Check if env variable is set

Is it possible to check if an env variable is set? I tired a workaround like this
In bash
$set DATE=a

if (["{DATE:a}"]=="a") { mutate { add_field => { "startTime" => "2019-10-19" }} } else{ mutate { add_field => { "startTime" => "{DATE}" } }

the output is

So, else statement was executed, instead of if statement.

Any ideas what is wrong here?

Thx, in advance.

See here.

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