are you convinced this script does not work as expected?

The fix

You have 2 equally viable options:

1. Use -gt

if [ $# -gt 0 ]

2. Use double brackets [[ (Does a lexicographic comparison but will work for this case)

if [[ $# > 0 ]]

Why?

When you did if [ $# > 0 ] the > was treated like an output redirection command similar to echo "foo" > file.txt. You might notice you have created a file named 0 someplace after executing:

if [ $# > 0 ]

When deciding between using [...] or [[...]] many find the answer is to use double brackets

Getting fancy

Now if what you’d really like to do is write a script that gives a default value to the u variable if none is provided by the first argument I would recommend using a neat bash syntax trick for implementing default values

u=${1:-${USER}}