That (like most of the answers) assumes that the lines in the second file are numbered consecutively with no numbers missing. I wasn't sure from the question whether that was the case, or if when he said, "The file which I am appending a single column has the line number in the first column," he meant it had the number of the line it needed to be attached to.

In other words, the second file could be:

2 red
4 brown
3 blue
6 orange
7 yellow
[download]

If 'yellow' is supposed to be appended to the 7th line of file1, then sorting file2 numerically and stepping through both files equally won't work, because 'lines' 1 and 5 are missing.

By using the numbers in file2 as keys in a lookup hash, it doesn't matter if file2 is missing a number somewhere. I guess it really just depends on how sure you are that you can count on the numbers truly being line numbers.

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