multiple sed operations creating empty file - shell

Question

When as part of shell script only one line is operating on a file using sed command the redirected file contains the updated data, as below
cat ${PROP_PATH}/${PROP_FILE} | sed "s!${ISTR_KEY}=.*!${ISTR_KEY}=${SIM_ISTR_KEY_VAL}!" > ${UPDATEDPROPS_DIR}/${PROP_FILE}
whereas when it is executed as part of a shell script, where after this another sed command updates the same file as in the below script at the end what i get is an empty file, why ? ..... ideas please.
(check 'switchAll2Sim()' function below)
#!/bin/ksh
#
SIM_ICR_KEY_VAL="http://www.example.com/sim/http/icr"
SIM_ISTR_KEY_VAL="http://www.example.com/sim/http/istr"
SIM_GT_KEY_VAL="http://www.example.com/sim/http/gtr"
#
ICR_KEY="interface.url.icr"
ISTR_KEY="interface.url.istr"
GT_KEY="interface.ws.url.gt"
## Property Files
PROP_PATH=""
PROP_FILE="properties"
##
DATE=`date +%m%d%Y`
DATETIME=`date +%m%d%Y-%T`
BCKUP_DIR=_bckup
UPDATEDPROPS_DIR=_updatedprops
# ----------------------------------
pause(){
echo "Press [Enter] key to continue..."
read fackEnterKey
}
permissions(){
chmod 777 ${UPDATEDPROPS_DIR}
}
backup(){
if [ ! -d "${BCKUP_DIR}" ]; then
mkdir ${BCKUP_DIR}
fi
if [ ! -d "${UPDATEDPROPS_DIR}" ]; then
mkdir ${UPDATEDPROPS_DIR}
fi
permissions
## keep backup of properties
cp ${PROP_PATH}/${PROP_FILE} ${BCKUP_DIR}/${PROP_FILE}_${DATETIME}
echo "Backup of property files completed at: " ${DATETIME}
}
#-------------------------------------------------------------
# switch all properties to SIM
#-------------------------------------------------------------
switchAll2Sim(){
backup
#
# update files
cat ${PROP_PATH}/${PROP_FILE} | sed "s!${ISTR_KEY}=.*!${ISTR_KEY}=${SIM_ISTR_KEY_VAL}!" > ${UPDATEDPROPS_DIR}/${PROP_FILE}
cat ${UPDATEDPROPS_DIR}/${PROP_FILE} | sed "s!${ICR_KEY}=.*!${ICR_KEY}=${SIM_ICR_KEY_VAL}!" > ${UPDATEDPROPS_DIR}/${PROP_FILE}
cat ${UPDATEDPROPS_DIR}/${PROP_FILE} | sed "s!${GT_KEY}=.*!${GT_KEY}=${SIM_GT_KEY_VAL}!" > ${UPDATEDPROPS_DIR}/${PROP_FILE}
echo "Switch all to SIM completed at: " ${DATETIME}
pause
}
# switch all properties to real
#-------------------------------------------------------------
switchAll2Real(){
pause
}
#-------------------------------------------------------------
dispCurrentStats(){
echo "Displaying current properties..."
echo "*********************************"
echo " File: " ${PROP_PATH}/${PROP_FILE}
grep ${ICR_KEY} ${PROP_PATH}/${PROP_FILE}
grep ${ISTR_KEY} ${PROP_PATH}/${PROP_FILE}
grep ${GT_KEY} ${PROP_PATH}/${PROP_FILE}
#
echo "*********************************"
pause
}
show_menus() {
clear
echo "~~~~~~~~~~~~~~~~~~~~~"
echo " M E N U"
echo "~~~~~~~~~~~~~~~~~~~~~"
echo "1. Display current properties"
echo "2. Switch all to real"
echo "3. Switch all to simulator"
echo "4. Exit"
}
# read input from the keyboard and take a action
read_options(){
read option
case $option in
1) dispCurrentStats ;;
2) switchAll2Real ;;
3) switchAll2Sim ;;
4) exit 0;;
*) echo "Please insert options 1 ~ 4";;
esac
}
# -----------------------------------
# Main - infinite loop
# ------------------------------------
while true
do
show_menus
read_options
done

Answer 1

Thanks, using '-i, says [sed: illegal option -- i]
Then you have to work with tmp files.
cp foo foo.tmp
sed "s/x/y/" foo.tmp > foo
/bin/rm foo.tmp
OR
sed "s/x/y/" foo > foo.tmp
/bin/mv -f foo.tmp foo
is probably more efficient.
I hope this helps.

Answer 2

Your problem is that cat is reading from the same file that sed is writing to.
cat foo | sed "s/x/y/" > foo
Will not work because cat and sed run at the same time, not one after the other.
To fix this try the -i option to sed.
sed -i "s/x/y/" foo