Add section on interpreter paths

Draft section based on notes from Brett Viren on the use case of scripts with shebang
lines. Use, as per notes, Python as the main example.

Author: drbenmorgan

RelocatableSoftware/README.md

@@ -112,8 +112,8 @@ compatibility are helpful for simplifying binary packaging and deployment.
     - Ensures software can be used by as wide a range of upstream clients as possible
     - Nevertheless, can be tricky for [languages like C++](https://community.kde.org/Policies/Binary_Compatibility_Issues_With_C%2B%2B)
     - There [are tools to help check compatibility](https://fedoraproject.org/wiki/How_to_check_for_ABI_changes_in_a_package) at least for ELF, but needs a more thorough survey.
-  - Program for multiple versions of any dependencies (assumes they have good API/ABI versioning!!)
-    - Dependecies should provide a versioning header [as per HSF (draft) guidelines](https://github.com/HEP-SF/documents/blob/master/HSF-TN/draft-2016-PROJ/draft-HSF-TN-2016-PROJ.md)
+  - Program for compatibility with multiple versions of any dependencies
+    - Dependencies should provide a versioning header [as per HSF (draft) guidelines](https://github.com/HEP-SF/documents/blob/master/HSF-TN/draft-2016-PROJ/draft-HSF-TN-2016-PROJ.md)
   - Hide dependencies as implementation details as far as possible.
   - Consider versioned symbols and/or inlined namespaces?
 - Building binaries
@@ -264,6 +264,49 @@ hard-coding or use of standard environment variables. On UNIX, these could inclu
 - `/var`
 - `/tmp` or `TMPDIR`
 
+(Re)Locating the Interpreter for Programs
+=========================================
+Programs implemented using intepreted languages such as Python are usually written as scripts using (on Unix platforms)
+a ["shebang"](https://en.wikipedia.org/wiki/Shebang_(Unix)) on the first line to define the interpreter program to pass the remainder of the script to. For example, a Python "hello world" program might be written as
+
+```Python
+#!/usr/bin/python
+
+print("hello world")
+```
+
+This hard codes the system interpreter into the program and whilst this program is relocatable (assuming a valid system
+Python install), it cannot be used with any other interpreter. Typical HEP software stacks install, and require use of,
+their own interpreters, whose paths may also end up hard coded into scripts:
+
+```Python
+#!/custom/stack/root/python/2.7/bin/python
+
+print("hello world")
+```
+
+The resulting stack is then not relocatable as the interpreter path will not exist after relocation. 
+
+Rather than hard coding system or custom interpreter paths, script authors should prefer the use of the
+[`env`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/env.html) program as the shebang, e.g.
+
+```Python
+#!/usr/bin/env python
+
+print("hello world")
+```
+
+Use of `env` makes the program relocatble, but defers location of the interpreter to the `PATH` environment variable,
+and consequently the configuration management system for the software stack. Whilst package authors should prefer
+usage of the `env` pattern, software stack managers can also consider rewriting the shebang line during install 
+and on any relocation to the absolute path of the required interpreter. As it is plain text, simple regular expression
+replacement can be used, but the chosen packaging system must support this, and care must be taken
+if the resultant stack is to be deployed over network file systems (and hence unknown mount points).
+
+**TODO?** Binaries *also* have an interpreter (on Linux, `ld-linux.so`, On macOS, `dyld`). These are also hardcoded, 
+though can be changed with, e.g., `patchelf` for ELF binaries.
+
+
 (Re)Locating Dynamic Libraries
 ==============================
 A non-trivial package will usually be partioned into a main
@@ -315,8 +358,15 @@ dependencies. At install time, rpaths are usually stripped, unless
 configured otherwise.
 
 
-Scripting/Development Support Tools
-===================================
+(Re)Locating Language Modules
+=============================
+**TODO** How to handle module lookup, e.g. `PYTHONPATH` for Python (other languages?). Things that package authors can do.
+Things that the packaging system should do (inc. any packaging system provided by the language, e.g. `pip`, `virtualenv`).
+Things best left to configuration management.
+
+
+Development Tools
+=================
 CMake
 -----
 To support use of a Project by a CMake based client project, scripts for
@@ -382,7 +432,6 @@ Relocatability with External Dependencies
 What happens to relocatability when we have two packages with a dependency?
 For example `Foo` and `Bar`, with `Foo` linking to `libbar` from `Bar`.
 
-
 1. Can move `Foo` if its `RPATH` contains absolute path to `libbar`.
 2. Cannot move `Bar` without updating `Foo`'s RPATH or using/updating dynamic
    loader paths
@@ -432,6 +481,11 @@ paths, so can only really be handled by a package manager system and would
 not work for deploying software over network file systems where final
 mount points are not guaranteed to be identical.
 
+Interpreter Paths
+-----------------
+Shebangs are plain text, so are straightforward to patch directly using regular expression
+find/replace directly, or via tooling at build or install time.
+
 Library RPATHs
 --------------
 1. RPATHs can be changed at install time by the packaging system/tools (`patchelf`, `otool`, `install_name_tool` etc)