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Cas
Asked: 2021-05-26 23:55:30 +0800 CST

for loop doesn't work when "{0..$var}" is used [duplicate]

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Simple problem but I don't know how to fix it. Here's the code:

missing_filters=('Sort key' 'Page' 'Pagesize' 'Sort direction')

for level in {0..$((${#missing_filters[@]}-1))}
do
    echo "${#missing_filters[$level]}^|^${missing_filters[$level]}"
done | column -tes "^"

It should show every element of the array with 1-4 before it like this:

1 | Sort key
2 | Page
3 | Pagesize
4 | Sort direction

However, I get the error ./advancedplexapi.sh: line 4608: {0..3}: syntax error: operand expected (error token is "{0..3}"). I find this strange as the error shows exactly how I want the shell to interpret it (that being {0..3}). I did nothing wrong because otherwise the shell wouldn't give me the correct output in the error. Why doesn't it work? And ofcourse how can I fix this?

command-line

1 Answers

  1. It is not working as expected, because brace expansion is done before variable substitution.

    You could use seq or a for-loop instead:

    for level in $(seq 0 $((${#missing_filters[@]}-1))); ...

    or

    for (( level=0;level<="$((${#missing_filters[@]}-1))";level++ )); ...

    For more details, see this answer to another question.

    Also "${#missing_filters[$level]}" will not give 1 2 3 4, but expand to the number of characters in each value --> 8 4 8 14. Better use $((level+1)).

    However, you could more easily loop the array keys:

    for k in "${!missing_filters[@]}"; do
    printf '%s | %s\n' "$((k+1))" "${missing_filters[$k]}"
done
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