RFC0040 - Enable use of pwsh as login shell (#186)

* draft to enable pwsh to be used as login shell

* update RFC per feedback

* address feedback on different paths to pwsh for different OS

* change proposal to exec to sh -l pwsh

* add additional implementation details regaring inspecting argv[0]

* add mention of potential future work to support other default shells for processing profile

* clarify `zsh` relationship

* Update and rename RFCnnnn-PowerShell-as-Login-Shell.md to RFC0040-PowerShell-as-Login-Shell.md

Author: SteveL-MSFT

2-Draft-Accepted/RFC0040-PowerShell-as-Login-Shell.md

@@ -0,0 +1,103 @@
+---
+RFC: RFC0040
+Author: Steve Lee
+Status: Draft-Accepted
+SupercededBy: N/A
+Version: 1.0
+Area: Console
+Comments Due: July 15, 2019
+Plan to implement: Yes, PS7
+---
+
+# Enable PowerShell as Login Shell on POSIX-based systems
+
+POSIX shells on Unix-based systems use a `-l` switch to start the shell as a login shell.
+This means the shell is not being started from an existing configured environment and
+needs to execute `/etc/profile` to populate environment variables along with other
+environment changes.
+
+## Motivation
+
+    As a PowerShell user,
+    I can use PowerShell as my login shell on Unix-based systems,
+    so that I can use PowerShell everywhere.
+
+## Specification
+
+Many existing tools expect `-l` to be accepted by a shell and to act as a login
+shell.
+
+`-l` (expanded form is `-Login`) will:
+
+- On Windows, do nothing as the login shell concept doesn't exist.
+  No error, this param is ignored.
+- On Unix based systems, exec `pwsh` using `sh -l` so that the normal login
+  shell processing occurs and the environment is pre-populated as expected.
+
+>[!NOTE]
+> The actual path to `sh` would depend on the operating system.
+> For Linux, the path would be `/usr/bin`.
+> For Snap, the path would be `/snap/bin`.
+> For macOS, there is an issue with `sh` not loading the login profile with
+> our usage, so `pwsh` will use `zsh` with `sh` emulation mode to enable
+> proper initialization of the environment.
+
+Since `pwsh` is simply starting an instance of itself, it should work the same
+regardless if a stable or preview release.
+
+Initial performance tests indicate that `pwsh -l` will incur a 6% startup hit.
+Compared to a pure Bourne shell script which has a 4% startup hit.
+Most of that is simply processing the Bourne shell scripts to setup the environment.
+As such, there is a relative 2% additional perf hit with this proposal.
+
+The impact seems acceptable given that the login shell is typically the first
+shell you create and subsequent use of `pwsh` would not be a login shell.
+
+In addition, `pwsh` will need to be updated to detect if it was started with
+a hyphen (for example, `-pwsh`) as that is another way that Unix based operating
+systems start the default login shell and doesn't necessarily use `-l`.
+
+On Linux, this will be implemented by inspecting `/proc` on the filesystem.
+On macOS, this will be implemented by calling `sysctl()` getting the process
+information for the current process and inspecting `argv[0]`.
+
+## Alternate Proposals and Considerations
+
+### Process /etc/profile using sh and copy the env vars
+
+This proposal would be that if `-l` is specified, pwsh would run `/bin/sh -l -c export`
+which would create a new shell process that does all the processing needed for
+a login shell and export the environment variables that would be exported.
+Additional code is needed to take those environment variables and copy them to
+the parent pwsh process.
+
+The downsides to this approach is additional code to maintain,
+but more importantly not getting a complete login shell environment as things
+like ulimit and umask would not be inherited into pwsh.
+
+### `pwsh-login` script
+
+This proposal would be to include a Bourne shell script called `pwsh-login`
+that would contain:
+
+```sh
+#!/bin/sh -l
+exec /usr/local/bin/pwsh "$@"
+```
+
+The location of `sh` would depend on the specific operating system.
+
+The perf impact of this compared to starting pwsh as a non-login shell is
+approximately 4%.
+
+### Supporting different default login shells
+
+In the future, if relying on `sh` (and thus `/etc/profile`) is not sufficient,
+we can allow an optional argument to `-login` to indicate the shell to use
+to process the script that creates the environment.
+
+### macOS moving to Zsh by default
+
+Expectation is that macOS intends to switch to `zsh` as the default shell so
+it may make sense to revisit this in the future to default to using `zsh` so
+that `zprofile` gets processed.