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Accepted
Ziyaddin Sadigov
Asked: 2014-02-27 10:54:01 +0800 CST

While loop in Shell

  • 772

I want to make a while...loop statement in my Shell script like:

while [ $line < $line_number ]; do
    echo $line
done

But there is an error. I think it is because I wrote it in bash code. But I need to do it in sh mode. How can I do it? Thanks!

command-line

1 Answers

  1. Best Answer

    You need to use, either

    while [ $line -lt $line_number ]; do
    echo $line
done

    or

    while [[ $line < $line_number ]]; do
    echo $line
done

    Disscussion[source]

    Single [] are posix shell compliant condition tests.

    Double [[]] are an extension to the standard [] and are supported by bash and other shells (e.g. zsh, ksh). They support extra operations (as well as the standard posix operations). For example: || instead of -o and regex matching with =~. A fuller list of differences can be found in the bash manual section on conditional constructs.

    Use [] whenever you want your script to be portable across shells. Use [[]] if you want conditional expressions not supported by [] and don't need to be portable.

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