vsim = subprocess.popen(['./run_vsim'], 
                        stdin=subprocess.pipe,
                        shell=true, 
                        cwd='path/to/program')
while flag:
    with open(log_file), 'r') as f:
        for l in f:
            if condition:
                break
    vsim.stdin.write(b'do something\n')
    vsim.stdin.flush()
vsim.stdin.write(b'do something else\n')
vsim.stdin.flush()

As is, the "do something" input is being sent multiple times even before the program finished starting up. Also, the log file is read before the program finishes running the command from the last while iteraction. That causes it to buffer the inputs, so I keeps executing the commands even after the condition as been met.

I could use time.sleep after each stdin.write but since the time needed to execute each command is variable, I would need to use times longer than necessary making the python script slower. Also, that's a dumb solution to this.

Thanks!

Answer 1

If you are using python3, you can try updating your code to use subprocess.run instead. It should wait for your task to complete and return the output.

Answer 2

As of 2019, you can use subprocess.getstatusoutput() to run a process and wait for the output, ie:

import subprocess
args = "echo 'Sleep for 5 seconds' && sleep 5"
status_output = subprocess.getstatusoutput(args)
if status_output[0] == 0: # exitcode 0 means NO error
  print("Ok:", status_output[1])
else:
  print("Error:", status_output[1])

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Python Demo

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From python docs :

subprocess.getstatusoutput(_cmd_)

Return (exitcode, output) of executing cmd in a shell.

Execute the string cmd in a shell with Popen.check_output() and return a 2-tuple (exitcode, output) . The locale encoding is used; see the notes on Frequently Used Arguments for more details.

A trailing newline is stripped from the output. The exit code for the command can be interpreted as the return code of subprocess. Example:

>>> subprocess.getstatusoutput('ls /bin/ls')
(0, '/bin/ls')
>>> subprocess.getstatusoutput('cat /bin/junk')
(1, 'cat: /bin/junk: No such file or directory')
>>> subprocess.getstatusoutput('/bin/junk')
(127, 'sh: /bin/junk: not found')
>>> subprocess.getstatusoutput('/bin/kill $$')
(-15, '')

Answer 3

You can use commands instead of subprocess. Here is an example with ls command:

import commands 
status_output = commands.getstatusoutput('ls ./')
print status_output[0] #this will print the return code (0 if everything is fine)
print status_output[1] #this will print the output (list the content of the current directory)