Example of different bash string tests

bash_conditionals.sh

#!/usr/bin/env bash

# a)
unset FOO
if [ ! -z ${FOO+x} ]; then
  echo "a) is true"
fi

# b)
FOO=""
if [ ! -z ${FOO+x} ]; then
  echo "b) is true"
fi

# c)
FOO="BAR"
if [ ! -z ${FOO+x} ]; then
  echo "c) is true"
fi

echo "###"

# d)
unset FOO
if [ -n ${FOO+x} ]; then
  echo "d) is true"
fi

# e)
FOO=""
if [ -n ${FOO+x} ]; then
  echo "e) is true"
fi

# f)
FOO="BAR"
if [ -n ${FOO+x} ]; then
  echo "f) is true"
fi

echo "###"

# g)
unset FOO
if [[ -v FOO ]]; then
  echo "g) is true"
fi

# h)
FOO=""
if [[ -v FOO ]]; then
  echo "h) is true"
fi

# i)
FOO="BAR"
if [[ -v FOO ]]; then
  echo "i) is true"
fi