I don't wanna be spoiler so do remove what i say here if I am...
I solve this after reading so much posts in this thread. These are the message I got from this mission:
1. the script allows ../ then u find out it allows ../../ too, so does it allows some more??
2. Next, everyone talks of the word "EXACT" and yes, u must be logically correct, be careful of what u are thinking, do u make any mistake in your thinking? Think of what command u are typing in and where does it leads u to. U are reminded to be aware of the DIRECTORY, which mission's password u wanna get, so... draw a flow chart of your command if u still don't get it, u will see what your command should look like.
Think of giving instruction to lead a person to some place, u will tell him "from HERE, go straight, turn left, turn right...".