Useful Things You Can Do with FVWM

by Ronan McGoran

FVWM is a window manager used with the X Window System, which is the standard GUI for UNIX. This article explains a few useful things you can do with FVWM, including how to take screenshots conveniently, how to easily change a window's title and how to reconfigure easily a running FVWM instance.

This article is aimed at fairly knowledgeable UNIX users. A little programming experience would be helpful, but you probably can manage without it. No knowledge of X or FVWM is assumed.

The ideas presented in this document were tested with FVWM 2.4.16. They should work with any FVWM 2 version, however. Scripts presented in this document are written in Bash; I used the 2.05a.0(1)-release, but they should work in any version. Finally, I run Debian Linux 3.0. This means the utilities invoked from the shell scripts are assumed to be the standard Linux ones or to behave like them.

Taking Screenshots Conveniently

The method I present here allows you to save a screenshot of the current window by selecting an option from one of FVWM's window menus. If you use FVWM, it's almost certainly configured to put one or more buttons on the titlebar of each window. When clicked, one of these buttons--probably the one in the top left corner--brings up a menu that lists window operations such as move, resize and so on. We're going to modify this menu to have two new options, Take screenshot and Take named screenshot. Selecting the former will cause a screenshot with a unique name to be put in a standard directory. Selecting the latter will do the same thing, except it will prompt you for a name for the new image.

You may ask, why is this useful? You may go on to ask, can I not simply use a standard window-dumping utility, such as xwd, to take my screenshot? These are good questions, but the method I'm outlining offers several advantages. First, my method is more convenient than switching to another window and giving a command to take a screenshot. Second, my method chooses a filename for you, relieving you of the burden. The filename chosen is unique, ensuring that screenshots can be ordered according to the time at which they were taken.

The savings in time and trouble may seem minor, but when you need to take a series of screenshots from a particular window, they add up. Imagine that you want to explain to a client how to perform a particular operation. A good way of doing so is to send him a series of screenshots showing him every step in the sequence. Saving such a series is much less trouble with the method I'm about to outline.

FVWM Menu Definitions

As mentioned above, we're going to add two new options to an FVWM menu. To begin, let's look at how FVWM menus are defined. The following code, taken from a sample configuration file shipped with FVWM 2.4.16, defines an FVWM menu called Window-Ops2. More precisely, it adds elements to the end of the menu.

  AddToMenu Window-Ops2   "&Move"          Move
  +                       "&Resize"        Resize
  +                       "&Raise"         Raise
  +                       "&Lower"         Lower
  +                       "(De)&Iconify"   Iconify
  +                       "(Un)&Stick"     Stick
  +                       "(Un)&Maximize"  Maximize
  +                       ""               Nop
  +                       "&Delete"        Delete
  +                       "&Close"         Close
  +                       "&Destroy"       Destroy
  +                       "&ScrollBar"     Module FvwmScroll 2 2
  +                       "&Print"         PrintFunction
  +                       "&Print Reverse" PrintReverseFunction

A full description of how to define menus in FVWM is unnecessary for our purposes, so I'm not going to give one. An outline is necessary, however. I begin by recapitulating the above command in the following table, labeling each component:

Command for Adding to Menu DefinitionName of MenuMenu LabelFVWM Code for Menu Action
AddToMenu Window-Ops2&MoveMove
Plus Sign Menu LabelFVWM Code for Menu Action
+ &ResizeResize
+ &RaiseRaise
+ &LowerLower
+ (De)&IconifyIconify
+ (Un)&StickStick
+ (Un)&MaximizeMaximize
+  Nop
+ &DeleteDelete
+ &CloseClose
+ &DestroyDestroy
+ &ScrollBarModule
+ &PrintPrint Function
+ &PrintReverseReverse Print Function

This table shows the structure of the command. The first element is AddToMenu, an FVWM command. Next comes Window-Ops2, the name of the menu. After that comes &Move, which defines a menu label; I explain the function of the ampersand in a minute. Then comes Move, which is an FVWM command that is executed if the user chooses Move from the menu. Following this are a number of lines. Each line begins with a plus sign, which is followed by a label and then an FVWM command, possibly with arguments. In summary, the essence of the AddToMenu command is to bind menu labels to actions.

Now, about those ampersands: they're used in labels to define hotkeys. FVWM looks at the character after the ampersand to find out what the hotkey is. For example, a label of &Move defines M as a hotkey. If the user presses the M key after bringing up the menu, this is equivalent to clicking on the Move option.

Adding Our Own Action to the Menu

Suppose we want a simple way of saving a screenshot of the current window, so that when we select the correct option from a menu, a screenshot is saved to the file /tmp/screenshot. We can achieve this by adding the following line to the end of the AddToMenu command listed above:

  + "Take screenshot" Exec xwd -id $w -out /tmp/screenshot

Let's examine how this works. As already explained, the first two elements of the line are a plus sign and a label. Then comes the FVWM command, which in this case is:

  Exec xwd -id $w -out /tmp/screenshot

The command name is Exec, which allows one to execute an external command. Its arguments are the external command, followed by the external command's arguments. In this case, the external command is xwd, and the arguments passed to it by FVWM are -id $w -out /tmp/screenshot.

The notation $w in these arguments needs some explanation. It's a special FVWM variable that stands for the window ID of the current window. When FVWM runs the above xwd command, it replaces the argument $w with the relevant window ID.

What is a window ID? The X Window System needs a way of identifying and referring to windows. It does so by assigning each window a unique number, known as the window ID.

For our purposes, you don't need to understand xwd's other arguments just yet. For now, all you need to know is that the command given above causes xwd to put a screenshot of the current window in the file /tmp/screenshot.

At this point, you may be asking, FVWM has many menus; how do I identify the one I need to modify? The answer is the correct one probably is the one called Window-Ops2, but it all depends on whoever packaged and configured FVWM for you. The best way to find the correct menu is to search your FVWM config file for a menu that uses all or most of the following functions: Move, Resize, Raise, Lower, Iconify, Close. If you find such a menu, it's almost certainly the right one.

Running Our Window-Dumping Program

We've completed the preliminaries and now are ready to start looking at the promised solution. It involves a program I call savescreenshot. To have FVWM run it to take a screenshot, we add the following line to the appropriate AddToMenu command:

  + "Take screens&hot" Exec savescreenshot $w
  + "Take &named screenshot" Exec savescreenshot -n $w

You need to make sure savescreenshot can be found by searching the PATH that was defined when FVWM was invoked. If it can't, you can use an absolute pathname for savescreenshot. Also, notice that the above labels make H and N into hotkeys.

xwd, xwud and Conversion Programs

To actually take the screenshot, savescreenshot relies on xwd. Although several screenshot utilities exist, I chose xwd because it comes packaged with X. xwd's basic usage is as follows:

xwd [-id window_ID] [-out output_file]

where window_ID and output_file should be replaced by the actual names of the window ID and output file, respectively.

The -out option, as the summary above suggests, directs xwd's output to the specified output file; if you don't supply it, xwd writes to its standard output. The -id option tells xwd to use the window with the specified window ID. If you don't supply it, xwd changes the mouse pointer's shape to indicate that it wants you to click in a window to select it. When you've done so, it takes a screenshot of that window.

xwd uses its own special image format. This format doesn't use compression, so output files often are quite large. You usually want to convert the output file to a format such as PNG; I'll cover this in a minute.

To view xwd's output files, you can use xwud, which also comes packaged with X. You can specify an input file with the -in option. The command xwud -in browser-window would display the screenshot stored in the file browser-window.

To convert files in xwd format, you can use convert from the ImageMagick suite. The following commands convert an image to various common formats and should be self-explanatory:

  convert browser-window browser-window.png
  convert browser-window browser-window.jpg

If you want to use the netpbm tools, the following two commands correspond to the two commands above:

  xwdtopnm < browser-window | pnmtopng > browser-window.png
  xwdtopnm < browser-window | pnmtojpeg > browser-window.jpg

Converting Made Easy

I have a script called convert-xwd that makes the process of converting a slew of XWD files much easier. If you supply arguments, it assumes they're the names of XWD files and converts them to PNG format. If you supply no arguments, it finds all of the XWD files in the current directory and converts them. After successfully converting an XWD file to PNG, it deletes the XWD file. Here's the script:


  main() {
    if [ $# = 0 ]
      do_files *
      do_files "$@"

  do_files() {
    local file

    for file in "$@"
      if file "$file" | fgrep -q ": XWD X-Windows Dump image data,"
	echo "Doing $file"
	convert "$file" "$file.png" && rm "$file"

  main "$@"

I've given two sample invocations of savescreenshot. Now it's time to explain what they do. As the accompanying labels suggest, when run with no arguments, savescreenshot simply saves a screenshot, and when passed the -n option, it saves and names a screenshot.

savescreenshot uses two directories, $HOME/.screenshots/t and $HOME/.screenshots/n. If they don't exist, it creates them. In the first directory, savescreenshot saves a screenshot whose name has the form of NNN-HHMM.SS. NNN is a three-digit number representing the number of the day in the year, HH is a two-digit number representing the hour, MM is a two-digit number for the minute and SS is a two-digit number for the second.

The second directory, $HOME/.screenshots/n, is left untouched unless the -n option was given. If it was, savescreenshot pops up a window to ask the user what name the screenshot should have. After the user has answered, savescreenshot creates a symbolic link, using the name entered in the popup window, in $HOME/.screenshots/n pointing to the screenshot in $HOME/.screenshots/t.

The user needs to know when the screenshot has been taken so that he can continue interacting with the window he's dumped. xwd beeps when it's finished, but in case you have the bell disabled, as I do, savescreenshot also makes the screen flash red when xwd has completed.

Here's the savescreenshot program. I don't believe it needs much comment. One thing to note, however, is it handles prompting the user for a name by invoking itself again with a private option.

Another interesting feature is the savescreenshot program attempts to delay the screenshot a little if it believes the window may be slow to update. This can happen with windows whose contents are fetched over a network. Examples of applications that create such windows are remote administration programs such as VNC and rdesktop.


  main() {

    test -e "$timedir" || mkdir -p "$timedir"
    test -e "$namedir" || mkdir -p "$namedir"


    # we wait this amount of time to allow for the menu obscuring the
    # window to disappear.


    timestamp=`date +%j-%H%M.%S`

    case "$1" in
	read -p "Please supply name for screenshot: " name
	ln -s "../t/$timestamp" "$namedir/$name" 


    sleep $menu_linger_time

    sleep `get_delay_for_update $window_id`

    xwd -id "$window_id" -out "$timedir/$timestamp"

    xrefresh -solid red
    # this is to inform the user that xwd has finished.

    sleep .5
    xrefresh -solid red

    # just in case the user blinked and missed the first flash.

    if $prompt_for_name
      title="Please supply name..."
      x rxvt -T "$title" -n "$title" -e "$0" -p "$timestamp"

  # this function estimates how long it will take the application to repaint
  # the part of the window uncovered when the FVWM menu obscuring it disappears.
  # most applications repaint almost instantaneously, but some applications which
  # need to fetch data over a network in order to repaint take a little time.
  # this method isn't failsafe, because the window's title may have been 
  # manually changed.

  get_delay_for_update() {
    local window_id=$1

    case "`window_title "$window_id"`" in
      TightVNC:\ *)
	echo 2
      rdesktop\ *)
	echo 1
	echo 0

  window_title() {
    local window_id=$1

    xwininfo -id "$window_id" | perl -ne 'if(/"(.*)"\s*$/) { print $1; exit }'

  main "$@"

Making screenshots is a good thing to know how to do, but sometimes you want to capture continuous video. Here's a link to an interesting article on Newsforge that describes how to do this task.

Changing a Window Title

Your FVWM undoubtedly is configured to put a titlebar at the top of every window. To be entirely accurate here, I should have said nearly every window. Some applications, such as xbiff, create small persistent windows, and putting a titlebar on them would clutter the screen needlessly. These titles also are used by window-listing facilities, such as FVWM's WindowList command or its FvwmWinList module.

Below I present a program that easily changes the title on a window. It is invoked by opening the window operations menu--the same one we modified in the previous section--choosing the Change title option and entering the new title when prompted, whereupon the window's title is changed.

I often find that I have many windows open at one time. They obscure one another, of course, so the only way to manage them and bring up the one I want when I need it is to use some window-listing facility.

In order for me to be able to find the window I want in the list, windows need to have informative titles. Applications don't always choose informative titles for their windows, however. Furthermore, sometimes the functions of windows change over time, making the initial title inappropriate. Other times, I start several instances of one application, and they all use the same title, making it impossible to distinguish between them.

I've covered modifying FVWM menu definitions in an earlier section (see FVWM Menu Definitions), so I won't go over it again. Instead, below is the line you need to add to the relevant menu definition in order to be able to change the title:

+   "Change &title" Exec set-new-title $w

As before, if set-new-title isn't in the PATH, you have to use an absolute pathname.

Here's the program, called set-new-title. It relies on xwit, which is the utility that actually changes the title.


  set -u

  main() {

    title=`xwininfo -id "$window_id" | perl -ne 'print $1 if /"(.*)"\s*$/'`

    if ! tempfile=`mktemp` 
      echo "Cannot create temporary file" 1>&2
      exit 1

    echo "$title" > "$tempfile"

    popup_window_title="Choose new title in editor, then save and exit..."

    rxvt -T "$popup_window_title" -n "$popup_window_title" \
      -e "${EDITOR:-vi}" "$tempfile"

    newtitle=`head -c 1000 "$tempfile"`

    xwit -id "$window_id" -name "$newtitle"

  main "$@"

Reconfiguring a Running FVWM Instance

Sometimes one wants to temporarily reconfigure FVWM, perhaps to test the effect of some configuration command. One way to do this is to modify the config file and then restart FVWM. You probably will find, depending, of course, on how your FVWM is configured, that if you click on the root window, you get a menu containing the option Restart FVWM. This is a little tedious, however. Maneuvering the mouse to the root window, clicking on it and choosing the option to restart FVWM takes time, particularly if the root window is completely obscured by windows. And the restart of FVWM takes a significant fraction of a second. There's a better and faster way--the program FvwmCommand, which comes packaged with FVWM.

FvwmCommand can perform a number of functions. We're only interested in one function, however: sending commands to FVWM. Sending commands is a simple task: one simply runs FvwmCommand command_to_send. For example, the command to restart FVWM is called Restart, so to restart FVWM, one only need give the command FvwmCommand Restart at the shell prompt.

In order for FvwmCommand to work, it needs to talk to FVWM. It doesn't do so directly, however; instead, it does so through an intermediary, called FvwmCommandS. FvwmCommandS is an FVWM module, which is a program started by FVWM that communicates with it by means of a special protocol.

The best way to get FVWM to start FvwmCommandS is to put a command to this effect in FVWM's config file. The command to use is Module, and it takes as its argument the name of the module to start. So in this case, the full command is Module FvwmCommandS. Once you've put this command into the FVWM config file and restarted FVWM, you should find that FvwmCommand works for you.

Ronan McGoran is a UNIX system administrator. His home page can be found here.

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