How to use a while loop to check if a variable is empty or not

Question

I am using following code to check whether variable is empty or not.

I'm using a while loop because I need to continue the loop while the variable is empty. The moment the variable is set to a value the loop should exit.

MR=[]
while [ -z "$MR" ]
do 
    echo "in while loop"
    sleep 10s
    MR="hi"
done

For some reason, it is not at all executing. What is the reason?

[] is not an empty array. It's a literal string [], which is not the empty string. — chepner
– chepner, Commented Dec 28, 2021 at 19:22
An empty array would be MR=() (ignoring the fact that MR=() doesn't actually define a variable; it only sets the array attribute on the name MR). — chepner
– chepner, Commented Dec 28, 2021 at 19:23
ok. got it. so can you suggest how to write while loop if any specific command is returning literal string or not . and continue the loop unless the variable is not empty literal string — user2315104
– user2315104, Commented Dec 28, 2021 at 19:30
@chepner What's the difference between defining a variable and setting the array attribute? I'm not familiar with the distinction. — John Kugelman
– John Kugelman, Commented Dec 28, 2021 at 19:31
MR=() is basically equivalent to declare -a MR. MR will still test as undefined with [[ -v MR ]], for example, but declare -p MR will show it as declare -a MR=(). — chepner
– chepner, Commented Dec 28, 2021 at 20:06

KamilCuk · Accepted Answer · 2021-12-28 19:48:21Z

5

What is the reason?

MR variable is not empty, it contains two characters [ and ].

$ MR=[]
$ echo "$MR"
[]

Because it is not empty, [ -z "$MR" ] returns nonzero, so while is never executed.

Instead set the variable to en empty string.

MR=
# or, does the same, but for some is more readable:
MR=""
# or
MR=''

answered Dec 28, 2021 at 19:48

KamilCuk

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