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Does your system use Pluggable Authentication Modules (PAM)? Most modern Linux or BSD use PAM. PAM allows you to hook into logins. There are a variety of PAM modules available which might meet your needs, or you can write your own in C. There is even a pam-python* binding which allows you to hook in Python code. Given that you want the daemon to be running ...
groups show you the groups you are in. So the problem is that your process thinks you still are in the now deleted 1001 group, from the deluser myuser 1001 command. You still would get that error message from groups as long as the process you started thinks you are in the group. When doing these kind of things in bash for my own account, I normally start a ...
It might be possible for a sufficiently crafty/devious user to defeat this, but you should be able to catch most logins if you put a command into /etc/profile to notify your daemon. It could be something simple, like running who am i with output redirected to a fifo that your daemon would read.
The file permissions specifically do not allow read, write or execute of that file to the owner (user1). If you were to change the owner to another user, then you would be able to read the file under the group permissions. Excert from File system permissions wiki page Classes ... The effective permissions are determined based on the user's class. ...
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