Ascending numbers

echo {0..10}

Descending numbers

echo {10..0}

Every second number

echo {0..10..2}

Letters

echo {a..z}

Combine

echo {0..2}{0..2}

00 01 02 10 11 12 20 21 22

Store as a variable.

$ LETTER_COMBOS=({a..z}{a..z})

$ echo $LETTER_COMBOS
aa ab ac ad ae af ag ah ai aj ak al am an ao ap aq ...