How can I write something into crontab file from Python file?

Question

I try to write from a Python file cronjobs into crontab. When calling the Python file which contains the code for writing into crontab from another Python file via sudo I expect that the cronjob gets added to the crontab file but instead I get this error:

PermissionError: [Errno 1] Operation not permitted: '/usr/bin/crontab'

File 1

with open('/usr/bin/crontab', 'w') as f2:
    f2.write("Hello World")

File 2

import os
command = "python3 /Users/myUser/Projects2022/CronjobManager/File1.py"
os.popen("sudo -S %s"%(command), 'w').write('password')

Answer 1

The binary (executable program) for adding cronjobs is stored in /usr/bin/crontab. That binary actually writes your crontab entries in a file whose location is dependent on your OS.

On macOS, the file is in /var/at/tabs/USERNAME, on some systems it is in /var/spool/cron/XXX.

You can, and should, no more overwrite the executable at usr/bin/crontab than you can, or should, overwrite /usr/bin/wc

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What I am saying is that you need to differentiate between:

• the program /usr/bin/crontab and
• the table it writes to in /var/spool/cron/XXX

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What you might want to do is:

• extract the user's current crontab to a temporary file, i.e. crontab -l > TEMPORARYFILE
• edit or append to that file
• re-load that file back to the system, i.e. crontab TEMPORARYFILE

There's a good description of the tempfile module here.