bash Cookbook

Summing a List of Numbers

Problem

You need to sum a list of numbers, including numbers that don’t appear on lines by themselves.

Solution

Use awk both to isolate the field to be summed and to do the summing. Here we’ll sum up the numbers that are the file sizes from the output of an ls -l command:

$ ls -l | awk '{sum += $5} END {print sum}'

Discussion

We are summing up the fifth field of the ls -l output. The output of ls-l looks like this:

-rw-r--r-- 1 albing users 267 2005-09-26 21:26 lilmax

and the fields are: permissions, links, owner, group, size (in bytes), date, time, and filename. We’re only interested in the size, so we use $5 in our awk program to reference that field.

We enclose the two bodies of our awk program in braces ({}); note that there can be more than one body (or block) of code in an awk program. A block of code preceded by the literal keyword END is only run once, when the rest of the program has finished. Similarly, you can prefix a block of code with BEGIN and supply some code that will be run before any input is read. The BEGIN block is useful for initializing variables, and we could have used one here to initialize sum, but awk guarantees that variables will start out empty.

If you look at the output of an ls -l command, you will notice that the first line is a total, and doesn’t fit our expected format for the other lines.

We have two choices for dealing with that. We can pretend it’s not there, which is the approach taken above. Since that undesired line doesn’t ...