existing empty null variable in bash - bash

Question

I am using the code below to determine if a variable in bash exists, if it is empty, or if it has length>0. The code works, but I can't find a good explanation for how if [ -n "${emptyvar+1}" ] can detect if emptyvar is not set. If I remove the +1 then the test fails for "". What is the purpose of the +1 in the test?
#!/bin/bash
emptyvar="a"
if [ -n "${emptyvar+1}" ]
then
echo "emptyvar is defined"
if [[ -z $emptyvar ]]
then
echo "emptyvar is empty";
else
echo "emptyvar is NOT empty";
if [[ -n $emptyvar ]]
then
echo "emptyvar has length > 0";
else
echo "emptyvar has length 0";
fi
fi
else
echo "emptyvar is not defined"
fi

Answer 1

From the bash documentation of Shell Parameter Expansion:
${parameter:+word} If parameter is null or unset, nothing is
substituted, otherwise the expansion of word is substituted.
Omitting the colon (:) makes it test only if the variable is unset, rather than null or unset.
So ${emptyvar+1} tests if $emptyvar is unset. If it is, it expands to the empty string; if not, it expands to 1.

Answer 2

You can also create sets of functions to test a variable passed by its name:
function is_var_set {
[[ -n ${!1+.} ]]
}
function is_var_empty {
[[ -z ${!1} ]]
}
Test:
> A=''
> is_var_set A && echo "A is set." || echo "A is unset."
A is set.
> is_var_empty A && echo "A is empty." || echo "A is not empty."
A is empty.

Answer 3

bash 4.2 added a new test operator, -v, that tests if a variable has been set.
# -v takes the name of the variable, not its values (since you are
# testing if it has a value or not).
if [[ -v emptyvar ]]; then
echo "emptyvar is defined"
if [[ -z $emptyvar ]]
then
echo "emptyvar is empty";
else
echo "emptyvar is NOT empty";
fi
else
echo "emptyvar is not defined"
fi
Note that an empty variable is one whose string has length 0, so your -n test is redundant.

Answer 4

Another way to express it:
$ unset var
$ if [[ -z ${var+x} ]]; then echo unset; elif [[ -z $var ]]; then echo empty; else echo not empty; fi
unset
$ var=
$ if [[ -z ${var+x} ]]; then echo unset; elif [[ -z $var ]]; then echo empty; else echo not empty; fi
empty
$ var=foo
$ if [[ -z ${var+x} ]]; then echo unset; elif [[ -z $var ]]; then echo empty; else echo not empty; fi
not empty
You'll never get "emptyvar has length 0" -- that's what "empty" is