I have two file to join. FILE 1: a A1 a A2 a A3 ... c C1 c C2 ... FILE 2: a feature1_of_a a feature2_of_a ... a featureN_of_a ... ... c feature1_of_c c feature2_of_c ... after join, i could get File like this: A1 feature1_of_a A2 feature1_of_a A3 feature1_of_a A1 feature2_of_a A2 feature2_of_a A3 feature2_of_a ... A1 featureN_of_a A2 featureN_of_a A3 featureN_of_a ... In order to do that: i wrote shell command join -11 -21 -o1.2,2.2 file1 file2. But the problem is: number N might be huge. So if join read all feautre of a into memory at once, memory might not be enough. I don't know how join is implemented. WQould the momery become a problem? If so, is there any way to get what I want?