Figuring out the next available userid for adding a user

crontab · 09-18-2007, 01:54 PM

Just as an exercise and to pass some time, I'm looking to recreate adduser in perl. The syntax of /etc/passwd requires a userid and groupid number for the new user. adduser will default the userid to the next available, but I don't know how it determines that. Does it look in /etc/passwd, or is there another file with every userid in use?

Thanks for any help you can spare

ChrisScott · 09-18-2007, 02:25 PM

You could use a regex to identify the UIDs from /etc/passwd:

Code:

cat /etc/passwd | perl -p -e "s/.*:.*:(.*):.*:.*:.*:.*/\1/g"

crontab · 09-18-2007, 02:30 PM

Hm, cool. And then figure out the largest number, and add 1 to it?

ChrisScott · 09-18-2007, 03:42 PM

Yeah. Or a double loop to look for the first free. Something like:

Code:

for ($X=1,1,$X++) {
   foreach $check(@arrayOfUIDs) {
      if ($check == $X) {
         $failed = 'true';
         }
      }
   if ($failed != 'true') {
      print $X;
      exit 0;
      }
   $failed = 'false';
   }

Although I'm not very up on perl so that may not work