my script deletes user accounts that have not been logged in for more than 30 days, in fact it works, but it deletes some system accounts - bash

Question

It works, but also deletes some system accounts...i don't know why
#!/bin/bash
# This script takes everyone with id>1000 from /etc/passwd and removes every user account in case if it hasn't been used for the last 30 days.
# Make sure that script is being executed with root priviligies.
if [[ "${UID}" -ne 0 ]]
then
echo "You should run this script as a root!"
exit 1
fi
# First of all we need to know id limit (min & max)
USER_MIN=$(grep "^UID_MIN" /etc/login.defs)
USER_MAX=$(grep "^UID_MAX" /etc/login.defs)
# Print all users accounts with id>=1000 and <=6000 (default).
awk -F':' -v "min=${USER_MIN##UID_MIN}" -v "max=${USER_MAX##UID_MAX}" ' { if ( $3 >= min && $3 <= max ) print $0}' /etc/passwd
# This function deletes users which hasn't log in in the last 30 days
# Make a color output message
for accounts in ` lastlog -b 30 | sed "1d" | awk ' { print $1 } '`
do
userdel $accounts 2>/dev/null
done
echo -e "\e[36mYou have successfully deleted all user's account which nobody logged in in the past 30 days.\e[0,"
exit 0

Answer 1

You do not apply the awk -F':' -v "min=${USER_MIN##UID_MIN}" -v "max=${USER_MAX##UID_MAX}" ' { if ( $3 >= min && $3 <= max ) print $0}' /etc/passwd filter to lastlog output....
Join the list from lastlog with the list of users from /etc/passwd and filter UID with proper number range.
lastlog -b30 | sed 1d | awk '{print $1}' |
sort | join -t: -11 -21 -o2.1,2.3 - <(sort /etc/passwd) |
awk -F: -v "min=${USER_MIN##UID_MIN}" -v "max=${USER_MAX##UID_MAX}" '$2 >= min && $2 <= max {print $1}' |
xargs -n1 echo userdel
Notes:
Do not use backticks `. Use $(...) instead. Obsolete and deprecated syntax
how to read a stream line by line in bash bashfaq

Answer 2

With bash and GNU grep:
#!/bin/bash
# I left out the check for root user
# get only the numbers from each line
# \K removes the matching part before \K (from left to right)
USER_MIN=$(grep -Po "^UID_MIN *\K.*" /etc/login.defs)
USER_MAX=$(grep -Po "^UID_MAX *\K.*" /etc/login.defs)
# create an associative array, see: help declare
declare -A users
# read all users with its uid to this associative array
while IFS=":" read -r user x uid x; do users[$user]="$uid"; done </etc/passwd
# see output of: declare -p users
# remove all unwanted lines including headline. NR is line number
lastlog -b 30 | awk '! /Never logged in/ && NR>1 {print $1}' |
while read -r user; do
# -ge: greater-than-or-equal
# -le: less-than-or-equal
if [[ ${users[$user]} -ge $USER_MIN ]] && [[ ${users[$user]} -le $USER_MAX ]]; then
echo "delete user $user with uid ${users[$user]}"
# add your code here to delete user $user
fi
done