#!/usr/bin/env bash

value=""
if [ -n $value ]
then
    echo "value is non-zero"
fi

#!/usr/bin/env bash

value=""
if [[ -n $value ]]
then
    echo "value is non-zero"
fi

[[ expression ]]
      Return a status of 0 or 1 depending on the evaluation of the conditional expression expression.  Expressions are composed of the pri‐
      maries described below under CONDITIONAL EXPRESSIONS.  Word splitting and pathname expansion are not performed on the  words  between
      the  [[  and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution,
      and quote removal are performed.  Conditional operators such as -f must be unquoted to be recognized as primaries.

test "$1"
  test ! "$1"

test -n "$1"
  test -z "$1"

[ "-${z:-n}" "$var" ]

[[ "-${z:-n}" "$var" ]]