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unixseq

Use output number of echo as input for seq

So as an input, i get echo [number]. For example: echo 5. I need to get as output the sequence of numbers from 1 to [number]. So if I get 5 as input, I need: 1 2 3 4 5 (all on a separate line).

I know I can use seq 5 to get 1 2 3 4 5 but the issue is that I need to use pipes.

So the final command should be like this: echo 5 | seq [number] which should give 1 2 3 4 5 as output. My issue is that I don't know how to get the output from echo as my input for seq.

Solution

  • Assuming that echo 5 is an example replacement of an unknown program that will write a single number to stdout and that this output should be used as an argument for seq, you could use a script like this:

    file seqstdin:

    #!/bin/sh
read num
seq "$num"

    You can use it like

    echo 5 | ./seqstdin

    to get the output

    1
2
3
4
5

    You can also write everything in a single line, e.g.

    echo '5'| { read num; seq "$num"; }

    Notes:
    This code does not contain any error handling. It uses the first line of input as an argument for seq. If seq does not accept this value it will print an error message.
    I did not use read -r or IFS= because I expect the input to be a number suitable for seq. With other input you might get unexpected results.