A common way of letting users supply data to a script is via its command-line arguments. In this case, it would, for example, be convenient to call your script with a list of domain names. Since the user needs to provide a password and some other key-related parameter, we could develop a script that takes two options and a list of domains.

./script -p 'password' -k 'keystore' domain1 domain2 domain3 ...

I don't know the keytool utility that you are using, but it seems that a user must supply a list of at least two domains rather than just one. The first domain name is used with the -dname and -alias options, while the -ext option takes the second and any other domain name in its value.

The following code uses the idiomatic way of parsing the command-line options and validates that the keystore and password variables have values. It then calls the keytool utility with the data given by the user.

#!/bin/sh

unset -v keystore password

while getopts k:p: opt; do
        case $opt in
                k)
                        keystore=$OPTARG
                        ;;
                p)
                        password=$OPTARG
                        ;;
                *)
                        echo 'Error' >&2
                        exit 1
        esac
done

shift "$(( OPTIND - 1 ))"

if [ -z "$keystore" ] || [ -z "$password" ]; then
        echo 'Missing keystore (-k) and/or password (-p)' >&2
        exit 1
fi

if [ "$#" -lt 2 ]; then
        echo 'Expecting at least two domain names' >&2
        echo '(a main one, and one for the -ext option)' >&2
        exit 1
fi

domain1=$1
shift

IFS=,

keytool -genkey \
        -keystore "$keystore"   \
        -keysize 2048   \
        -keypass "$password"    \
        -storepass "$password"  \
        -keyalg RSA     \
        -dname "CN=$domain1,OU=Devteam,O=Softech,L=Chicago,ST=IL,C=US"  \
        -alias "$domain1"       \
        -ext "san=dns:$*"

After the two if statements, the first domain name will be in $1 and the others in $2, $3, and so on. We assign $1 to the domain1 variable and call shift. After the shift, we may use "$*" with IFS set to a comma to generate a comma-delimited string of the rest of the domain names. We use this in the value for the -ext option.