can I customize any .conf file by my own line or is it predefined by system ?

short answer: yes with an if
long answer: no with a but

Could you be less cryptic when replying to new users please?

It depends, which .conf file you want to customize. But as far as I can understand, you're perhaps asking about your profile files i.e. .bashrc or .cshrc etc, for customization of your working enviornment & to set your own defined variables.. ain't you?

bubai, .conf files are usually plain text files so it is possible to edit them with any text editor.
but whether one should or not depends on which config file and what the outcome one is expecting.

can you please provide a specific example and maybe we can provide better assistance. i have in the past made alterations to grub.conf , httpd.conf , sshd.conf , ... eventhough they are automatically generated, doing something like manually disabling root login is usually preferble.

unspawn: i'll remember to check post count before making a sarcastic reply.

thanks,

At the very least, make a backup copy before you start.

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