Why does bash ignore a '-' in a sequence brace expansion?

This yields all numbers between 1 and 10

echo {1..10}

This one yields all odd numbers between 1 and 10 (increment/step value is 2)

echo {1..10..2}

I experimented a bit, and it turned out if I prefix the increment by a - sign, it has no effect

echo {1..10..-2}

Why is th开发者_StackOverflow中文版is accepted, rather than being an error?

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From the bash(1) man page:

When the increment is supplied, it is used as the difference between each term.

So... technically the output does have a difference of -2 between each term. But you still told it to increment rather than decrement in the sequence.

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Experimentation with Bash 4.1 (as opposed to the 3.2 version installed by default on the machine I'm using, which does not recognize the notation as special) shows:

$ echo {12..10..2}
12 10
$ echo {12..10..-2}
12 10
$ echo {12..-10..2}
12 10 8 6 4 2 0 -2 -4 -6 -8 -10
$ echo {12..-10..-2}
12 10 8 6 4 2 0 -2 -4 -6 -8 -10
$ echo {-12..-10..-2}
-12 -10
$ echo {-12..-10..2}
-12 -10
$

So, it seems that the direction of the incrementing is controlled by the first two numbers; the magnitude of the incrementing is controlled by the third (defaulting to 1 if the third is missing).