install.sh: Fix all shellcheck warnings | Misc improvements

* modify sed command to work in both gnu sed and mac os sed

    https://stackoverflow.com/a/4247319

* use printf everywhere for more compatibility

* add some error checking

install.sh

@@ -1,55 +1,74 @@
 #!/bin/sh
 
-get_platform() {
-  case "$(uname -s)" in
-  Linux*) platform=Linux ;;
-  Darwin*) platform=Mac ;;
-  *) platform="UNKNOWN:${unameOut}" ;;
-  esac
-  echo $platform
+# check if git command is installed
+command -v git > /dev/null || {
+  printf "Install git before proceeding\n"
+  exit 1
 }
 
-echo "installing packer"
+printf "%s\n" "Installing packer"
 
 if [ -d ~/.local/share/nvim/site/pack/packer ]; then
-  echo "Clearning previous packer installs"
+  printf "%s\n" "Clearning previous packer installs"
   rm -rf ~/.local/share/nvim/site/pack
 fi
 
-echo "\n=> Installing packer"
-git clone https://github.com/wbthomason/packer.nvim \
-  ~/.local/share/nvim/site/pack/packer/start/packer.nvim
-echo "=> packer installed!"
+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!"
+else
+  printf "Error: Couldn't clone packer\n"
+  exit 1
+fi
 
-echo "Linking config"
-echo "old nvim config will be changed to nvim.bak if exists! :0"
+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
-   echo "Nvim Directory exists"
-   echo "Changing nvim to nvim.bak"
+   printf "%s\n" "Nvim Directory exists"
+   printf "%s\n" "Changing nvim to nvim.bak"
    mv ~/.config/nvim/ ~/.config/nvim.bak/
-   echo "Creating new nvim directory"
+   printf "%s\n" "Creating new nvim directory"
    mkdir -p ~/.config/nvim
 else
-   echo "Nvim Config doesn't exist so creating one"
+   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/
+{ cp -r init.lua ~/.config/nvim/ && cp -r lua ~/.config/nvim/ ;} || {
+  printf "Error: Couldn't copy nvim config\n"
+  exit 1
+}
 
 # change shell in nvim config
-read -p "which shell do you use?: " shellname
-echo "$shellname"
+_CURRENT_SHELL="${SHELL##*/}"
+printf "%s\n: " "Which shell do you want to use ? [ Enter nothing for current shell ( $_CURRENT_SHELL ) ]"
+read -r shellname
+shellname="${shellname:-${_CURRENT_SHELL}}"
+printf "%s\n" "$shellname"
 
-if [ "$(get_platform)" = "Mac" ]; then
-  gsed -i "s/bash/$shellname/g" ~/.config/nvim/lua/mappings.lua
+# 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
+    printf "\n%s\n" "=> Shell changed to $shellname on nvim successfully!"
   else
-  sed -i "s/bash/$shellname/g" ~/.config/nvim/lua/mappings.lua
+    printf "\n%s\n" "Cannot edit with sed, edit ~/.config/nvim/lua/mappings.lua manually to replace bash with $shellname."
+  fi
+  rm -f ~/.config/nvim/lua/mappings.lua.bak # delete backup file created by sed
+else
+  printf "\n%s\n" "=> Shell changed to $shellname on nvim successfully!"
 fi
 
-echo "\n=> shell changed to $shellname on nvim successfully!"
-echo "\n=> neovim will open with some errors , just press enter" && sleep 1
+printf "\n%s\n" "=> Neovim will open with some errors, just press enter" && sleep 1
 
 # install all plugins + compile them
-nvim +PackerSync
+if _NVIM="$(command -v nvim)"; then
+  "${_NVIM}" +PackerSync
+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