Move logging functionality into new cmd.Log type

cmd/logging.go

+package cmd
+
+import (
+        "io"
+        "launchpad.net/gnuflag"
+        "launchpad.net/juju/go/log"
+        stdlog "log"
+        "os"
+)
+
+// Log supplies the necessary functionality for Commands that wish to set up
+// logging.
+type Log struct {
+        Path    string
+        Verbose bool
+        Debug   bool
+}
+
+// AddFlags adds appropriate flags to f.
+func (c *Log) AddFlags(f *gnuflag.FlagSet) {
+        f.StringVar(&c.Path, "log-file", "", "path to write log to")
+        f.BoolVar(&c.Verbose, "v", false, "if set, log additional messages")
+        f.BoolVar(&c.Verbose, "verbose", false, "if set, log additional messages")
+        f.BoolVar(&c.Debug, "debug", false, "if set, log debugging messages")
+}
+
+// Start starts logging using the given Context.
+func (c *Log) Start(ctx *Context) (err error) {
+        log.Debug = c.Debug
+        var target io.Writer
+        if c.Path != "" {
+                path := ctx.AbsPath(c.Path)
+                target, err = os.OpenFile(path, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0644)
+                if err != nil {
+                        return
+                }
+        } else if c.Verbose || c.Debug {
+                target = ctx.Stderr
+        }
+        if target != nil {
+                log.Target = stdlog.New(target, "", stdlog.LstdFlags)
+        } else {
+                log.Target = nil
+        }
+        return
+}