Shell: check for a NULL var

Return true if a bash variable is unset or set to the null (empty) string:

if [ -z "$var" ]; then
   echo "NULL"; 
else 
   echo "Not NULL"; 
fi

Another option to find if bash variable set to NULL:

[ -z "$var" ] && echo "NULL"
[ -z "$my_var" ] && echo "NULL" || echo "Not NULL"

[[ -z "$my_var" ]] && echo "NULL"
[[ -z "$my_var" ]] && echo "NULL" || echo "Not NULL"

Check if $my_var is NULL using ! so expr is false

[ ! -z "$my_var" ] || echo "NULL"
[ ! -z "$my_var" ] && echo "Not NULL" || echo "NULL"
 
[[ ! -z "$my_var" ]] || echo "NULL"
[[ ! -z "$my_var" ]] && echo "Not NULL" || echo "NULL"

The -n returns TRUE if the length of STRING is nonzero

if [ ! -n "$var" ]
then
   echo "NULL"; 
else 
   echo "Not NULL"; 
fi

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