Here I will show you how to check if a Linux user exists (or does not exist) in a POSIX-compatible shell like Bash or Dash. I will use the getent command along with the /etc/passwd file for this.
Example Bash script:
if getent passwd "$username" >/dev/null; then echo "User $username exists" else echo "User $username does not exist" fi
Replace the variable $username with the name of the user you want to check.
If you want to check if a username does not exist, you can negate the result of getent command like this:
if ! getent passwd "$username" >/dev/null; then echo "User $username does not exist" fi
The ! operator in front of the getent command is used to negate the exit code and the if statement then checks the negated exit code.