What means the -z value into this if expression of this Linux Shell script?

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http://stackoverflow.com – I am very new in shell scripting and in Ubuntu packaing. I have to work on a script that create a Ubuntu package for a software. In this script I found this if expression: if [ -z $1 ]; then echo "Usage: createpkg.sh <rev package>" exit else CURRENT_VERSION=$1 fi My problem is that I can't find what exactly means this -z value. From the content of the echo I can deduct that (maybe) $1 variable represents the sotware version. and that (maybe) -z is a void value. So if I execute the script without passing to it the version of the software that I would packing it print me (HowTos)