The test command has certain rules depending on the number of arguments. The most pertinent rule is: For one argument, the expression is true if, and only if,

The test command has certain rules depending on the number of arguments. The most pertinent rule is: For one argument, the expression is true if, and only if, the argument is not null. In this case [ -n $var ] is the same as test -n $var $var is not quoted, so when this command is run, word splitting occurs and therefore $var is null. Which there falls into the one argument rule above. Therefore, always quote your variables.