Fvwm ChangeSize crashes

Question

I use simple script to make dynamic status bar of cpu usage (see below). After starting fvwm it works but after 3-4 minutes the bar disappears! The cpu.sh is simple (see below). Without ChangeSize there is no crashing (but there is no dynamic bar, static only). The cpu.sh gives integer number because ChangeSize needs for integer number. What maybe reason of this crashing? I spent already two days and do not understand reason.

WindowTitle {Status}
WindowSize 120 30

##### Global Style
Font "xft:DejaVu Sans:size=8:bold"

Init
Begin
Set $probarColor = {#cccccc} 
##### Widgets
ChangeBackColor 1  $probarColor
ChangeBackColor 3  $probarColor
End

PeriodicTasks
Begin
##### CPU Status
If (RemainderOfDiv (GetTime) 2) == 0 Then
 Begin
Set $length =  (GetOutput {echo $(bash    $HOME/.fvwm/scripts/StaTux/cpu.sh)} 1 -1) 
ChangeSize 3 $length 3
 End
End

Widget      1
Property
Type        ItemDraw
Size        120 4
Position        0 0
Flags       NoFocus NoReliefString
Main
Case message of
End

Widget      2
Property
Type        ItemDraw
Size        118 2
Position        1 1
Flags       NoFocus NoReliefString
Main
Case message of
End

Widget      3
Property
Type        ItemDraw
Size        118 2
Position        1 1
Flags       NoFocus NoReliefString
Main
Case message of
End

cpu.sh

#!/bin/sh
DELAY=${1:-1}
{ cat /proc/stat; sleep "$DELAY"; cat /proc/stat; } | awk '/^cpu / {usr=$2-usr; sys=$4-sys; idle=$5-idle; iow=$6-iow} END {total=usr+sys+idle+iow; print int((total-idle)*118/total)}'

Answer 1

As discussed in the comments, it seems FvwmScript crashes due to an X error BadValue when the value returned by the shell script is 0. This is probably because it is trying to configure the window to a size of 0.

An alternative solution is to use the HDipstick widget, which is a horizontal bar in a box of fixed length. Here is a short but complete example:

WindowTitle    {Status}
WindowSize     120 30
WindowPosition 900 1
Init
 Begin
  ChangeBackColor 3 {#000}
 End

PeriodicTasks
Begin
 If (RemainderOfDiv (GetTime) 2) == 0 Then
  Begin
   Set $length = (GetOutput {bash $HOME/.fvwm/scripts/StaTux/cpu.sh} 1 -1)
   ChangeValue 3 $length
 End
End

Widget 3
Property
 Type        HDipstick
 Value       0
 MinValue    0
 MaxValue    200
 ForeColor   {#f00}
 Size        110 20
 Position    5 5
Main
 Case message of
 End

The MinValue and MaxValue set the expected limits on the number you are getting back from your script. I chose 200 arbitrarily. The PeriodicTasks call of your script gets this number and uses ChangeValue to set the Value property to it. It will be scaled by dividing by 200 and multiplying by the width of the horizontal bar, given here in Size as 110 pixels.

When playing with this I noticed a bug in my version of FvwmScript which would draw a full bar for certain low values. For example, setting a MaxValue of 700 meant values 7 to 26 gave a full bar.