Add `BROWSER` support to `OpenURL()`

If the `BROWSER` environment variable is set, open the given URL using
the command specified in the variable.

Run the command in a shell process, either from the `SHELL` environment
variable or `/bin/sh` on Unix OSes. Run it through `cmd` on Windows.

Essentially repurposes my code from
https://github.com/pkg/browser/pull/14.

4 files changed, 97 insertions, 0 deletions

diff --git a/browserenv.go b/browserenv.go
index 8685fc8..2a7abcd 100644
--- a/browserenv.go
+++ b/browserenv.go
@@ -3,6 +3,7 @@ package browserenv
 import (
 	"io"
 	"os"
+	"os/exec"
 
 	"github.com/pkg/browser"
 )
@@ -19,5 +20,32 @@ func OpenReader(r io.Reader) error {
 }
 
 func OpenURL(url string) error {
+	envCommand := envBrowserCommand()
+	if envCommand != "" {
+		return runBrowserCommand(envCommand, url)
+	}
+
 	return browser.OpenURL(url)
 }
+
+// TODO
+func envBrowserCommand() string {
+	return os.Getenv("BROWSER")
+}
+
+// TODO
+func runBrowserCommand(command, url string) error {
+	return browserCommand(command, url).Run()
+}
+
+// TODO
+func browserCommand(command, url string) *exec.Cmd {
+	shellArgs := shell()
+	shell := shellArgs[0]
+	args := shellArgs[1:]
+
+	command = fmtBrowserCommand(command, url)
+	args = append(args, command)
+
+	return exec.Command(shell, args...)
+}
diff --git a/browserenv_unix.go b/browserenv_unix.go
new file mode 100644
index 0000000..e6416f8
--- /dev/null
+++ b/browserenv_unix.go
@@ -0,0 +1,25 @@
+// +build !windows
+
+package browserenv
+
+import (
+	"fmt"
+	"os"
+)
+
+// TODO
+func shell() (args []string) {
+	shell := os.Getenv("SHELL")
+
+	if shell == "" {
+		shell = "/bin/sh"
+	}
+
+	return []string{shell, "-c"}
+}
+
+// TODO
+func fmtBrowserCommand(browser, url string) string {
+	// TODO: handle %s in browser command
+	return fmt.Sprintf("%s '%s'", browser, url)
+}
diff --git a/browserenv_unix_test.go b/browserenv_unix_test.go
new file mode 100644
index 0000000..bcc7c2d
--- /dev/null
+++ b/browserenv_unix_test.go
@@ -0,0 +1,29 @@
+// +build !windows
+
+package browserenv
+
+import (
+	"fmt"
+	"os"
+	"reflect"
+	"testing"
+)
+
+func TestBrowserCommand(t *testing.T) {
+	envValue := "open -a Firefox"
+	url := "https://duckduckgo.com"
+
+	cmd := browserCommand(envValue, url)
+
+	shell := os.Getenv("SHELL")
+	if shell == "" {
+		shell = "/bin/sh"
+	}
+
+	browserCommand := fmt.Sprintf("%s '%s'", envValue, url)
+
+	wantArgs := []string{shell, "-c", browserCommand}
+	if !reflect.DeepEqual(cmd.Args, wantArgs) {
+		t.Errorf("got args '%#v' want '%#v'", cmd.Args, wantArgs)
+	}
+}
diff --git a/browserenv_windows.go b/browserenv_windows.go
new file mode 100644
index 0000000..ef9c2b7
--- /dev/null
+++ b/browserenv_windows.go
@@ -0,0 +1,15 @@
+package browserenv
+
+import "fmt"
+
+var shellArgs = []string{"cmd", "/c"}
+
+// TODO
+func shell() (args []string) {
+	return shellArgs
+}
+
+// TODO
+func fmtBrowserCommand(browser, url string) string {
+	return fmt.Sprintf("%s %s", browser, url)
+}