seq - invalid floating point argument error

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http://unix.stackexchange.com – I have a file with many numbers, with each number on one line. My goal is to find the numbers that are missing. I'm trying to generate the sequence of all the numbers with seq start=$(head -1 numbers.txt) finish=$(tail -1 numbers.txt) seq $start $finish > all_numbers.txt I get the following error message seq: invalid floating point argument: 4106 Try 'seq --help' for more information. I'm baffled as 4106 is clearly not a floating point number. After this I plan to use diff to find the missing numbers. Can someone tell me why seq is behaving this way? (HowTos)