gistfile1.txt

BEGIN { LETTERS = "abcdefghijklmnopqrstuvwxyz" }

{
    len = length($0);
    if (len == 5) {
        for (i = 1; i <= len; i++) {
            c = substr($0, i, 1);
            ltr = index(LETTERS, c);

            if (ltr > 0) {
                ++count[ltr];
            }
        }
    }
}

END {
    for (i = 1; i <= 26; i++) {
        c = substr(LETTERS, i, 1);
        print(c, count[i])
    }
}

# cat /usr/share/dict/words | awk -f /tmp/count.awk | sort -k2,2rn
#
# a 5634
# e 4844
# r 3508
# o 3277
# i 3258
# n 2726
# l 2713
# s 2710
# t 2636
# u 2271
# y 1836
# c 1724
# d 1598
# h 1534
# m 1498
# p 1385
# b 1271
# g 1228
# k 1015
# w 722
# f 696
# v 514
# z 294
# x 232
# j 198
# q 95