+ Users and Groups
Một OS bao giờ cũng tạo ra các User và gom nhiều user có điểm chung lại với nhau thành Group, các user và group được định danh theo con số duy nhất gọi là UID (User IDentifier) và GID (Group IDentifier). Unix/Linux cung cấp các API để giúp chưng trình lấy được thông tin về User và Group mà nó thuộc về.
1. User and Groups
#include <sys/types.h>
#include <unistd.h>
uid_t getuid(void); //trả về UID
gid_t getgid(void); //trả về GID
int setuid(uid_t uid);
int setgid(gid_t gid);
char *getlogin(void); //trả về login name của user
Các thông tin trên được lưu trong /etc/passwd có dạng:
account:password:UID:GID:GECOS:directory:shell
account is the user name
password is the user password
UID is the numerical user ID
GID is the numerical primary group ID for the user
GECOS is an optional field used for informational purposes; usually it contains the full user name
directory is the user's $HOME directory
shell is the user command interpreter (defaults to /bin/sh)
Ex:
$ cat /etc/passwd | grep ninhld
ninhld:x:1000:1000:ninhld:/home/ninhld:/bin/bash
2. Password Database
#include <sys/types.h>
#include <pwd.h>
struct passwd *getpwuid(uid_t uid);
struct passwd *getpwnam(const char *name);
struct passwd:
| passwd Member | Description |
|---|---|
| char *pw_name | The user’s login name |
| uid_t pw_uid | The UID number |
| gid_t pw_gid | The GID number |
| char *pw_dir | The user’s home directory |
| char *pw_gecos | The user’s full name |
| char *pw_shell | The user’s default shell |
Thông tin về Password Database được lưu trong /etc/passwd
Để lấy tât cả thông tin trong Password Database:
#include <pwd.h>
#include <sys/types.h>
void setpwent(void); //trở về đầu database
struct passwd *getpwent(void); //sacn tất cả entry, khi không còn thì trả về NULL
void endpwent(void); //đóng database khi scan xong
Ex1:
user.c
#include <sys/types.h>
#include <pwd.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int main()
{
uid_t uid;
gid_t gid;
struct passwd *pw;
uid = getuid();
gid = getgid();
printf("User is %s\n", getlogin());
printf("User IDs: uid=%d, gid=%d\n", uid, gid);
pw = getpwuid(uid);
printf("UID passwd entry:\n name=%s, uid=%d, gid=%d, home=%s, shell=%s\n",
pw->pw_name, pw->pw_uid, pw->pw_gid, pw->pw_dir, pw->pw_shell);
pw = getpwnam("root");
printf("root passwd entry:\n");
printf("name=%s, uid=%d, gid=%d, home=%s, shell=%s\n",
pw->pw_name, pw->pw_uid, pw->pw_gid, pw->pw_dir, pw->pw_shell);
exit(0);
}
Compile & Execute:
$ gcc user.c
$ ./a.out
User is ninhld
User IDs: uid=1000, gid=1000
UID passwd entry:
name=ninhld, uid=1000, gid=1000, home=/home/ninhld, shell=/bin/bash
root passwd entry:
name=root, uid=0, gid=0, home=/root, shell=/bin/bash
Ex2:
getpwent.c
/*
* This program loops, reading a login name from standard
* input and checking to see if it is a valid name. If it
* is not valid, the entire contents of the name in the
* password database are printed.
*/
#include <stdio.h>
#include <sys/types.h>
#include <pwd.h>
void main()
{
struct passwd *pw;
setpwent( );
while( ( pw=getpwent( ) ) != ( struct passwd * )0 )
printf( "%s\n", pw->pw_name );
endpwent();
}
Compile & Execute:
$ gcc getpwent.c
$ ./a.out
root
bin
daemon
adm
lp
sync
shutdown
halt
mail
operator
games
ftp
nobody
avahi-autoipd
dbus
polkitd
usbmuxd
rtkit
chrony
tss
unbound
openvpn
avahi
pulse
lightdm
nm-openconnect
colord
sshd
tcpdump
ninhld
abrt
rpc
rpcuser
nfsnobody
stap-server