Answer by Luca Borrione for How to determine if a bash variable is empty?

The question asks how to check if a variable is an empty string and the best answers are already given for that.
But I landed here after a period passed programming in php and what I was actually searching was a check like the empty function in php working in a bash shell.
After reading the answers I realized I was not thinking properly in bash, but anyhow in that moment a function like empty in php would have been soooo handy in my bash code.
As I think this can happen to others, I decided to convert the php empty function in bash

According to the php manual:
a variable is considered empty if it doesn't exist or if its value is one of the following:

• "" (an empty string)
• 0 (0 as an integer)
• 0.0 (0 as a float)
• "0" (0 as a string)
• an empty array
• a variable declared, but without a value

Of course the null and false cases cannot be converted in bash, so they are omitted.

function empty
{
    local var="$1"

    # Return true if:
    # 1.    var is a null string ("" as empty string)
    # 2.    a non set variable is passed
    # 3.    a declared variable or array but without a value is passed
    # 4.    an empty array is passed
    if test -z "$var"
    then
        [[ $( echo "1" ) ]]
        return

    # Return true if var is zero (0 as an integer or "0" as a string)
    elif [ "$var" == 0 2> /dev/null ]
    then
        [[ $( echo "1" ) ]]
        return

    # Return true if var is 0.0 (0 as a float)
    elif [ "$var" == 0.0 2> /dev/null ]
    then
        [[ $( echo "1" ) ]]
        return
    fi

    [[ $( echo "" ) ]]
}

Example of usage:

if empty "${var}"
    then
        echo "empty"
    else
        echo "not empty"
fi

Demo:
the following snippet:

#!/bin/bash

vars=(
    ""
    0
    0.0
    "0"
    1
    "string"
    " "
)

for (( i=0; i<${#vars[@]}; i++ ))
do
    var="${vars[$i]}"

    if empty "${var}"
        then
            what="empty"
        else
            what="not empty"
    fi
    echo "VAR \"$var\" is $what"
done

exit

outputs:

VAR "" is empty
VAR "0" is empty
VAR "0.0" is empty
VAR "0" is empty
VAR "1" is not empty
VAR "string" is not empty
VAR " " is not empty

Having said that in a bash logic the checks on zero in this function can cause side problems imho, anyone using this function should evaluate this risk and maybe decide to cut those checks off leaving only the first one.