install.sh: Refactor install script, Add new flags

Giving either --install or --update flag is neccessary for the script to proceed

    -i | --install       => Install the config.

    -u | --update        => Update the existing config without removing existing stuff.

    -b | --backup 'path' => Custom config backup path.

    -nb | --no-backup    => Don't take existing config backup. Use with caution.

Update formatting using following command
    shfmt -i 2 -sr -p -bn -ci -w install.sh

install.sh

@@ -1,74 +1,200 @@
 #!/bin/sh
 
-# check if git command is installed
-command -v git > /dev/null || {
-  printf "Install git before proceeding\n"
-  exit 1
+_usage() {
+  printf "%s" \
+    "Usage: sh ${0##*/} [ options ]
+    -h | --help          => Show this help.
+
+    -i | --install       => Install the config.
+
+    -u | --update        => Update the existing config without removing existing stuff.
+
+    -b | --backup 'path' => Custom config backup path.
+
+    -nb | --no-backup    => Don't take existing config backup. Use with caution.
+"
+  exit 0
 }
 
-printf "%s\n" "Installing packer"
+_check_install_dependencies() {
+  _error_dependencies() {
+    printf "%s\n" "Error: Install ${1} before proceeding."
+    exit 1
+  }
+  _GIT="$(command -v git)" || _error_dependencies git
+  _SED="$(command -v sed)" || _error_dependencies sed
+  return 0
+}
 
-if [ -d ~/.local/share/nvim/site/pack/packer ]; then
-  printf "%s\n" "Clearning previous packer installs"
-  rm -rf ~/.local/share/nvim/site/pack
-fi
+_check_nvim_version() {
+  version_string="$(nvim --version | grep -Eo 'NVIM.*v[0-9.]+' -m 1 | grep -Eo '[0-9.]+')" || return 1
+  printf "%s\n" "${version_string}" | while IFS=. read -r num1 num2 num3; do
+    { [ "${num1}" -ge 0 ] && [ "${num2}" -ge 5 ] && [ "${num3}" -ge 0 ]; } || return 1
+  done
+}
+
+_setup_packer() {
+  _clone_packer() {
+    [ -d "${_PACKER_PATH}" ] && rm -rf "${_PACKER_PATH}"
 
     printf "\n%s\n" "=> Cloning packer.."
-if git clone https://github.com/wbthomason/packer.nvim \
-  ~/.local/share/nvim/site/pack/packer/start/packer.nvim; then
-  printf "%s\n" "=> Packer installed!"
+    if "${_GIT}" clone "${_PACKER_REPO_URL}" \
+      "${_PACKER_PATH}"; then
+      printf "%s\n" "=> Packer Installed!"
     else
       printf "Error: Couldn't clone packer\n"
       exit 1
     fi
+  }
 
+  if [ "${_UPDATE}" = "true" ]; then
+    printf "%s\n" "Updating packer"
+    { [ -d "${_PACKER_PATH}" ] \
+      && "${_GIT}" -C "${_PACKER_PATH}" pull "${_PACKER_REPO_URL}"; } || _clone_packer
+  else
+    printf "%s\n" "Installing packer"
+    if [ -d ~/.local/share/nvim/site/pack/packer ]; then
+      printf "%s\n" "Clearning previous packer installs"
+      rm -rf ~/.local/share/nvim/site/pack
+    fi
+    _clone_packer
+  fi
+  printf "\n"
+  return 0
+}
+
+_copy_config() {
   printf "%s\n" "Linking config"
   printf "%s\n" "Old nvim config will be changed to nvim.bak if exists! :0"
 
   # copying config
-if [ -d ~/.config/nvim ]; then
+  if [ -d "${_CONFIG_PATH}" ]; then
     printf "%s\n" "Nvim Directory exists"
-   printf "%s\n" "Changing nvim to nvim.bak"
-   mv ~/.config/nvim/ ~/.config/nvim.bak/
+    if [ "${_NO_BACKUP}" = "true" ]; then
+      printf "%s\n" "Skipping backup as --no-backup flag was passed.."
+    else
+      printf "%s\n" "Taking backup of existing config.."
+      mv "${_CONFIG_PATH}" "${_BACKUP_PATH}" || {
+        printf "\n%s\n" "Error: Cannot move ${_CONFIG_PATH} to ${_BACKUP_PATH}"
+        printf "\n%s\n" "Take backup manually or use -b/--backup to provide non-empty path or use -nb/--no-backup to skip backup."
+        exit 1
+      }
+    fi
     printf "%s\n" "Creating new nvim directory"
-   mkdir -p ~/.config/nvim
   else
     printf "%s\n" "Nvim Config doesn't exist so creating one"
-   mkdir -p ~/.config/nvim/
   fi
 
-{ cp -r init.lua ~/.config/nvim/ && cp -r lua ~/.config/nvim/ ;} || {
-  printf "Error: Couldn't copy nvim config\n"
+  mkdir -p "${_CONFIG_PATH}" || {
+    printf "%s\n" "Error: Cannot create folder ${_CONFIG_PATH}"
     exit 1
   }
 
-# change shell in nvim config
-_CURRENT_SHELL="${SHELL##*/}"
+  { cp -r init.lua lua "${_CONFIG_PATH}"/; } || {
+    printf "Error: Couldn't copy nvim config\n"
+    exit 1
+  }
+  return 0
+}
+
+_setup_terminal_shell() {
+  _mappings_file="${_CONFIG_PATH}/lua/mappings.lua"
+  # only ask for shellname if running in terminal
+  if [ -t 1 ]; then
     printf "%s\n: " "Which shell do you want to use ? [ Enter nothing for current shell ( $_CURRENT_SHELL ) ]"
     read -r shellname
+  fi
   shellname="${shellname:-${_CURRENT_SHELL}}"
   printf "%s\n" "$shellname"
 
   # don't try to do any changes if given shellname is same as bash
   if ! [ bash = "$shellname" ]; then
     # Reference: https://stackoverflow.com/a/4247319
-  if "$(command -v sed)" -i'.bak' -e "s/bash/$shellname/g" ~/.config/nvim/lua/mappings.lua; then
+    if "${_SED}" -i'.bak' -e "s/bash/$shellname/g" "${_mappings_file}"; then
       printf "\n%s\n" "=> Shell changed to $shellname on nvim successfully!"
     else
-    printf "\n%s\n" "Cannot edit with sed, edit ~/.config/nvim/lua/mappings.lua manually to replace bash with $shellname."
+      printf "\n%s\n" "Cannot edit with sed, edit ${_mappings_file} manually to replace bash with $shellname."
     fi
-  rm -f ~/.config/nvim/lua/mappings.lua.bak # delete backup file created by sed
+    rm -f "${_mappings_file}".bak # delete backup file created by sed
   else
     printf "\n%s\n" "=> Shell changed to $shellname on nvim successfully!"
   fi
+  return 0
+}
 
-printf "\n%s\n" "=> Neovim will open with some errors, just press enter" && sleep 1
+_setup_arguments() {
+  # default variables to be used
+  _PACKER_PATH="${HOME}/.local/share/nvim/site/pack/packer/start/packer.nvim"
+  _PACKER_REPO_URL="https://github.com/wbthomason/packer.nvim"
+  _CONFIG_PATH="${HOME}/.config/nvim"
+  _UPDATE=""
+  _BACKUP_PATH="${_CONFIG_PATH}.bak"
+  _NO_BACKUP="false"
+  _CURRENT_SHELL="${SHELL##*/}"
+
+  _check_longoptions() {
+    [ -z "${2}" ] \
+      && printf '%s: %s: option requires an argument\nTry '"%s -h/--help"' for more information.\n' "${0##*/}" "${1}" "${0##*/}" \
+      && exit 1
+    return 0
+  }
+
+  while [ $# -gt 0 ]; do
+    case "${1}" in
+      -h | --help) _usage ;;
+      -i | --install) _UPDATE="false" ;;
+      -u | --update) _UPDATE="true" ;;
+      -b | --backup)
+        _check_longoptions "${1}" "${2}"
+        _BACKUP_PATH="${2}" && shift
+        ;;
+      -nb | --nobackup) _NO_BACKUP="true" ;;
+    esac
+    shift
+  done
+
+  [ -z "${_UPDATE}" ] && {
+    printf "%s\n" "Error: Either -i/--install or -u/--update flag must be used."
+    printf "%s\n" "See -h/--help for more info."
+    exit 1
+  }
+
+  printf "%s\n" \
+    "CONFIG PATH: ${_CONFIG_PATH}
+$(if [ "${_NO_BACKUP}" = "true" ]; then
+      printf "SKIPPING CONFIG BACKUP !\n"
+    else
+      printf "%s\n" "CONFIG BACKUP PATH: ${_BACKUP_PATH}"
+    fi)
+CURRENT SHELL: ${_CURRENT_SHELL}
+"
+
+  return 0
+}
+
+main() {
+  [ $# = 0 ] && _usage
+
+  _check_install_dependencies
+
+  _setup_arguments "${@}"
+  _setup_packer
+  _copy_config
+  _setup_terminal_shell
 
   # install all plugins + compile them
   if _NVIM="$(command -v nvim)"; then
+    if _check_nvim_version; then
+      printf "\n%s\n" "=> Neovim will open with some errors, just press enter" && sleep 1
       "${_NVIM}" +PackerSync
+    else
+      printf "Error: Neovim is installed, but version is lower than 0.5.x, install Neovim >= 5.x and then run the below command.\n"
+      printf "  nvim +PackerSync\n"
+    fi
   else
     printf "Error: Neovim is not installed, install Neovim >= 5.x and then run the below command\n"
     printf "  nvim +PackerSync\n"
-  exit 1
   fi
+}
+
+main "${@}"