====== Shell: check for a NULL var ======
\\


**Return true if a bash variable is unset or set to the null (empty) string:**
<file bash>
if [ -z "$var" ]; then
   echo "NULL"; 
else 
   echo "Not NULL"; 
fi
</file>
    
**Another option to find if bash variable set to NULL:**
<file bash>
[ -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"
</file>

**Check if $my_var is NULL using ! so expr is false**
<file bash>
[ ! -z "$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"
</file>

**The -n returns TRUE if the length of STRING is nonzero**
<file bash>
if [ ! -n "$var" ]
then
   echo "NULL"; 
else 
   echo "Not NULL"; 
fi
</file>




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