????1.???????????
???????????????  /etc/password
?????????????      /etc/shadow
??????????????    /etc/group
??????????????????/etc/gshadow
????1.1 /etc/password???
????vim /etc/password
????fubh:x:1020:1000::/home/fubh:/bin/bash
????man 5 password  #?????????????
????(???????)????????????????????????????????
????account:password:UID:GID:GECOS:directory:shell
????(???:????:???ID:??ID:???????:??????:shell)
???????????????:

 

account   ??????????е?????????????????д???.
password  ????????????????????
UID       ??? ID ????
GID       ?????????? ID ????
GECOS     ????????????????????????????????????????????????????.
directory ????? $HOME ??.
shell     ???????е??????????????/bin/sh???????????????У?????????????????login(1) ???.??

????1.1.1 ???????
????UID=0??       ????????
????UID=500~60000 ???????
????UID=1~499     ??α?????????????????????
????1.2 /etc/shadow ???
????root:#21312sd$44:wd323%cds:14945:0:99999:7:::
?????????:????????:???????????:С?????:???????:???????:??????????:?Ч???:
????1.3 /etc/group ???
????vim /etc/group
????sudo:x:27:web??yanghuang??zhoumin??duyp??taofh??luanqq
????group_name:password:GID:user_list
????(????:??????:??ID:????)
????2??????????
???????????????useradd [-ugGdsce] ?????
????useradd -g webadmin -G root??web -c 'test suer' bob
????-u UID
????-g ???????????GID
????-G ??????????????
????-d ??????
????-s ?????????Shell
????-c ???????
????-e ???????Ч???