Make launcher easier wrappable for different install locations (#467)

* add ELS_INSTALL_PREFIX to launch scripts

* add ELS_INSTALL_PREFIX to debug scripts

* add ELS_INSTALL_PREFIX to README

* fix copy and paste error in debugger start script

Author: NobbZ

README.md

@@ -251,6 +251,10 @@ accomplished by adding `>/dev/null` and/or `2>/dev/null` at the end of any line
 output, and for a windows batch script you will want `@echo off` at the top and `>nul` on every
 line.
 
+## Environment variables
+
+* `ELS_INSTALL_PREFIX`: The folder where the language server got installed to. If set eq. through a wrapper script, it makes maintaining multiple versions/instances on the same host much easier. If not set or empty, a heuristic will be used to discover the install location.
+
 ## Acknowledgements and related projects
 
 ElixirLS isn't the first frontend-independent server for Elixir language support. The original was [Alchemist Server](https://github.com/tonini/alchemist-server/), which powers the [Alchemist](https://github.com/tonini/alchemist.el) plugin for Emacs. Another project, [Elixir Sense](https://github.com/elixir-lsp/elixir_sense), builds upon Alchemist and powers the [Elixir plugin for Atom](https://github.com/msaraiva/atom-elixir) as well as another VS Code plugin, [VSCode Elixir](https://github.com/fr1zle/vscode-elixir). ElixirLS uses Elixir Sense for several code insight features. Credit for those projects goes to their respective authors.

apps/elixir_ls_utils/priv/debugger.sh

@@ -1,7 +1,11 @@
 #!/bin/sh
 # Launches the debugger. This script must be in the same directory as the compiled .ez archives.
 
-dir=$(dirname "$0")
+if [ -z "${ELS_INSTALL_PREFIX}" ]; then
+  dir=$(dirname "$0")
+else
+  dir=${ELS_INSTALL_PREFIX}
+fi
 
 export ELS_MODE=debugger
 export ELS_SCRIPT="ElixirLS.Debugger.CLI.main()"

apps/elixir_ls_utils/priv/language_server.sh

@@ -1,7 +1,11 @@
 #!/bin/sh
 # Launches the language server. This script must be in the same directory as the compiled .ez archives.
 
-dir=$(dirname "$0")
+if [ -z "${ELS_INSTALL_PREFIX}" ]; then
+  dir=$(dirname "$0")
+else
+  dir=${ELS_INSTALL_PREFIX}
+fi
 
 export ELS_MODE=language_server
 export ELS_SCRIPT="ElixirLS.LanguageServer.CLI.main()"

apps/elixir_ls_utils/priv/launch.sh

@@ -61,8 +61,13 @@ readlink_f () {
   fi
 }
 
-SCRIPT=$(readlink_f "$0")
-SCRIPTPATH=$(dirname "$SCRIPT")
+if [ -z "${ELS_INSTALL_PREFIX}" ]; then
+  SCRIPT=$(readlink_f "$0")
+  SCRIPTPATH=$(dirname "$SCRIPT")
+else
+  SCRIPTPATH=${ELS_INSTALL_PREFIX}
+fi
+
 export ERL_LIBS="$SCRIPTPATH:$ERL_LIBS"
 
 exec elixir --erl "+sbwt none +sbwtdcpu none +sbwtdio none"  -e "$ELS_SCRIPT"