Adding User and Group in Unix - unix

Question

Does anyone know the api for adding users and groups in unix and removing them ? I want to do this programatically.
Thanks,
Frank

Answer 1

I started looking at some system calls and found the following. Note that they are of varying standards, so not all may work on your Unix version:
getpwent
setpwent
putpwent
These however, all assume a password file. Out of curiosity, I straced useradd to find out what he did. Here's a small section of it's output:
# grep -E 'passwd|shadow' useradd.log.20283
...
open("/etc/shadow", O_RDONLY|O_CLOEXEC) = 3
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 3
open("/etc/passwd.20283", O_WRONLY|O_CREAT|O_EXCL, 0600) = 4
link("/etc/passwd.20283", "/etc/passwd.lock") = 0
stat("/etc/passwd.20283", {st_mode=S_IFREG|0600, st_size=6, ...}) = 0
unlink("/etc/passwd.20283") = 0
open("/etc/passwd", O_RDWR) = 4
open("/etc/shadow.20283", O_WRONLY|O_CREAT|O_EXCL, 0600) = 5
link("/etc/shadow.20283", "/etc/shadow.lock") = 0
stat("/etc/shadow.20283", {st_mode=S_IFREG|0600, st_size=6, ...}) = 0
unlink("/etc/shadow.20283") = 0
open("/etc/shadow", O_RDWR) = 5
open("/etc/gshadow.20283", O_WRONLY|O_CREAT|O_EXCL, 0600) = 7
link("/etc/gshadow.20283", "/etc/gshadow.lock") = 0
stat("/etc/gshadow.20283", {st_mode=S_IFREG|0600, st_size=6, ...}) = 0
unlink("/etc/gshadow.20283") = 0
open("/etc/gshadow", O_RDWR) = 7
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 8
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 9
open("/etc/passwd-", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 9
utime("/etc/passwd-", [2010/09/02-07:07:34, 2010/09/02-07:07:34]) = 0
open("/etc/passwd+", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4
lstat("/etc/passwd", {st_mode=S_IFREG|0644, st_size=2479, ...}) = 0
rename("/etc/passwd+", "/etc/passwd") = 0
open("/etc/shadow-", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4
utime("/etc/shadow-", [2010/09/02-07:07:34, 2010/09/02-07:07:34]) = 0
open("/etc/shadow+", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4
lstat("/etc/shadow", {st_mode=S_IFREG|0600, st_size=1429, ...}) = 0
r ename("/etc/shadow+", "/etc/shadow") = 0
open("/etc/gshadow-", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4
utime("/etc/gshadow-", [2010/09/02-07:07:34, 2010/09/02-07:07:34]) = 0
open("/etc/gshadow+", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4
lstat("/etc/gshadow", {st_mode=S_IFREG|0400, st_size=1069, ...}) = 0
rename("/etc/gshadow+", "/etc/gshadow") = 0
unlink("/etc/shadow.lock") = 0
unlink("/etc/passwd.lock") = 0
unlink("/etc/gshadow.lock") = 0
Although you get a better idea what's going on with the full context, note that it links a temporary file it created (/etc/passwd.20283) to /etc/passwd.lock. useradd does similarly with the shadow, and gshadow files as well.
It's also important to note that useradd made four calls out to nscd. Two of these were for passwd, and two were for group:
execve("/usr/sbin/nscd", ["/usr/sbin/nscd", "nscd", "-i", "passwd"], [/* 0 vars */]) = 0
If there isn't an API (and I can't seem to find one), it may be because there's many more ways to store users than simple passwd files. Indeed, it's possible that the machine has no control at all over the users.
EDIT: I suppose it's also important to note that useradd consulted /etc/nsswitch.conf as well, likely to verify the origin of the user database. Furthermore, userdel behaved almost identically, creating similarly named temporary and lock files.
I tested under Linux using the following command:
strace -o useradd.log -f -ff -s 1024 useradd tempuser
strace may also appear as truss and ktrace on other unix systems.

Answer 2

I found this question while looking for a way to list all groups on a Unix.
But to create and delete users you can programatically call the utility programs useradd, userdel, groupadd, groupdel since you would know the user name been sent. But I guess you will need superuser rights to invoke them. Anyway you can check the process exit code for command status.
useradd xxx // status zero operation completed
useradd xxx // status nine user already exists
Hope it helps.