Issue with WINCH signal - what am I overlooking?

Question

I am having an issue with the WINCH signal in the following code which is from a tutorial located at developerWorks®:

#!/bin/bash
trap 'get_window_size' WINCH                    # trap when a user has resized the window

_UNDERLINE_ON=`tput smul`                       # turn on underline
_UNDERLINE_OFF=`tput rmul`                      # turn off underline

get_window_size() {
  _WINDOW_X=`tput lines`
  _WINDOW_Y=`tput cols`

  _FULL_SPACES=`echo ""|awk '
  {
    _SPACES = '${_WINDOW_Y}'
    while (_SPACES-- > 0) printf (" ")
  }'`
  _FULL_UNDERLINE=`echo "${_UNDERLINE_ON}${_FULL_SPACES}${_UNDERLINE_OFF}"`

  unset _FULL_SPACES
  show_menu

  return 0
}

set_color() {
  tput clear
  PS3="Enter Selection[1-9]:"
  select _COLOR in "Black" "Blue" "Green" "Cyan" "Red" "Magenta" "Yellow" "White" "Exit"
  do
    case ${REPLY} in
       [1-8])  _X=`expr ${REPLY} - 1`;;
           9)  break;;
           *)  echo "Invalid Color"; continue;;
    esac

    if [[ ${1} = "b" ]]
    then
      tput setb ${_X}
        else
      tput setf ${_X}
    fi
  done
}

show_menu() {
  while [[ -z ${_ANS} ]]
  do
    tput civis
    tput clear

    cat <<- EOF
Window Size: ${_WINDOW_X} / ${_WINDOW_Y}


Select => ${_UNDERLINE_ON}     ${_UNDERLINE_OFF}

${_FULL_UNDERLINE}
B) Background Text Color
F) Foreground Text Color

X) Exit
EOF

    tput rc
    tput smul
    tput cnorm

    read _ANS
    tput rmul

    case ${_ANS} in
      [Bb])  set_color "b";;
      [Ff])  set_color "f";;
      [Xx])  tput clear; exit;;
         *)
             echo -e "Invalid Selection: ${_ANS}\c"
             sleep 2
             ;;
    esac
    unset _ANS
  done
}

tput sgr0
tput civis
tput clear
tput cup 3 10
tput sc
tput cup 0 0

[[ -n ${_ANS} ]] && unset _ANS
get_window_size

exit 0

Since I am trapping the WINCH (window change) signal, I expect that (each time I adjust the window which contains this running script) the get_window_size function will be invoked.

However, on my machine, it only traps the WINCH signal once (under specific conditions). The trap happens only if I use the xterm (in this case, gnome-terminal) Terminal menu to choose one of the default sizes (80x24, 80x43, 132x24, 132x43); further, it happens only the first such time. If I subsequently change the window size (using the Terminal menu item), no trapping occurs.

Also, if I don't use the Terminal menu item, but re-size using a pointing device, even the first trap does not happen.

Can someone help me to understand what is wrong (either with the code or with my expectation)?

Answer 1

I suspect there's something goofy in the way the script waits for a user action and/or a signal. Note that get_window_size calls show_menu, which is what waits for user input (read _ANS).

The trap command also causes get_window_size to be called by the SIGWINCH. I don't know if shell functions are re-entrant or not.

My expectations of how SIGWINCH should get generated, and when the trap function should get executed are basically the same as yours. I cut down the example script a bit, and made it non-reentrant. See below.

You may also want to consider trying your script with a different window manager. I used the venerable, speedy and highly configurable twm in my experiment.

#!/bin/bash
trap 'get_window_size' WINCH                    # trap when a user has resized the window
get_window_size() {
        _WINDOW_X=`tput lines`
        _WINDOW_Y=`tput cols`

        echo "X: $_WINDOW_X"
        echo "Y: $_WINDOW_Y"

        return 0
}

while read ALINE
do
        echo "Read: '$ALINE'"
done