Conditional Construct Syntax Issue - Square or Round Brackets

Question

I'm studying basic Bash scripting on a Debian System 8.6. I've read an example I found on a book that shows how to build a conditional construct. I let you it below:

#!/bin/bash
#Filname: CondConst.sh

if [$UID -ne 0]; then
        echo "Non root User."
else
        echo "Root User."
fi

When I run the script containing this code I get the following error message and result:

[1000: command not found
Root User.

I've tried to modify the code in many ways (using quotes and spaces), but I could not understand how to use square brackets syntax for the comparision. I worked out the issue by using a different syntax, that I show you below:

#!/bin/bash
#Filname: CondConst.sh

if (($UID!=0)); then
        echo "Non root User."
else
        echo "Root User."
fi

Can anyone explain me, if possible, the reason why I can not use square brackets?

Thank you, really, in advance.

Answer 1

Your error is because you are not typing a space after [ so it is see it as a one command [1000, so just type space:

#!/bin/bash
#Filname: CondConst.sh

if [ $UID -ne 0 ]; then
        echo "Non root User."
else
        echo "Root User."
fi