From 1bfbbe241553881a6416364ccc9a5af2a1fd7f13 Mon Sep 17 00:00:00 2001
From: Tom Hubrecht <github@mail.hubrecht.ovh>
Date: Thu, 27 Nov 2025 15:55:53 +0100
Subject: [PATCH] nix-shell: Add NIX_SHELL_LEVEL variable

This contains the depth of nix shells nesting.

Fixes #826

Co-authored-by: Qyriad <qyriad@qyriad.me>

Change-Id: If584c9d02730d6c857636dafdeab0c01f4ec8e0f
---
 doc/manual/rl-next/nix-shell-level.md |  9 +++++++++
 lix/legacy/nix-build.cc               |  9 +++++++++
 lix/nix/develop.cc                    |  4 ++++
 lix/nix/run.cc                        | 11 +++++++++++
 4 files changed, 33 insertions(+)
 create mode 100644 doc/manual/rl-next/nix-shell-level.md

diff --git a/doc/manual/rl-next/nix-shell-level.md b/doc/manual/rl-next/nix-shell-level.md
new file mode 100644
index 000000000..8c2661db0
--- /dev/null
+++ b/doc/manual/rl-next/nix-shell-level.md
@@ -0,0 +1,9 @@
+---
+synopsis: "Add an indication of nix-shell nesting depth"
+cls: [4657]
+issues: [fj#826]
+category: "Improvements"
+credits: [thubrecht]
+---
+
+When in a nix shell (either via a `nix-shell` or a `nix develop` invocation), a variable `NIX_SHELL_LEVEL` is exported to indicate the nesting depth of nix shells.
diff --git a/lix/legacy/nix-build.cc b/lix/legacy/nix-build.cc
index 71bf426e1..e0f0916e8 100644
--- a/lix/legacy/nix-build.cc
+++ b/lix/legacy/nix-build.cc
@@ -25,6 +25,7 @@
 #include "lix/libutil/shlex.hh"
 #include "nix-build.hh"
 #include "lix/libstore/temporary-dir.hh"
+#include "lix/libutil/strings.hh"
 
 extern char * * environ __attribute__((weak)); // Man what even is this
 
@@ -425,6 +426,14 @@ static int main_nix_build(AsyncIoRoot & aio, std::string programName, Strings ar
             env["__ETC_PROFILE_SOURCED"] = "1";
         }
 
+        // Set NIX_SHELL_LEVEL
+        env["NIX_SHELL_LEVEL"] = std::to_string(
+            getEnvNonEmpty("NIX_SHELL_LEVEL")
+                .and_then([](std::string lvl) { return string2Int<size_t>(lvl); })
+                .value_or(0)
+            + 1
+        );
+
         // Don't use defaultTempDir() here! We want to preserve the user's TMPDIR for the shell
         env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] =
             getEnvNonEmpty("TMPDIR").value_or(buildTopTmpDir);
diff --git a/lix/nix/develop.cc b/lix/nix/develop.cc
index b9640d2e2..264286e0c 100644
--- a/lix/nix/develop.cc
+++ b/lix/nix/develop.cc
@@ -356,6 +356,10 @@ struct Common : InstallableCommand, MixProfile
 
         buildEnvironment.toBash(out, ignoreVars);
 
+        // Add NIX_SHELL_LEVEL
+        out << "printf -v NIX_SHELL_LEVEL \"%d\" \"${NIX_SHELL_LEVEL:-0}\" || NIX_SHELL_LEVEL=0\n";
+        out << "export NIX_SHELL_LEVEL=$(( NIX_SHELL_LEVEL + 1 ))\n";
+
         for (auto & var : savedVars)
             out << fmt("%s=\"$%s${nix_saved_%s:+:$nix_saved_%s}\"\n", var, var, var, var);
 
diff --git a/lix/nix/run.cc b/lix/nix/run.cc
index aeada699b..e58eb2d26 100644
--- a/lix/nix/run.cc
+++ b/lix/nix/run.cc
@@ -150,6 +150,17 @@ struct CmdShell : InstallablesCommand, MixEnvironment
         auto unixPathString = concatStringsSep(":", unixPath);
         (void) sys::setenv("PATH", unixPathString, 1);
         (void) sys::setenv("IN_NIX_SHELL", ignoreEnvironment ? "pure" : "impure", 1);
+        // Set NIX_SHELL_LEVEL
+        (void) sys::setenv(
+            "NIX_SHELL_LEVEL",
+            std::to_string(
+                getEnvNonEmpty("NIX_SHELL_LEVEL")
+                    .and_then([](std::string lvl) { return string2Int<size_t>(lvl); })
+                    .value_or(0)
+                + 1
+            ),
+            1
+        );
 
         Strings args;
         for (auto & arg : command) args.push_back(arg);