IT-230

Linux Administration

Exercise

Creating Users from a File Exercise

Task

You have been handed a list of new hires and asked to create an account for each of them. Write a script that reads the list line by line, reports what it did, refuses to run without root privileges, and is safe to run twice.

Host
servera
Script location
~/scripts/user_creator.sh
Input file
~/scripts/new_users.txt

  1. Stage the input file

    Create the scripts directory and write the list of accounts, one name per line. This stands in for a list handed to you by someone else. 

    mkdir -p ~/scripts
cd ~/scripts
vim new_users.txt
    thufir_hawat
lady_jessica
paul_atreides
    cat new_users.txt

  2. Create an executable script

    vim user_creator.sh
    #!/bin/bash
echo "Starting $0"
    chmod a+x user_creator.sh
sudo ./user_creator.sh

  3. Read the file line by line

    A while read loop with input redirection processes one line per iteration. 

    vim user_creator.sh
    #!/bin/bash
while read -r new_user; do
    echo "Processing: $new_user"
    useradd "$new_user"
done < "new_users.txt"
    sudo ./user_creator.sh

    Now run it a second time.

    sudo ./user_creator.sh

    Every useradd fails because the accounts already exist, but the script says Processing either way. The output is misleading.

  4. Report whether each account was created

    useradd returns 0 on success like any other command, so an if can branch on it directly. 

    vim user_creator.sh
    while read -r new_user; do
    echo "Processing: $new_user"
    if useradd "$new_user"; then
        echo "User created."
    else
        echo "User NOT created."
    fi
done < "new_users.txt"

    Add one more name to the list and run it again. The new account is created and the existing ones are reported honestly. 

    echo "feyd_rautha" >> new_users.txt
sudo ./user_creator.sh

  5. Refuse to run without root privileges

    EUID is the effective user ID of the running process. Root is always 0. Compare the two values yourself first. 

    echo $EUID
sudo bash -c 'echo $EUID'

    Add a guard immediately below the hashbang so the script stops before doing any work. 

    vim user_creator.sh
    #!/bin/bash
if [[ $EUID -ne 0 ]]; then
    echo "You must be root to run this script!"
    exit 9
fi

    Without sudo the script now stops immediately with a clear message and a nonzero exit code, instead of failing halfway through. 

    ./user_creator.sh
echo $?
sudo ./user_creator.sh

  6. Skip blank lines

    A stray blank line at the end of an input file is very common. Add one and watch what happens. 

    vim new_users.txt
sudo ./user_creator.sh

    The script tries to create an account with an empty name. Add a check at the top of the loop body: -z is true when a string is empty, and continue skips to the next iteration. 

    vim user_creator.sh
    while read -r new_user; do
    if [[ -z "$new_user" ]]; then
        echo "Skipping empty line"
        continue
    fi
    echo "Processing: $new_user"
    if useradd "$new_user"; then
        echo "User created."
    else
        echo "User NOT created."
    fi
done < "new_users.txt"
    sudo ./user_creator.sh

  7. Clean up

    Remove the accounts and their home directories so the host returns to its starting state, then delete the script and its input file. 

    sudo userdel -r thufir_hawat
sudo userdel -r lady_jessica
sudo userdel -r paul_atreides
sudo userdel -r feyd_rautha
rm ~/scripts/user_creator.sh ~/scripts/new_users.txt
cd ~