Check whether Linux user's password is correct.

passwd-check.sh

#!/bin/bash

checkPassword () {
    user="${1}"
    passwordClear="${2}"

    shadowLine=$(grep "^${user}:" /etc/shadow)
    passwordSalted=$(strField "$shadowLine" ":" 2)

    enc=$(strField "$passwordSalted" "$" 2)
    params=$(strField "$passwordSalted" "$" 3)
    salted=$(strField "$passwordSalted" "$" 4)
    next=$(strField "$passwordSalted" "$" 5)
    if [[ "$next" != '' ]]; then
        params="$params\$$salted"
        salted="$next"
    fi

    # openssl >= 1.1: openssl passwd -${enc} -salt ${salt} -stdin
    # mkpasswd (debian: whois, centos: expect):
    # python 2 & 3: python -c 'import crypt; print(crypt.crypt("password", "$6$saltsalt$"))'
    export FV_CHECK_PASSWORD_ENC="${enc}"
    export FV_CHECK_PASSWORD_PARAMS="${params}"
    export FV_CHECK_PASSWORD_CLEAR="${passwordClear}"
    crypt=$(echo "${passwordClear}" | python3 -c 'import crypt,os; print(crypt.crypt(os.getenv("FV_CHECK_PASSWORD_CLEAR"), "$" + os.getenv("FV_CHECK_PASSWORD_ENC") + "$" + os.getenv("FV_CHECK_PASSWORD_PARAMS") + "$"))')
    unset FV_CHECK_PASSWORD_CLEAR

    if [[ "$passwordSalted" = "$crypt" ]]; then
        echo "OK"
        exit 0
    else
        echo "Error!"
	echo "Params=$FV_CHECK_PASSWORD_PARAMS"
        echo "Shadow=$passwordSalted"
        echo "Crypt=$crypt"
        exit 1
    fi
}

strField () {
    str="$1"
    sep="$2"
    pos="$3"
    echo $(echo "$str" | cut -d"$sep" -f "$pos")
}

main() {
    user="$1"
    [[ "$user" = "" ]] && user=root
    echo -n "test password for $user: "
    read -s pass
    echo "checking password: ${pass:0:4}****** ..."

    checkPassword "$user" "$pass"
}

main "$@"