May 10, 2011

We have today another exercise in our continuing series of Unix V7 commands:


comm — select or reject lines common to two sorted files


comm [ -[123] ] file1 file2


Comm reads file1 and file2, which should be ordered in ASCII collating sequence, and produces a three column output: lines only in file1; lines only in file2; and lines in both files. The filename ‘-‘ means the standard input. Flags 1, 2, or 3 suppress printing of the corresponding column. Thus comm -12 prints only the lines common to the two files; comm -23 prints only lines in the first file but not in the second; comm -123 is a no-op.

Your task is to implement the comm command. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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