46.4 Stop Syntax Errors in Numeric Tests

The test and [ (square bracket) commands ( 44.20 ) can compare two numbers. But it's an error if one of the numbers you test is stored in a shell variable that's empty or doesn't exist. For example, an empty num variable here will give you a Syntax error :

if [ "$num" -gt 0 ]
then ...

To stop syntax errors, add a leading zero, like this:

if [ "0$num" -gt 0 ]
then ...

In that case, if $num is empty, the test will compare 0 to 0 . If $num is 1 , the test will be true (because 01 is greater than 0 )-and so on, just as it should be.