add support for any external command as "cmd:command goes here"

README.md

@@ -4,12 +4,16 @@ The `getpass` module provides a simple way to retrieve a password from
 the user by specifying a number of different password sources:
 
 ```
-func Getpass(passfrom) (pass string, err error)
+func Getpass(passfrom string) (pass string, err error)
 ```
 
 Getpass retrieves a password from the user using a method defined by
 the `passfrom` string.  The following methods are supported:
 
+`cmd:command` -- Obtain the password by running the given command.  The
+command will be passed to the shell for execution via "/bin/sh -c
+'command'".
+
 `env:var` -- Obtain the password from the environment variable var.
 Since the environment of other processes may be visible
 via e.g. ps(1), this option should be used with caution.
@@ -78,7 +82,7 @@ func main() {
 	}
 
 	// Using a custom prompt:
-	p, err = getpass.Getpass("tty:Please enter your secret passphrase: ")
+	p, err = getpass.Getpass("tty", "Please enter your secret passphrase: ")
 	if err != nil {
 		log.Fatal("Unable to get password from user: %v\n", err)
 	} else {

example/getpass_example.go

@@ -57,5 +57,12 @@ func main() {
 		fmt.Printf("%s\n", p)
 	}
 
+	// Using a command:
+	p, err = getpass.Getpass("cmd:env | grep -i mysecret")
+	if err != nil {
+		log.Fatal("Unable to get password from user: ", err)
+	} else {
+		fmt.Printf("%s\n", p)
+	}
 	// etc. etc.
 }

getpass.go

@@ -15,17 +15,20 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
-	"os/user"
 	"os/signal"
+	"os/user"
 	"regexp"
 	"runtime"
 	"strings"
 	"syscall"
 )
 
-// Getpass retrieves a password from the user using a
-// method defined by the 'passfrom' string.  The
-// following methods are supported:
+// Getpass retrieves a password from the user using a method defined by
+// the 'passfrom' string.  The following methods are supported:
+//
+//  cmd:command    Obtain the password by running the given command.
+//                 The command will be passed to the shell for execution
+//                 via "/bin/sh -c 'command'".
 //
 //  env:var        Obtain the password from the environment variable var.
 //                 Since the environment of other processes may be visible
@@ -77,6 +80,8 @@ func Getpass(passfrom ...string) (pass string, err error) {
 	}
 
 	switch source {
+	case "cmd":
+		return getpassFromCommand(passin[1])
 	case "env":
 		return getpassFromEnv(passin[1])
 	case "file":
@@ -105,6 +110,15 @@ func Getpass(passfrom ...string) (pass string, err error) {
 	return pass, nil
 }
 
+func getpassFromCommand(command string) (pass string, err error) {
+	cmd := []string{"/bin/sh", "-c", command}
+	out, err := runCommand(cmd, "", true)
+	if err != nil {
+		return "", err
+	}
+	return out, nil
+}
+
 func getpassFromEnv(varname string) (pass string, err error) {
 	errMsg := fmt.Sprintf("environment variable '%v' not set", varname)
 	pass = os.Getenv(varname)

getpass_test.go

@@ -52,6 +52,24 @@ func TestGetpassPass(t *testing.T) {
 	}
 }
 
+// TestGetpassPass tests that Getpass with a passfrom set to 'cmd:'
+// executes the given command with full shell evaluation..
+func TestGetpassCmd(t *testing.T) {
+	want := os.Getenv("PATH")
+	cmd := "echo $PATH"
+	p, err := Getpass("cmd:" + cmd)
+	if err != nil || p != want {
+		t.Fatalf(`Getpass("cmd:%s") = %q, %v, want %s, nil`, cmd, p, err, want)
+	}
+
+	want = "/dev/null"
+	cmd = "ls -l /dev/null | awk '{print $NF}'"
+	p, err = Getpass("cmd:" + cmd)
+	if err != nil || p != want {
+		t.Fatalf(`Getpass("cmd:%s") = %q, %v, want %s, nil`, cmd, p, err, want)
+	}
+}
+
 // TestGetpassFail tests that Getpass with an invalid
 // passfrom fails.
 func TestGetpassFail(t *testing.T) {