shellubuntuuidetcpasswd

How do I extract the biggest UID value from /etc/passwd?

I want to predict the next UID before creating a new user. Since the new one will take the biggest ID value yet and adds 1 to it, I thought of the following script:

biggestID=0
cat /etc/passwd | while read line
do
if test [$(echo $line | cut -d: -f3) > $biggestID]
then
biggestID=$(echo $line | cut -d: -f3)
fi
echo $biggestID
done
let biggestID=$biggestID+1
echo $biggestID

As a result I get 1. This confused me and I thought that the problem is with the loop, so I added the echo $biggestID just below fi to check if its value is truly changing and it turns out there is no problem with the loop as I got many values up to 1000. So why is biggestID's value returning to 0 after the loop?

Solution

  • It's because of this line:

    cat /etc/passwd | while read line

    That runs the while loop in a sub-shell, so biggestID is being set in the sub-shell, not in the parent shell.

    If you change your loop to the following, it will work:

    while read line
...
done < /etc/passwd

    This is because the while loop is now running in the same shell as the main script, and you're just redirecting the contents of /etc/passwd into the loop.