find all the files in a directory. Take out the first dot . put in by find. Remove slashes (can’t be a character in a filename). Use fold -w 1 (–width) the width option limits column output to 1 character, which puts each character on it’s own line. Don’t count spaces (we don’t care about them). Sort the output, count how many occurrences of each character happened. Sort output by least to most occurrences of characters.

find . -type f | sed -e 's/\.//' -e 's/\// /g' | fold -w 1 | grep -v '^ $' | sort | uniq -c | sort -nk1
      1 '
      7 ^
     22 ,
     29 (
     29 )
     40 #
     51 =
     72 ~
    214 @
    312 :
    672 Y
   1141 +
   1217 J
   1497 Z
   2813 G
   3696 U
   3727 H
   5168 O
   5654 N
   5700 X
   5721 K
  10185 R
  10590 W
  11414 F
  12412 A
  13114 E
  13424 C
  13904 z
  15369 Q
  15698 j
  18746 I
  20582 S
  30232 M
  39547 q
  44301 B
  44946 P
  54675 7
  74749 9
  74777 L
  78077 T
  83720 8
  86739 D
  87151 4
  92824 k
  93168 y
  94261 5
  96495 w
 105734 V
 135527 6
 193306 f
 215943 0
 239003 g
 274810 3
 284082 v
 291777 1
 305769 h
 329499 _
 353852 2
 397075 b
 493086 m
 513388 p
 523439 d
 539160 x
 654812 -
 697485 l
 717868 a
 728134 n
 843460 t
 862742 u
 883640 .
1059771 i
1060749 c
1109991 o
1227620 r
1326244 s
1440326 e