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up vote 4 down vote favorite

I'm getting an strange error.

#!/bin/bash
echo "Please enter a number"
read var
declare -i num
num=0
        while ($num<$var)
        do
                echo "$num"
        done

./loop: line 5: 6: No such file or directory

What am i mistaking?

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up vote 13 down vote

(...) starts a subshell and runs the specified commands inside it. This is why ($num<$var) generates that error message; it can't find the file corresponding to $var to pass into the command corresponding to $num.

You probably wanted something like

while (( num < var )); do
  echo "$num"
done

((...)) is an arithmetic expansion. It will compute the value of the expression inside. In this case it will compare the values of the two variables (the $ in front of them are not needed here). If the comparison is true, then the while-loop will run one more iteration.

The while-loop is also an endless loop, since you do not increment num nor decrement var.

In the end, you might want to try

for (( num = 0; num < var; ++num )); do
  echo "$num"
done

or

for (( num = 0; num < var; ++num )); do
  printf '%d\n' "$num"
done

If you know C or a language with C-like syntax, then you'll recognize this type of for-loop.

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