How to append something to a running file in Popen?

Question

Here's the deal:

#!/usr/bin/env python3

import subprocess

from subprocess import Popen, PIPE

var = '* * * * * mkdir /etc/STATISTICS\n'


try:
  with open('./.file_temp', 'w') as f:
    f.write(var)
    p1 = Popen(['crontab', '-e'], stdout=PIPE, stdin=open('./.file_temp', 'r'), shell=True)
    print ('The first one got executed.')
except FileNotFoundError:
    subprocess.run(['touch', './.file_temp'])
    f.write(var)
    p1 = Popen(['crontab', '-e'], stdout=PIPE, stdin=open('./.file_temp', 'r'), shell=True)
    print ('The second got executed.')
    
delete = Popen(['rm', '-f', './.file_temp'])

print ('The temp file has been deleted.')

Firstly, the code that you are looking at is a dummy code I made just so you can see what's bugging me - the real one is somewhat nicer.

So, I wanted to use Popen to start crontab -e and append whatever is found in the variable called var. I tried stdin=var but that gives me AttributeError: 'str' object has no attribute 'fileno', which led me to make a file that will store my variable. Now, that file serves as an input parameter for crontab -e, which is initiated by the Popen itself.

When I run it, it works like a charm. In the crontab, I see my variable that is indeed executed. Awesome!

Buuut, here's the catch: whenever I start this small program of mine, it would delete everything that was in the crontab file prior to the program execution and write the content of the .file_temp to the crontab file. I don't want that. I want to append to the crontab file, instead of deleting everything and inputting only what's in that temporary file.

That .file_temp is no longer a temp file since it has to exists on the system with all commands (exclusively given by my program), otherwise, it will get rid of everything...which is not the desired effect. Imagine someone manually writes something to the crontab file and then after some time decides to use my program. Well in that case, if my program gets initiated, it will load only things defined within itself (referring to the .file_temp), and nicely run over manually given commands.

For short, how to append something to a program run by Popen?

NOTE: This is done in Linux (Ubuntu). I am kind of a beginner, so yeah, I might have missed something obvious. I've checked everything before coming here but I haven't found any concrete answers.

It is possible that I have messed something up with stdout and stdin since I am not 100% clear on those concepts.

Open the file in append mode. Right now you are opening it in write mode, which truncates it first. instead of `open(filename, 'w')` use `open(filename, 'a')` — Z4-tier, Mar 21 '21 at 16:29
Does this answer your question? [How do you append to a file?](https://stackoverflow.com/questions/4706499/how-do-you-append-to-a-file) — Z4-tier, Mar 21 '21 at 16:31
using `a` will not give you an empty file, unless it was empty anyway. Please read https://docs.python.org/3/library/functions.html#open. You asked how to append to a file, the answer is: *use the `a` mode and not the `w` mode* — Z4-tier, Mar 21 '21 at 18:36
Actually you are correct, it will not be empty but it will just be loaded with whatever my variable is. Changing the mode won't do much -- having 'a' or 'w' isn't effective, since my temp file gets deleted and new empty one gets made. This cycle happens everytime you start this program. In the crontab file nothing is appended rather flushed and injected with whatever's in the temp file. — Fostgen, Mar 21 '21 at 19:07

How to append something to a running file in Popen?

0 Answers0