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shopt -s nullglob # Non-matching globs are removed ('*.foo' => '')
shopt -s failglob # Non-matching globs throw errors
shopt -s nocaseglob # Case insensitive globs
shopt -s dotglob # Wildcards match dotfiles ("*.sh" => ".foo.sh")
shopt -s globstar # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
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if [[ -z "$string" ]]; then
echo "String is empty"
elif [[ -n "$string" ]]; then
echo "String is not empty"
fi