Those who don't understand Unix are condemned to reinvent it, poorly.
According to Cambridge dictionary, condemn means to criticize something or someone strongly.
In the above sentence how is the word "condemn" used and what does this statement mean.
"Condemn" has another meaning as well:
to severely punish someone who has committed a crime, or to force someone to suffer
(from Cambridge online dictionary)
This is the way "condemn" is meant in the sentence you are asking about. I guess it means that Unix is so important that not understanding it is almost like a crime, and if you commit that "crime", you will be forced to re-invent Unix yourself because it is so necessary.
FYI, I think this quote about Unix is referencing another famous quote, namely:
"Those who cannot remember the past are condemned to repeat it." **
** According to the Internet, G. Santayana originated that other quote, and in case you hadn't heard it, it might shed some light on the Unix quote.
