what is the rationale behind putting double square brackets in an if clause?

for e.g.

if [[ ${APPNAME} = '' ]] || [[ ${SRCSYSNAME} = '' ]] || [[ ${FILENAME} = '' ]];
then
echo some fields are null
fi

What shell is it running under?
What UNIX do you have?

See this post

The following is a segment from that post.
"There is a command called [ and like many commands it takes parameters. It checks its last parameter and gets mad unless it is ]. If the last parameter is a ], it looks at everything else and tries to determine what you are trying to test. If you do:
lines=""
if [ $lines -eq 0 ]
you will have a problem. All that the [ command will see is the -eq and the 0 so it will complain. Because of these concerns, Dave Korn invented another syntax. Now you can do:
if [[ $lines = 0 ]]
and it won't get mixed up as easily."

'All that the [ command will see is the -eq and the 0'...why so? why wont it see the ']' ? isnt ']' the last argument for command '[' ? plz explain. and by 'it looks at everything else' , u mean that it looks inside the square brackets? if so, then i dont see, why the first format shudnt work. (it isnt working, obviously, but i cant get the reason)

i am using the bash shell by the way. solaris

I carelessly phrased that. Yes it sees the ] but the $lines just disappears. It starts out:
if [ $lines -eq 0 ]
The shell replaces $lines with nothing. Now we have:
if [ -eq 0 ]
So yes the ] is there but the test no longer makes sense. This cannot happen with the [[ style.