How to count occurrences of each character?

You could use this:

sed 's/./&\n/g' 1.txt | sort | uniq -ic
  4  
  5 a
  1 c
  1 k
  1 M
  1 n
  5 o
  2 s
  4 t
  2 w
  1 y

The sed part places a newline after every character. Then we sort the ouput alphabetically. And at last uniq counts the number of occurences. The -i flag of uniq can be ommited if you don't want case insensitivity.

A bit late, but to complete the set, another python(3) approach, sorted result:

#!/usr/bin/env python3
import sys

chars = open(sys.argv[1]).read().strip().replace("\n", "")
[print(c+" -", chars.count(c)) for c in sorted(set([c for c in chars]))]

A - 1
M - 1
O - 1
T - 1
a - 4
c - 1
k - 1
n - 1
o - 4
s - 2
t - 3
w - 2
y - 1

Explanation

1. Read the file, skip spaces and returns as "characters":

   chars = open(sys.argv[1]).read().strip().replace("\n", "")
   

2. Create a (sorted) set of uniques:

   sorted(set([c for c in chars]))
   

3. Count and print the occurrence for each of the characters:

   print(c+" -", chars.count(c)) for c in <uniques>
   

How to use

1. Paste the code into an empty file, save it as chars_count.py

2. Run it with the file as an argument by either:

   /path/to/chars_count.py </path/to/file>
   

   if the script is executable, or:

   python3 /path/to/chars_count.py </path/to/file>
   

   if it isn't

By default in awk the Field Separator (FS) is space or tab. Since we want to count each character, we will have to redefine the FS to nothing(FS="") to split each character in separate line and save it into an array and at the end insideEND{..} block, print their total occurrences by the following awk command:

$ awk '{for (i=1;i<=NF;i++) a[$i]++} END{for (c in a) print c,a[c]}' FS="" file
A 1
M 1
O 1
T 1
a 4
c 1
k 1
n 1
o 4
s 2
t 3
w 2
y 1

In {for (i=1;i<=NF;i++) a[$i]++} ... FS="" ... block we just splits the characters. And
in END{for (c in a) print c,a[c]} block we are looping to array a and printing saved character in it print c and its number of occurrences a[c]