How to get all packages’ code coverage together in Go?

EDIT: Things have changed since I wrote this answer. See the release notes of Go 1.10: https://golang.org/doc/go1.10#test :

The go test -coverpkg flag now interprets its argument as a
comma-separated list of patterns to match against the dependencies of
each test, not as a list of packages to load anew. For example, go
test -coverpkg=all is now a meaningful way to run a test with coverage
enabled for the test package and all its dependencies. Also, the go
test -coverprofile option is now supported when running multiple
tests.

You can now run

go test -v -coverpkg=./... -coverprofile=profile.cov ./...
go tool cover -func profile.cov

Old answer

Here is a bash script extracted from https://github.com/h12w/gosweep :

#!/bin/bash
set -e

echo 'mode: count' > profile.cov

for dir in $(find . -maxdepth 10 -not -path './.git*' -not -path '*/_*' -type d);
do
if ls $dir/*.go &> /dev/null; then
    go test -short -covermode=count -coverprofile=$dir/profile.tmp $dir
    if [ -f $dir/profile.tmp ]
    then
        cat $dir/profile.tmp | tail -n +2 >> profile.cov
        rm $dir/profile.tmp
    fi
fi
done

go tool cover -func profile.cov