From 2dd972f3a4cfca4f112defe47bec26e6940fec78 Mon Sep 17 00:00:00 2001
From: Trent Huber <trentmhuber@gmail.com>
Date: Mon, 27 Oct 2025 23:32:16 -0400
Subject: [PATCH] INSTALL.md added

---
 INSTALL.md | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)
 create mode 100644 INSTALL.md

diff --git a/INSTALL.md b/INSTALL.md
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+# thus is not a POSIX compliant shell
+
+thus uses a distinctly different syntax than other POSIX compliant shells. While fine for an end user to learn, some programs rely on POSIX compilant shells to run internal commands or scripts. Therefore, **it is not recommended that thus be set to the user's default shell**. Instead, thus should be automatically run by a separate POSIX compilant shell which in turn is set as the user's default shell. This allows utilities to function as before while presenting thus as the default shell in an interactive session.
+
+# "Installing" thus
+
+Here is a minimal example which assumes sh as the user's default shell (a similar approach can be used for other shells). We'll need to modify the scripts that are executed at login and during an interactive session. For sh, the login configuration file is `.profile`.
+
+```console
+# .profile
+
+ENV=$HOME/.shrc
+export ENV
+
+THUSLOGIN=-l
+```
+
+`ENV` is used by sh as the path of the user's configuration file used for interactive sessions, in this case, `.shrc`.
+
+```console
+# .shrc
+
+test -z "$EXEUNT" && echo $- | grep -q i && exec thus $THUSLOGIN
+```
+
+`THUSLOGIN` will be set to the `-l` flag only in the case when sh is running as a login shell. Running thus with the `-l` flag starts it up as a login shell.
+
+Notice if sh is called from within thus, then it will run *as* thus which makes it impossible to open sh as an interactive subshell. This is why the `EXEUNT` environment variable is used as a guard in the command list. Defining `EXEUNT` will make all subsequent calls to sh as an interactive shell actually open sh, not thus.
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2.51.0