I want to know what are the implications of create an account using
# useradd -r ...
The help says:
-r, --system Create a system account. System users will be created with no aging information in /etc/shadow, and their numeric identifiers are chosen in the SYS_UID_MIN-SYS_UID_MAX range, defined in /etc/login.defs, instead of UID_MIN-UID_MAX (and their GID counterparts for the creation of groups). Note that useradd will not create a home directory for such an user, regardless of the default setting in /etc/login.defs (CREATE_HOME). You have to specify the -m options if you want a home directory for a system account to be created.
But, beyond assign lower values to uid, gid and groups.
Question 1 What files are affected?
Question 2 What additional performance this type system account presents?
Question 3 What behavior ignores or stops submit?
Question 4 Can I change an account created with the "-r" option to an account as if it were created without that option?