xmonad calls the logHook with every internal state update, which is useful for (among other things) outputting status information to an external status bar program such as xmobar or dzen.

This module provides a composable interface for (re)starting these status bars and logging to them, either using pipes or X properties. There's also XMonad.Hooks.StatusBar.PP which provides an abstraction and some utilities for customization what is logged to a status bar. Together, these are a modern replacement for XMonad.Hooks.DynamicLog, which is now just a compatibility wrapper.

You can use this module with the following in your xmonad.hs:

import XMonad
import XMonad.Hooks.StatusBar
import XMonad.Hooks.StatusBar.PP

The easiest way to use this module with xmobar, as well as any other status bar that supports property logging, is to use statusBarProp with withEasySB; these take care of the necessary plumbing:

mySB = statusBarProp "xmobar" (pure xmobarPP)
main = xmonad $ withEasySB mySB defToggleStrutsKey def

You can read more about X11 properties here or here, although you don't have to understand them in order to use the functions mentioned above.

Most users will, however, want to customize the logging and integrate it into their existing custom xmonad configuration. The withSB function is more appropriate in this case: it doesn't touch your keybindings, layout modifiers, or event hooks; instead, you're expected to configure XMonad.Hooks.ManageDocks yourself. Here's what that might look like:

mySB = statusBarProp "xmobar" (pure myPP)
main = xmonad . withSB mySB . ewmh . docks $ def {...}

You then have to tell your status bar to read from the _XMONAD_LOG property of the root window. In the case of xmobar, this is achieved by simply using the XMonadLog plugin instead of StdinReader in your .xmobarrc:

Config { ...
       , commands = [ Run XMonadLog, ... ]
       , template = "%XMonadLog% }{ ..."
       }

If you don't have an .xmobarrc, create it; the XMonadLog plugin is not part of the default xmobar configuration and your status bar will not show workspace information otherwise!

With statusBarProp, you need to use property logging. Make sure the status bar you use supports reading a property string from the root window, or use some kind of wrapper that reads the property and pipes it into the bar (e.g. xmonadpropread | dzen2, see scripts/xmonadpropread.hs). The default property is _XMONAD_LOG, which is conveniently saved in xmonadDefProp. You can use another property by using the function statusBarPropTo.

If your status bar does not support property-based logging, you may also try statusBarPipe. It can be used in the same way as statusBarProp above (for xmobar, you now have to use the StdinReader plugin in your .xmobarrc). Instead of writing to a property, this function opens a pipe and makes the given status bar read from that pipe. Please be aware that this kind of setup is very bug-prone and hence is discouraged: if anything goes wrong with the bar, xmonad will freeze!

Also note that statusBarPipe returns 'IO StatusBarConfig', so you need to evaluate it before passing it to withSB or withEasySB:

main = do
  mySB <- statusBarPipe "xmobar" (pure myPP)
  xmonad $ withSB mySB myConf

StatusBarConfig is a Monoid, which means that multiple status bars can be combined together using <> or mconcat and passed to withSB.

Here's an example of what such declarative configuration of multiple status bars may look like:

-- Make sure to setup the xmobar configs accordingly
xmobarTop    = statusBarPropTo "_XMONAD_LOG_1" "xmobar -x 0 ~/.config/xmobar/xmobarrc_top"    (pure ppTop)
xmobarBottom = statusBarPropTo "_XMONAD_LOG_2" "xmobar -x 0 ~/.config/xmobar/xmobarrc_bottom" (pure ppBottom)
xmobar1      = statusBarPropTo "_XMONAD_LOG_3" "xmobar -x 1 ~/.config/xmobar/xmobarrc1"       (pure pp1)

main = xmonad $ withSB (xmobarTop <> xmobarBottom <> xmobar1) myConfig

And here is an example of the related xmobar configuration for the multiple status bars mentioned above:

xmobarrc_top
Config { ...
       , commands = [ Run XPropertyLog "_XMONAD_LOG_1", ... ]
       , template = "%_XMONAD_LOG_1% }{ ..."
       }

The above example also works if the different status bars support different logging methods: you could mix property logging and logging via pipes. One thing to keep in mind is that if multiple bars read from the same property, their content will be the same. If you want to use property-based logging with multiple bars, they should read from different properties.

XMonad.Util.Loggers includes loggers that can be bound to specific screens, like logCurrentOnScreen, that might be useful with multiple screens.

Long-time xmonad users will note that the above config is equivalent to the following less robust and more verbose configuration that they might find in their old configs:

main = do
  -- do not use this, this is an example of a deprecated config
  xmproc0 <- spawnPipe "xmobar -x 0 ~/.config/xmobar/xmobarrc_top"
  xmproc1 <- spawnPipe "xmobar -x 0 ~/.config/xmobar/xmobarrc_bottom"
  xmproc2 <- spawnPipe "xmobar -x 1 ~/.config/xmobar/xmobarrc1"
  xmonad $ def {
    ...
    , logHook = dynamicLogWithPP ppTop { ppOutput = hPutStrLn xmproc0 }
             >> dynamicLogWithPP ppBottom { ppOutput = hPutStrLn xmproc1 }
             >> dynamicLogWithPP pp1 { ppOutput = hPutStrLn xmproc2 }
    ...
  }

By using the new interface, the config becomes more declarative and there's less room for errors.

The only *problem* now is that the status bars will not be updated when your screen configuration changes (by plugging in a monitor, for example). Check the section on dynamic status bars for how to do that.

Using multiple status bars by just combining them with <> works well as long as the screen configuration does not change often. If it does, you should use dynamicSBs: by providing a function that creates status bars, it takes care of setting up the event hook, the log hook and the startup hook necessary to make the status bars, well, dynamic.

xmobarTop    = statusBarPropTo "_XMONAD_LOG_1" "xmobar -x 0 ~/.config/xmobar/xmobarrc_top"    (pure ppTop)
xmobarBottom = statusBarPropTo "_XMONAD_LOG_2" "xmobar -x 0 ~/.config/xmobar/xmobarrc_bottom" (pure ppBottom)
xmobar1      = statusBarPropTo "_XMONAD_LOG_3" "xmobar -x 1 ~/.config/xmobar/xmobarrc1"       (pure pp1)

barSpawner :: ScreenId -> IO StatusBarConfig
barSpawner 0 = pure $ xmobarTop <> xmobarBottom -- two bars on the main screen
barSpawner 1 = pure $ xmobar1
barSpawner _ = mempty -- nothing on the rest of the screens

main = xmonad $ dynamicSBs barSpawner (def { ... })

Make sure you specify which screen to place the status bar on (in xmobar, this is achieved by the -x argument). In addition to making sure that your status bar lands where you intended it to land, the commands are used internally to keep track of the status bars.

Note also that this interface can be used with one screen, or if the screen configuration doesn't change.