UNIX for Dummies Questions & Answers

what if useradd ? or useradd * is given as input????
if it wont execute.
then what are the values for name shud be avoided
like ? *

See "man useradd" for your computer.

On my system this just states that the login field:
Must be printable characters.
Must not contain colon ":" or newline \n .


See also "man 4 passwd" (about the /etc/passwd file) for more rules about login name - including the length of the field.
Avoid plus sign "+" because it has special meaning to NIS.
Avoid space characters " " and anything recognised by the shell because it will give you grief.

Personally I just stick to lower case alphanumeric for compatibility with older systems (and some versions of ftp).