Share a single screen (when having multi-screens) on slack through webcam (bad quality though)

v4l2loopback-install.sh

#!/bin/bash

export V4L2L_DIR=$(mktemp -d)
sudo apt-get install git v4l-utils dialog libelf-dev
echo ========================================================================
echo == REMOVING ANY PREVIOUS INSTALLATION OF v4l2loopback ==================
echo ========================================================================
sudo apt-get purge v4l2loopback-dkms v4l2loopback-utils
find "/lib/modules/$(uname -r)" | grep v4l2loopback | sudo xargs rm -f
echo ========================================================================
echo == CLONING AND ISNTALLING v4l2loopback FROM GITHUB =====================
echo ========================================================================
git clone https://github.com/umlaeute/v4l2loopback.git $V4L2L_DIR
make "--directory=$V4L2L_DIR"
make "--directory=$V4L2L_DIR" install
echo ========================================================================
echo == REBUILDING KERNEL MODULES DEPENDENCY TREE ===========================
echo ========================================================================
sudo depmod -ae

v4l2loopback-run.sh

#!/bin/bash

# global options
DO_FLIP_HORIZONTALLY=1 # slack shares webcam horizontally-fliped

echo =============================================================================
echo == Loading V4L2-Loopback kernel module with 2 devices \(to simulate webcam\) ==
echo =============================================================================
sudo modprobe v4l2loopback devices=2 exclusive_caps=1,1

echo =============================================================================
echo == Fetching V4L2-Loopback device id =========================================
echo =============================================================================
FIRST_VIDEO_LOOPBACK=$(v4l2-ctl --list-devices | grep --after 1 v4l2loopback-000 | tail -n -1 | awk '{ print $1 }')

echo =============================================================================
echo == Fetching screen region information =======================================
echo =============================================================================
ALL_SCREENS_DATA=/tmp/v4l2loopback-all-screens-names.log
xrandr | grep -w connected | sed -E 's|(\S+) connected (primary )?([0-9]+)x([0-9]+)\+([0-9]+)\+([0-9]+).*|\1 \3x\4 \5,\6|' > "$ALL_SCREENS_DATA"

echo =============================================================================
echo == Showing curses dialog to select which screen to share ====================
echo =============================================================================
HEIGHT=15
WIDTH=40
CHOICE_HEIGHT=4
BACKTITLE="Select the screen that you would like to share"
TITLE="Display Select"
MENU="Choose one of the displays:"
CHOICE=$(dialog --clear \
                --backtitle "$BACKTITLE" \
                --title "$TITLE" \
                --menu "$MENU" \
                $HEIGHT $WIDTH $CHOICE_HEIGHT \
                `awk '{ print ++i " " $1 "_(" $3 ")" }' "$ALL_SCREENS_DATA"` \
                2>&1 >/dev/tty)

# clean screen
printf "\033c"
echo
tput reset

echo "                                                             "
echo "  ==========================================================="
echo "                                                             "
echo "       Casting screen.. you can watch it by running:         "
echo "                                                             "
echo "       vlc v4l2://$FIRST_VIDEO_LOOPBACK                      "
echo "                                                             "
echo "  ==========================================================="
echo "                                                             "
ffmpeg \
    -loglevel error \
    -f x11grab \
    -r 15 \
    -s $(sed -n "${CHOICE}p" "$ALL_SCREENS_DATA" | awk '{print $2}') \
    -i :0+$(sed -n "${CHOICE}p" "$ALL_SCREENS_DATA" | awk '{print $3}') \
    -vcodec rawvideo -pix_fmt yuv420p \
    $([ "$DO_FLIP_HORIZONTALLY" == 1 ] && echo "-vf hflip -c:a copy" || echo "") \
    -threads 0 \
    -f v4l2 $FIRST_VIDEO_LOOPBACK

Thank you, everything works but I had to protect (escape) the parentheses : (to simulate webcam) => \ (to simulate webcam \ ).