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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)?

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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
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  • Essentially, you were on to something in raising the re-entrancy question. I removed the call to show_menu from the get_window_size function and simply placed a call to show_menu beneath the call to get_window_size in the main section. This, indeed, resolves the issue! Thank you for your input! Commented Jan 11, 2012 at 6:01

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