Select range of users from /etc/password in a script?

While this task might initially sound daunting, it's actually quite easy to accomplish by using either Perl or awk. My preference is awk, simply because I know that it's included in every single version of Unix or Linux, however old or obsolete, so let's look at how that'd help us here.

First off, I'm going to assume that the only thing you need is the username itself, not any of the other fields in the password file. I think that's safe!

A typical entry in the /etc/passwd file looks like this:

In order, the fields, separated by colons, are: account name, encrypted password (a "*" shows that the password's actually stored elsewhere, as it should be), user ID, group ID, so-called "gecos field" which is the full name of the user, home directory and default login shell. Here you can see that root has a UID of zero and that it uses /bin/sh as the login shell and starts out at /var/root.

There are various tricks we could do in a shell script using cut and similar utilities, but let's make it easy and let awk do all the work.

To start, awk lets you specify the field separator with the -F flag, so awk -F: tells the program that it should consider a colon as the field separator, easily enough! Then you specify the mini-program you want awk to run. To pull out just the third field, for example, add '{ print $3 }'. Combine it all and you have a simple way of pulling out all the UIDs in the password file:

That's not really useful yet, though, so let's take the next step and give awk a conditional statement to evaluate, line by line, before it determines whether to run the code block subsequent. Let's start by just printing the account names of accounts with a UID > 99:

That's easy enough, isn't it?

All that's left is to make the conditional test check for an upper and lower bound simultaneously:

That's the solution. That'll give you a list of userIDs associated with accounts between 99 and 120. I'll let you take the final step of changing these values to 5000 and 5999, respectively.

Good luck with your script!