Count occurances of a character in a file

Shell Programming and Scripting
1

I want to find the number of occurences of a character in a file.
How do i do it.

Eg:
$cat file1.txt
Welcome to World of Unix.
$

If i want to find the occurences of 'o' then I should be getting 3.

Thanks.

2 
echo "start with this" | awk '{ for ( i=1; i<=length; i++ ) arr[substr($0, i, 1)]++ }END{ for ( i in arr ) { print i, arr } }'
3

Thanks buddy,

Can you explain me where the actual conting of string happens..
I am unable to understand the script

4 
 # awk 'BEGIN{FS=""}{array[$1]++}END{ # code to print values}' file
5

Extract each character and create an associative array which maps between the character and the count of character

6

Hi, Shivdatta.

Do you wish to count occurrences of all characters or of a specific character? ... cheers, drl

7 
tr -cd o file1.txt | wc -c