gistfile1.txt

#!/bin/bash
# lazily modified from here: http://unix.stackexchange.com/questions/204069/all-possible-combinations-of-characters-and-numbers

for c1 in {A..Z} {a..z} {0..9}
do
    for c2 in {A..Z} {a..z} {0..9}
    do
        for c3 in {A..Z} {a..z} {0..9}
        do
            for c4 in {A..Z} {a..z} {0..9}
            do
                printf "%s\n" "$c1$c2$c3$c4"
            done
        done
    done
done