Support custom shell in nix-shell/nix run

* Use `$NIX_SHELL_INTERACTIVE_SHELL` (if present) as interactive shell for `nix-shell`
* Use `$NIX_RUN_INTERACTIVE_SHELL` (if present) as interactive shell for `nix run`

If `--run`, `--command` or `-c` are given, then bash is still used.

One could also consider those environment variables to be arbitrary
default commands that are executed when `nix-shell`/`nix run` are ran
interactive and without a command. However, I think the most common
use case (by far) is to set them to a shell, hence the name
`[...]_INTERACTIVE_SHELL`.

Tests:

I was unsuccessful adding tests to *tests/nix-shell.sh*. Any
non-interactive test doesn't trigger the feature since it's only
triggered in interactive shells.

Previous work:

Issue #419 (nix-shell ignoring SHELL enviroment variable) proposed to
use `$SHELL` as interactive shell for `nix-shell`. This is of course
not desirable if people depend on `nix-shell` always using bash. With
this change however, the feature is optional since it uses previously
unused environment variables.

doc/manual/command-ref/nix-shell.xml

@@ -109,7 +109,11 @@ also <xref linkend="sec-common-options" />.</phrase></para>
     <literal>return</literal> at the end; e.g. <literal>--command
     "echo Hello; return"</literal> will print <literal>Hello</literal>
     and then drop you into the interactive shell. This can be useful
-    for doing any additional initialisation.</para></listitem>
+    for doing any additional initialisation. If ran interactive and
+    none of <option>--run</option> and <option>--command</option> are
+    used but <envar>NIX_SHELL_INTERACTIVE_SHELL</envar> is set, then
+    <literal>--command "$NIX_SHELL_INTERACTIVE_SHELL"</literal> is
+    implied.</para></listitem>
 
   </varlistentry>
 

src/nix-build/nix-build.cc

@@ -113,6 +113,7 @@ static void _main(int argc, char * * argv)
             if (std::regex_search(lines.front(), std::regex("^#!"))) {
                 lines.pop_front();
                 inShebang = true;
+                interactive = false;
                 for (int i = 2; i < argc; ++i)
                     savedArgs.push_back(argv[i]);
                 args.clear();
@@ -193,7 +194,6 @@ static void _main(int argc, char * * argv)
 
         else if (inShebang && *arg == "-i") {
             auto interpreter = getArg(*arg, arg, end);
-            interactive = false;
             auto execArgs = "";
 
             // Überhack to support Perl. Perl examines the shebang and
@@ -403,6 +403,10 @@ static void _main(int argc, char * * argv)
 
         restoreAffinity();
 
+        if (interactive && envCommand.empty()) {
+            envCommand = "[ ! -z \"$NIX_SHELL_INTERACTIVE_SHELL\" ] && \"$NIX_SHELL_INTERACTIVE_SHELL\"\nexit";
+        }
+
         /* Run a shell using the derivation's environment.  For
            convenience, source $stdenv/setup to setup additional
            environment variables and shell functions.  Also don't

src/nix/run.cc

@@ -21,7 +21,7 @@ std::string chrootHelperName = "__run_in_chroot";
 
 struct CmdRun : InstallablesCommand
 {
-    std::vector<std::string> command = { "bash" };
+    std::vector<std::string> command;
     StringSet keep, unset;
     bool ignoreEnvironment = false;
 
@@ -30,7 +30,8 @@ struct CmdRun : InstallablesCommand
         mkFlag()
             .longName("command")
             .shortName('c')
-            .description("command and arguments to be executed; defaults to 'bash'")
+            .description("command and arguments to be executed; defaults to "
+                "\"$NIX_RUN_INTERACTIVE_SHELL\" if set, otherwise to 'bash'")
             .labels({"command", "args"})
             .arity(ArityAny)
             .handler([&](std::vector<std::string> ss) {
@@ -147,6 +148,11 @@ struct CmdRun : InstallablesCommand
 
         setenv("PATH", concatStringsSep(":", unixPath).c_str(), 1);
 
+        if (command.empty()) {
+            auto interactiveShell = getenv("NIX_RUN_INTERACTIVE_SHELL");
+            command = { interactiveShell ? interactiveShell : "bash" };
+        }
+
         std::string cmd = *command.begin();
         Strings args;
         for (auto & arg : command) args.push_back(arg);