generating frequency table from file

Question

Given an input file containing one single number per line, how could I get a count of how many times an item occurred in that file?

cat input.txt
1
2
1
3
1
0
         


        

Answer 1

Another option:

awk '{n[$1]++} END {for (i in n) print i,n[i]}' input.txt | sort -n > output.txt