Improve bash code

- more modern bash code (e.g. replace backtick expansions with
  `$([...])`)
- handle more paths (e.g. with spaces); this boils down to simply
  quoting every variable
- prefer clearer `${BASH_SOURCE[0]}` over `$BASH_SOURCE`

activate.sh

 #!/bin/bash
-RELATIVE_PATH=`dirname ${BASH_SOURCE}`
-ABSOLUTE_PATH=`realpath ${RELATIVE_PATH}`
 
-[[ $0 != $BASH_SOURCE ]] && echo "The activation script must be sourced, otherwise the virtual environment will not work." || ( echo "Vars script must be sourced." && exit 1) ;
+RELATIVE_PATH="$(dirname "${BASH_SOURCE[0]}")"
+ABSOLUTE_PATH="$(realpath "${RELATIVE_PATH}")"
 
-source ${ABSOLUTE_PATH}/config.sh
-source ${ABSOLUTE_PATH}/modules.sh
+[[ "$0" != "${BASH_SOURCE[0]}" ]] && echo "The activation script must be sourced, otherwise the virtual environment will not work." || ( echo "Vars script must be sourced." && exit 1) ;
 
-export PYTHONPATH=`echo ${ENV_DIR}/lib/python*/site-packages`:${PYTHONPATH}
+source "${ABSOLUTE_PATH}"/config.sh
+source "${ABSOLUTE_PATH}"/modules.sh
 
-source ${ENV_DIR}/bin/activate
+export PYTHONPATH="$(echo "${ENV_DIR}"/lib/python*/site-packages):${PYTHONPATH}"
+
+source "${ENV_DIR}"/bin/activate

config.sh

 ## Check if this script is sourced
-[[ $0 != $BASH_SOURCE ]] && echo "Setting vars" || ( echo "Vars script must be sourced." && exit 1) ;
+[[ "$0" != "${BASH_SOURCE[0]}" ]] && echo "Setting vars" || ( echo "Vars script must be sourced." && exit 1) ;
 ## Determine location of this file
-RELATIVE_PATH=`dirname ${BASH_SOURCE}`
-ABSOLUTE_PATH=`realpath ${RELATIVE_PATH}`
+RELATIVE_PATH="$(dirname "${BASH_SOURCE[0]}")"
+ABSOLUTE_PATH="$(realpath "${RELATIVE_PATH}")"
 ####################################
 
 ### User Configuration
-export ENV_NAME=`basename $ABSOLUTE_PATH`             # Default Name of the venv is the directory that contains this file
-export ENV_DIR=${ABSOLUTE_PATH}/venv         # Default location of this VENV is "./venv"
+export ENV_NAME="$(basename "$ABSOLUTE_PATH")"             # Default Name of the venv is the directory that contains this file
+export ENV_DIR="${ABSOLUTE_PATH}"/venv         # Default location of this VENV is "./venv"

create_kernel.sh

 #!/bin/bash
 
-RELATIVE_PATH=`dirname ${BASH_SOURCE}`
-ABSOLUTE_PATH=`realpath ${RELATIVE_PATH}`
-source ${ABSOLUTE_PATH}/config.sh
+RELATIVE_PATH="$(dirname "${BASH_SOURCE[0]}")"
+ABSOLUTE_PATH="$(realpath "${RELATIVE_PATH}")"
+source "${ABSOLUTE_PATH}"/config.sh
 
-KERNELFILE=${ENV_DIR}/kernel.sh
+KERNELFILE="${ENV_DIR}"/kernel.sh
 
-echo the name is $ENV_NAME
+echo the name is "$ENV_NAME"
 
-echo "Setting up the kernel script in the following dir: " ${KERNELFILE}
+echo "Setting up the kernel script in the following dir: " "${KERNELFILE}"
 
 echo '#!/bin/bash
 
-source '"${ABSOLUTE_PATH}"'/activate.sh
+source "'"${ABSOLUTE_PATH}"'"/activate.sh
 
-exec python -m ipykernel $@' > ${KERNELFILE}
+exec python -m ipykernel "$@"' > "${KERNELFILE}"
 
-chmod a+x ${KERNELFILE}
+chmod a+x "${KERNELFILE}"
 
-mkdir -p ~/.local/share/jupyter/kernels/${ENV_NAME}
+mkdir -p ~/.local/share/jupyter/kernels/"${ENV_NAME}"
 echo '{
  "argv": [
   "'"${KERNELFILE}"'",
   "-f",
   "{connection_file}"
  ],
- "display_name": "'${ENV_NAME}'",
+ "display_name": "'"${ENV_NAME}"'",
  "language": "python"
-}' > ~/.local/share/jupyter/kernels/${ENV_NAME}/kernel.json
+}' > ~/.local/share/jupyter/kernels/"${ENV_NAME}"/kernel.json

setup.sh

 #!/bin/bash
-RELATIVE_PATH=`dirname ${BASH_SOURCE}`
-ABSOLUTE_PATH=`realpath ${RELATIVE_PATH}`
 
-source ${ABSOLUTE_PATH}/config.sh
-source ${ABSOLUTE_PATH}/modules.sh
+RELATIVE_PATH="$(dirname "${BASH_SOURCE[0]}")"
+ABSOLUTE_PATH="$(realpath "${RELATIVE_PATH}")"
 
-python -m venv --prompt $ENV_NAME --system-site-packages ${ENV_DIR}
+source "${ABSOLUTE_PATH}"/config.sh
+source "${ABSOLUTE_PATH}"/modules.sh
 
-source ${ABSOLUTE_PATH}/activate.sh
+python -m venv --prompt "$ENV_NAME" --system-site-packages "${ENV_DIR}"
 
-python -m pip install -r ${ABSOLUTE_PATH}/requirements.txt
+source "${ABSOLUTE_PATH}"/activate.sh
+
+python -m pip install -r "${ABSOLUTE_PATH}"/requirements.txt