Best way to simulate "group by" from bash?

sort ip_addresses | uniq -c

This will print the count first, but other than that it should be exactly what you want.

The quick and dirty method is as follows:

cat ip_addresses | sort -n | uniq -c

If you need to use the values in bash you can assign the whole command to a bash variable and then loop through the results.

PS

If the sort command is omitted, you will not get the correct results as uniq only looks at successive identical lines.

for summing up multiple fields, based on a group of existing fields, use the example below : ( replace the $1, $2, $3, $4 according to your requirements )

cat file

US|A|1000|2000
US|B|1000|2000
US|C|1000|2000
UK|1|1000|2000
UK|1|1000|2000
UK|1|1000|2000

awk 'BEGIN { FS=OFS=SUBSEP="|"}{arr[$1,$2]+=$3+$4 }END {for (i in arr) print i,arr[i]}' file

US|A|3000
US|B|3000
US|C|3000
UK|1|9000