Evaluating strings

As mentioned in the variables subsection, numeric values are different from strings. Strings are typically evaluated like this:

#!/bin/bash
MY_NAME="John"
NAME_1="Bob"
NAME_2="Jane"
NAME_3="Sue"
Name_4="Kate"

if [ "${MY_NAME}" == "Ron" ]; then
    echo "Ron is home from vacation"
elif [ "${MY_NAME}" != ${NAME_1}" && "${MY_NAME}" != ${NAME_2}" && "${MY_NAME}" == "John" ]; then
    echo "John is home after some unnecessary AND logic"
elif [ "${MY_NAME}" == ${NAME_3}" || "${MY_NAME}" == ${NAME_4}" ]; then
    echo "Looks like one of the ladies are home"
else
    echo "Who is this stranger?"
fi

In the preceding snippet, you might notice that the MY_NAME variable will be executed and the string John is home after some unnecessary AND logic will be echoed to the console. In the snippet, the logic flows like this:

1. If MY_NAME is equal to Ron, then echo "Ron is home from vacation"
2. Else if MY_NAME is not equal to NAME_1 AND MY_NAME is not equal to NAME_2 AND MY_NAME is equal to John, then echo "John is home after some unnecessary AND logic"
3. Else if MY_NAME is equal to NAME_3 OR MY_NAME is equal to NAME_4, then echo "Looks like one of the ladies"
4. Else echo "Who is this stranger?"

Notice the operators: &&, ||, ==, and != 

• && (means and)
• || (means or)
• == (is equal to)
• != (not equal to)
• -n (is not null or is not set)
• -z (is null and zero length)