nix-build: Port to c++

This was a dumb line-for-line rewrite, because nix build/nix run/etc.
will replace it.

6 files changed, 495 insertions, 362 deletions

diff --git a/.gitignore b/.gitignore
index a524e9b6a29b..2252e512af8c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -40,7 +40,6 @@ Makefile.config
 /scripts/nix-prefetch-url
 /scripts/nix-install-package
 /scripts/nix-channel
-/scripts/nix-build
 /scripts/nix-copy-closure
 /scripts/NixConfig.pm
 /scripts/NixManifest.pm
@@ -77,6 +76,9 @@ Makefile.config
 # /src/download-via-ssh/
 /src/download-via-ssh/download-via-ssh
 
+# /src/nix-build/
+/src/nix-build/nix-build
+
 # /tests/
 /tests/test-tmp
 /tests/common.sh
diff --git a/Makefile b/Makefile
index 90dca473f390..52286b1fa673 100644
--- a/Makefile
+++ b/Makefile
@@ -12,6 +12,7 @@ makefiles = \
   src/nix-daemon/local.mk \
   src/nix-collect-garbage/local.mk \
   src/nix-prefetch-url/local.mk \
+  src/nix-build/local.mk \
   perl/local.mk \
   scripts/local.mk \
   corepkgs/local.mk \
diff --git a/scripts/local.mk b/scripts/local.mk
index edaf44cc492d..142188ad5f47 100644
--- a/scripts/local.mk
+++ b/scripts/local.mk
@@ -1,5 +1,4 @@
 nix_bin_scripts := \
-  $(d)/nix-build \
   $(d)/nix-channel \
   $(d)/nix-copy-closure \
   $(d)/nix-install-package \
@@ -26,6 +25,5 @@ $(eval $(call install-program-in, $(d)/build-remote.pl, $(libexecdir)/nix))
 ifeq ($(OS), Darwin)
   $(eval $(call install-program-in, $(d)/resolve-system-dependencies.pl, $(libexecdir)/nix))
 endif
-$(eval $(call install-symlink, nix-build, $(bindir)/nix-shell))
 
 clean-files += $(nix_bin_scripts) $(nix_noinst_scripts)
diff --git a/scripts/nix-build.in b/scripts/nix-build.in
deleted file mode 100755
index 2d45e37c52d6..000000000000
--- a/scripts/nix-build.in
+++ /dev/null
@@ -1,359 +0,0 @@
-#! @perl@ -w @perlFlags@
-
-use utf8;
-use strict;
-use Nix::Config;
-use Nix::Store;
-use Nix::Utils;
-use File::Basename;
-use Text::ParseWords;
-use Cwd;
-
-binmode STDERR, ":encoding(utf8)";
-
-my $dryRun = 0;
-my $verbose = 0;
-my $runEnv = $0 =~ /nix-shell$/;
-my $pure = 0;
-my $fromArgs = 0;
-my $packages = 0;
-my $interactive = 1;
-
-my @instArgs = ();
-my @buildArgs = ();
-my @exprs = ();
-
-my $shell = $ENV{SHELL} || "/bin/sh";
-my $envCommand = ""; # interactive shell
-my @envExclude = ();
-
-my $myName = $runEnv ? "nix-shell" : "nix-build";
-
-my $inShebang = 0;
-my $script;
-my @savedArgs;
-
-my $tmpDir = mkTempDir($myName);
-
-my $outLink = "./result";
-my $drvLink = "$tmpDir/derivation";
-
-# Ensure that the $tmpDir is deleted.
-$SIG{'INT'} = sub { exit 1 };
-
-
-# Heuristic to see if we're invoked as a shebang script, namely, if we
-# have a single argument, it's the name of an executable file, and it
-# starts with "#!".
-if ($runEnv && defined $ARGV[0] && $ARGV[0] !~ /nix-shell/) {
-    $script = $ARGV[0];
-    if (-f $script && -x $script) {
-        open SCRIPT, "<$script" or die "$0: cannot open ‘$script’: $!\n";
-        my $first = <SCRIPT>;
-        if ($first =~ /^\#\!/) {
-            $inShebang = 1;
-            @savedArgs = @ARGV; shift @savedArgs;
-            @ARGV = ();
-            while (<SCRIPT>) {
-                chomp;
-                if (/^\#\!\s*nix-shell (.*)$/) {
-                    push @ARGV, shellwords($1);
-                }
-            }
-        }
-        close SCRIPT;
-    }
-}
-
-
-for (my $n = 0; $n < scalar @ARGV; $n++) {
-    my $arg = $ARGV[$n];
-
-    if ($arg eq "--help") {
-        exec "man $myName" or die;
-    }
-
-    elsif ($arg eq "--version") {
-        print "$myName (Nix) $Nix::Config::version\n";
-        exit 0;
-    }
-
-    elsif ($arg eq "--add-drv-link") {
-        $drvLink = "./derivation";
-    }
-
-    elsif ($arg eq "--no-out-link" || $arg eq "--no-link") {
-        $outLink = "$tmpDir/result";
-    }
-
-    elsif ($arg eq "--drv-link") {
-        $n++;
-        die "$0: ‘$arg’ requires an argument\n" unless $n < scalar @ARGV;
-        $drvLink = $ARGV[$n];
-    }
-
-    elsif ($arg eq "--out-link" || $arg eq "-o") {
-        $n++;
-        die "$0: ‘$arg’ requires an argument\n" unless $n < scalar @ARGV;
-        $outLink = $ARGV[$n];
-    }
-
-    elsif ($arg eq "--attr" || $arg eq "-A" || $arg eq "-I") {
-        $n++;
-        die "$0: ‘$arg’ requires an argument\n" unless $n < scalar @ARGV;
-        push @instArgs, ($arg, $ARGV[$n]);
-    }
-
-    elsif ($arg eq "--arg" || $arg eq "--argstr") {
-        die "$0: ‘$arg’ requires two arguments\n" unless $n + 2 < scalar @ARGV;
-        push @instArgs, ($arg, $ARGV[$n + 1], $ARGV[$n + 2]);
-        $n += 2;
-    }
-
-    elsif ($arg eq "--option") {
-        die "$0: ‘$arg’ requires two arguments\n" unless $n + 2 < scalar @ARGV;
-        push @instArgs, ($arg, $ARGV[$n + 1], $ARGV[$n + 2]);
-        push @buildArgs, ($arg, $ARGV[$n + 1], $ARGV[$n + 2]);
-        $n += 2;
-    }
-
-    elsif ($arg eq "--max-jobs" || $arg eq "-j" || $arg eq "--max-silent-time" || $arg eq "--cores" || $arg eq "--timeout" || $arg eq '--add-root') {
-        $n++;
-        die "$0: ‘$arg’ requires an argument\n" unless $n < scalar @ARGV;
-        push @buildArgs, ($arg, $ARGV[$n]);
-    }
-
-    elsif ($arg eq "--dry-run") {
-        push @buildArgs, "--dry-run";
-        $dryRun = 1;
-    }
-
-    elsif ($arg eq "--show-trace") {
-        push @instArgs, $arg;
-    }
-
-    elsif ($arg eq "-") {
-        @exprs = ("-");
-    }
-
-    elsif ($arg eq "--verbose" || substr($arg, 0, 2) eq "-v") {
-        push @buildArgs, $arg;
-        push @instArgs, $arg;
-        $verbose = 1;
-    }
-
-    elsif ($arg eq "--quiet" || $arg eq "--repair") {
-        push @buildArgs, $arg;
-        push @instArgs, $arg;
-    }
-
-    elsif ($arg eq "--check") {
-        push @buildArgs, $arg;
-    }
-
-    elsif ($arg eq "--run-env") { # obsolete
-        $runEnv = 1;
-    }
-
-    elsif ($arg eq "--command" || $arg eq "--run") {
-        $n++;
-        die "$0: ‘$arg’ requires an argument\n" unless $n < scalar @ARGV;
-        $envCommand = "$ARGV[$n]\nexit";
-        $interactive = 0 if $arg eq "--run";
-    }
-
-    elsif ($arg eq "--exclude") {
-        $n++;
-        die "$0: ‘$arg’ requires an argument\n" unless $n < scalar @ARGV;
-        push @envExclude, $ARGV[$n];
-    }
-
-    elsif ($arg eq "--pure") { $pure = 1; }
-    elsif ($arg eq "--impure") { $pure = 0; }
-
-    elsif ($arg eq "--expr" || $arg eq "-E") {
-        $fromArgs = 1;
-        push @instArgs, "--expr";
-    }
-
-    elsif ($arg eq "--packages" || $arg eq "-p") {
-        $packages = 1;
-    }
-
-    elsif ($inShebang && $arg eq "-i") {
-        $n++;
-        die "$0: ‘$arg’ requires an argument\n" unless $n < scalar @ARGV;
-        my $interpreter = $ARGV[$n];
-        my $execArgs = "";
-
-        sub shellEscape {
-            my $s = $_;
-            $s =~ s/'/'\\''/g;
-            return "'" . $s . "'";
-        }
-
-        # Überhack to support Perl. Perl examines the shebang and
-        # executes it unless it contains the string "perl" or "indir",
-        # or (undocumented) argv[0] does not contain "perl". Exploit
-        # the latter by doing "exec -a".
-        if ($interpreter =~ /perl/) {
-            $execArgs = "-a PERL";
-        }
-
-        if ($interpreter =~ /ruby/) {
-            # Hack for Ruby. Ruby also examines the shebang. It tries to
-            # read the shebang to understand which packages to read from. Since
-            # this is handled via nix-shell -p, we wrap our ruby script execution
-            # in ruby -e 'load' which ignores the shebangs.
-            $envCommand = "exec $execArgs $interpreter -e 'load(\"$script\")' -- ${\(join ' ', (map shellEscape, @savedArgs))}";
-        } else {
-            $envCommand = "exec $execArgs $interpreter $script ${\(join ' ', (map shellEscape, @savedArgs))}";
-        }
-    }
-
-    elsif (substr($arg, 0, 1) eq "-") {
-        push @buildArgs, $arg;
-    }
-
-    elsif ($arg eq "-Q" || $arg eq "--no-build-output") {
-        push @buildArgs, $arg;
-        push @instArgs, $arg;
-    }
-
-    else {
-        push @exprs, $arg;
-    }
-}
-
-die "$0: ‘-p’ and ‘-E’ are mutually exclusive\n" if $packages && $fromArgs;
-
-if ($packages) {
-    push @instArgs, "--expr";
-    @exprs = (
-        'with import <nixpkgs> { }; runCommand "shell" { buildInputs = [ '
-        . (join " ", map { "($_)" } @exprs) . ']; } ""');
-} elsif (!$fromArgs) {
-    @exprs = ("shell.nix") if scalar @exprs == 0 && $runEnv && -e "shell.nix";
-    @exprs = ("default.nix") if scalar @exprs == 0;
-}
-
-$ENV{'IN_NIX_SHELL'} = 1 if $runEnv;
-
-
-foreach my $expr (@exprs) {
-
-    # Instantiate.
-    my @drvPaths;
-    if ($expr !~ /^\/.*\.drv$/) {
-        # If we're in a #! script, interpret filenames relative to the
-        # script.
-        $expr = dirname(Cwd::abs_path($script)) . "/" . $expr
-            if $inShebang && !$packages && $expr !~ /^\//;
-
-        # !!! would prefer the perl 5.8.0 pipe open feature here.
-        my $pid = open(DRVPATHS, "-|") || exec "$Nix::Config::binDir/nix-instantiate", "--add-root", $drvLink, "--indirect", @instArgs, $expr;
-        while (<DRVPATHS>) {chomp; push @drvPaths, $_;}
-        if (!close DRVPATHS) {
-            die "nix-instantiate killed by signal " . ($? & 127) . "\n" if ($? & 127);
-            exit 1;
-        }
-    } else {
-        push @drvPaths, $expr;
-    }
-
-    if ($runEnv) {
-        die "$0: a single derivation is required\n" if scalar @drvPaths != 1;
-        my $drvPath = $drvPaths[0];
-        $drvPath = (split '!',$drvPath)[0];
-        $drvPath = readlink $drvPath or die "cannot read symlink ‘$drvPath’" if -l $drvPath;
-        my $drv = derivationFromPath($drvPath);
-
-        # Build or fetch all dependencies of the derivation.
-        my @inputDrvs = grep { my $x = $_; (grep { $x =~ $_ } @envExclude) == 0 } @{$drv->{inputDrvs}};
-        system("$Nix::Config::binDir/nix-store", "-r", "--no-output", "--no-gc-warning", @buildArgs, @inputDrvs, @{$drv->{inputSrcs}}) == 0
-            or die "$0: failed to build all dependencies\n";
-
-        # Set the environment.
-        my $tmp = $ENV{"TMPDIR"} // $ENV{"XDG_RUNTIME_DIR"} // "/tmp";
-        if ($pure) {
-            foreach my $name (keys %ENV) {
-                next if grep { $_ eq $name } ("HOME", "USER", "LOGNAME", "DISPLAY", "PATH", "TERM", "IN_NIX_SHELL", "TZ", "PAGER", "NIX_BUILD_SHELL");
-                delete $ENV{$name};
-            }
-            # NixOS hack: prevent /etc/bashrc from sourcing /etc/profile.
-            $ENV{'__ETC_PROFILE_SOURCED'} = 1;
-        }
-        $ENV{'NIX_BUILD_TOP'} = $ENV{'TMPDIR'} = $ENV{'TEMPDIR'} = $ENV{'TMP'} = $ENV{'TEMP'} = $tmp;
-        $ENV{'NIX_STORE'} = $Nix::Config::storeDir;
-        $ENV{$_} = $drv->{env}->{$_} foreach keys %{$drv->{env}};
-
-        # Run a shell using the derivation's environment.  For
-        # convenience, source $stdenv/setup to setup additional
-        # environment variables and shell functions.  Also don't lose
-        # the current $PATH directories.
-        my $rcfile = "$tmpDir/rc";
-        writeFile(
-            $rcfile,
-            "rm -rf '$tmpDir'; " .
-            'unset BASH_ENV; ' .
-            '[ -n "$PS1" ] && [ -e ~/.bashrc ] && source ~/.bashrc; ' .
-            ($pure ? '' : 'p=$PATH; ' ) .
-            'dontAddDisableDepTrack=1; ' .
-            '[ -e $stdenv/setup ] && source $stdenv/setup; ' .
-            ($pure ? '' : 'PATH=$PATH:$p; unset p; ') .
-            'set +e; ' .
-            '[ -n "$PS1" ] && PS1="\n\[\033[1;32m\][nix-shell:\w]$\[\033[0m\] "; ' .
-            'if [ "$(type -t runHook)" = function ]; then runHook shellHook; fi; ' .
-            'unset NIX_ENFORCE_PURITY; ' .
-            'unset NIX_INDENT_MAKE; ' .
-            'shopt -u nullglob; ' .
-            'unset TZ; ' . (defined $ENV{'TZ'} ? "export TZ='${ENV{'TZ'}}'; " : '') .
-            $envCommand);
-        $ENV{BASH_ENV} = $rcfile;
-        my @args = ($ENV{NIX_BUILD_SHELL} // "bash");
-        push @args, "--rcfile" if $interactive;
-        push @args, $rcfile;
-        exec @args;
-        die;
-    }
-
-    # Ugly hackery to make "nix-build -A foo.all" produce symlinks
-    # ./result, ./result-dev, and so on, rather than ./result,
-    # ./result-2-dev, and so on.  This combines multiple derivation
-    # paths into one "/nix/store/drv-path!out1,out2,..." argument.
-    my $prevDrvPath = "";
-    my @drvPaths2;
-    foreach my $drvPath (@drvPaths) {
-        my $p = $drvPath; my $output = "out";
-        if ($drvPath =~ /(.*)!(.*)/) {
-            $p = $1; $output = $2;
-        } else {
-            $p = $drvPath;
-        }
-        my $target = readlink $p or die "cannot read symlink ‘$p’";
-        print STDERR "derivation is $target\n" if $verbose;
-        if ($target eq $prevDrvPath) {
-            push @drvPaths2, (pop @drvPaths2) . "," . $output;
-        } else {
-            push @drvPaths2, $target . "!" . $output;
-            $prevDrvPath = $target;
-        }
-    }
-
-    # Build.
-    my @outPaths;
-    my $pid = open(OUTPATHS, "-|") || exec "$Nix::Config::binDir/nix-store", "--add-root", $outLink, "--indirect", "-r",
-        @buildArgs, @drvPaths2;
-    while (<OUTPATHS>) {chomp; push @outPaths, $_;}
-    if (!close OUTPATHS) {
-        die "nix-store killed by signal " . ($? & 127) . "\n" if ($? & 127);
-        exit ($? >> 8 || 1);
-    }
-
-    next if $dryRun;
-
-    foreach my $outPath (@outPaths) {
-        my $target = readlink $outPath or die "cannot read symlink ‘$outPath’";
-        print "$target\n";
-    }
-}
diff --git a/src/nix-build/local.mk b/src/nix-build/local.mk
new file mode 100644
index 000000000000..91532411a505
--- /dev/null
+++ b/src/nix-build/local.mk
@@ -0,0 +1,9 @@
+programs += nix-build
+
+nix-build_DIR := $(d)
+
+nix-build_SOURCES := $(d)/nix-build.cc
+
+nix-build_LIBS = libmain libstore libutil libformat
+
+$(eval $(call install-symlink, nix-build, $(bindir)/nix-shell))
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
new file mode 100755
index 000000000000..4d80a0c533a5
--- /dev/null
+++ b/src/nix-build/nix-build.cc
@@ -0,0 +1,482 @@
+#include <cstring>
+#include <regex>
+#include "util.hh"
+#include <unistd.h>
+#include "shared.hh"
+#include <sstream>
+#include <vector>
+#include <iostream>
+#include <fstream>
+#include "store-api.hh"
+#include "globals.hh"
+#include "derivations.hh"
+
+using namespace nix;
+using std::stringstream;
+
+extern char ** environ;
+
+std::vector<string> shellwords(const string & s)
+{
+    auto whitespace = std::regex("^(\\s+).*");
+    auto begin = s.cbegin();
+    auto res = std::vector<string>{};
+    auto cur = stringstream{};
+    enum state {
+        sBegin,
+        sQuote
+    };
+    state st = sBegin;
+    auto it = begin;
+    for (; it != s.cend(); ++it) {
+        if (st == sBegin) {
+            auto match = std::smatch{};
+            if (regex_search(it, s.cend(), match, whitespace)) {
+                cur << string(begin, it);
+                res.push_back(cur.str());
+                cur = stringstream{};
+                it = match[1].second;
+                begin = it;
+            }
+        }
+        switch (*it) {
+            case '"':
+                cur << string(begin, it);
+                begin = it + 1;
+                st = st == sBegin ? sQuote : sBegin;
+                break;
+            case '\\':
+                /* perl shellwords mostly just treats the next char as part of the string with no special processing */
+                cur << string(begin, it);
+                begin = ++it;
+                break;
+        }
+    }
+    cur << string(begin, it);
+    auto last = cur.str();
+    if (!last.empty()) {
+        res.push_back(std::move(last));
+    }
+    return res;
+}
+
+int main(int argc, char ** argv)
+{
+    return handleExceptions(argv[0], [&]() {
+        initNix();
+        auto store = openStore();
+        auto dryRun = false;
+        auto verbose = false;
+        auto runEnv = std::regex_search(argv[0], std::regex("nix-shell$"));
+        auto pure = false;
+        auto fromArgs = false;
+        auto packages = false;
+        auto interactive = true;
+
+        auto instArgs = Strings{};
+        auto buildArgs = Strings{};
+        auto exprs = Strings{};
+
+        auto shell = getEnv("SHELL", "/bin/sh");
+        auto envCommand = string{}; // interactive shell
+        auto envExclude = Strings{};
+
+        auto myName = runEnv ? "nix-shell" : "nix-build";
+
+        auto inShebang = false;
+        auto script = string{};
+        auto savedArgs = std::vector<string>{};
+
+        auto tmpDir = AutoDelete{createTempDir("", myName)};
+
+        auto outLink = string("./result");
+        auto drvLink = (Path) tmpDir + "/derivation";
+
+        auto args = std::vector<string>{};
+        for (int i = 1; i < argc; ++i)
+            args.push_back(argv[i]);
+        // Heuristic to see if we're invoked as a shebang script, namely, if we
+        // have a single argument, it's the name of an executable file, and it
+        // starts with "#!".
+        if (runEnv && argc > 1 && !std::regex_search(argv[1], std::regex("nix-shell"))) {
+            script = argv[1];
+            if (access(script.c_str(), F_OK) == 0 && access(script.c_str(), X_OK) == 0) {
+                auto SCRIPT = std::ifstream(script);
+                string first;
+                std::getline(SCRIPT, first);
+                if (std::regex_search(first, std::regex("^#!"))) {
+                    inShebang = true;
+                    for (int i = 2; i < argc - 1; ++i)
+                        savedArgs.push_back(argv[i]);
+                    args = std::vector<string>{};
+                    for (string line; std::getline(SCRIPT, line);) {
+                        line = chomp(line);
+                        std::smatch match;
+                        if (std::regex_match(line, match, std::regex("^#!\\s*nix-shell (.*)$")))
+                            for (const auto & word : shellwords(match[1].str()))
+                                args.push_back(word);
+                    }
+                }
+            }
+        }
+
+        for (auto n = decltype(args)::size_type{0}; n < args.size(); ++n) {
+            auto arg = args[n];
+
+            if (arg == "--help") {
+                deletePath(tmpDir);
+                tmpDir.cancel();
+                execlp("man", "man", myName, NULL);
+                throw SysError("executing man");
+            }
+
+            else if (arg == "--version") {
+                std::cout << myName << " (Nix) " << nixVersion << '\n';
+                return;
+            }
+
+            else if (arg == "--add-drv-link") {
+                drvLink = "./derivation";
+            }
+
+            else if (arg == "--no-out-link" || arg == "--no-link") {
+                outLink = (Path) tmpDir + "/result";
+            }
+
+            else if (arg == "--drv-link") {
+                n++;
+                if (n >= args.size()) {
+                    throw UsageError("--drv-link requires an argument");
+                }
+                drvLink = args[n];
+            }
+
+            else if (arg == "--out-link" || arg == "-o") {
+                n++;
+                if (n >= args.size()) {
+                    throw UsageError(format("%1% requires an argument") % arg);
+                }
+                outLink = args[n];
+            }
+
+            else if (arg == "--attr" || arg == "-A" || arg == "-I") {
+                n++;
+                if (n >= args.size()) {
+                    throw UsageError(format("%1% requires an argument") % arg);
+                }
+                instArgs.push_back(arg);
+                instArgs.push_back(args[n]);
+            }
+
+            else if (arg == "--arg" || arg == "--argstr") {
+                if (n + 2 >= args.size()) {
+                    throw UsageError(format("%1% requires two arguments") % arg);
+                }
+                instArgs.push_back(arg);
+                instArgs.push_back(args[n + 1]);
+                instArgs.push_back(args[n + 2]);
+                n += 2;
+            }
+
+            else if (arg == "--option") {
+                if (n + 2 >= args.size()) {
+                    throw UsageError(format("%1% requires two arguments") % arg);
+                }
+                instArgs.push_back(arg);
+                instArgs.push_back(args[n + 1]);
+                instArgs.push_back(args[n + 2]);
+                buildArgs.push_back(arg);
+                buildArgs.push_back(args[n + 1]);
+                buildArgs.push_back(args[n + 2]);
+                n += 2;
+            }
+
+            else if (arg == "--max-jobs" || arg == "-j" || arg == "--max-silent-time" || arg == "--cores" || arg == "--timeout" || arg == "--add-root") {
+                n++;
+                if (n >= args.size()) {
+                    throw UsageError(format("%1% requires an argument") % arg);
+                }
+                buildArgs.push_back(arg);
+                buildArgs.push_back(args[n]);
+            }
+
+            else if (arg == "--dry-run") {
+                buildArgs.push_back("--dry-run");
+                dryRun = true;
+            }
+
+            else if (arg == "--show-trace") {
+                instArgs.push_back(arg);
+            }
+
+            else if (arg == "-") {
+                exprs = Strings{"-"};
+            }
+
+            else if (arg == "--verbose" || (arg.size() >= 2 && arg.substr(0, 2) == "-v")) {
+                buildArgs.push_back(arg);
+                instArgs.push_back(arg);
+                verbose = true;
+            }
+
+            else if (arg == "--quiet" || arg == "--repair") {
+                buildArgs.push_back(arg);
+                instArgs.push_back(arg);
+            }
+
+            else if (arg == "--check") {
+                buildArgs.push_back(arg);
+            }
+
+            else if (arg == "--run-env") { // obsolete
+                runEnv = true;
+            }
+
+            else if (arg == "--command" || arg == "--run") {
+                n++;
+                if (n >= args.size()) {
+                    throw UsageError(format("%1% requires an argument") % arg);
+                }
+                envCommand = args[n] + "\nexit";
+                if (arg == "--run")
+                    interactive = false;
+            }
+
+            else if (arg == "--exclude") {
+                n++;
+                if (n >= args.size()) {
+                    throw UsageError(format("%1% requires an argument") % arg);
+                }
+                envExclude.push_back(args[n]);
+            }
+
+            else if (arg == "--pure") { pure = true; }
+            else if (arg == "--impure") { pure = false; }
+
+            else if (arg == "--expr" || arg == "-E") {
+                fromArgs = true;
+                instArgs.push_back("--expr");
+            }
+
+            else if (arg == "--packages" || arg == "-p") {
+                packages = true;
+            }
+
+            else if (inShebang && arg == "-i") {
+                n++;
+                if (n >= args.size()) {
+                    throw UsageError(format("%1% requires an argument") % arg);
+                }
+                auto interpreter = args[n];
+                auto execArgs = "";
+
+                auto shellEscape = [](const string & s) {
+                    return "'" + std::regex_replace(s, std::regex("'"), "'\\''") + "'";
+                };
+
+                // Überhack to support Perl. Perl examines the shebang and
+                // executes it unless it contains the string "perl" or "indir",
+                // or (undocumented) argv[0] does not contain "perl". Exploit
+                // the latter by doing "exec -a".
+                if (std::regex_search(interpreter, std::regex("perl"))) {
+                        execArgs = "-a PERL";
+                }
+
+                auto joined = stringstream{};
+                for (const auto & i : savedArgs)
+                    joined << shellEscape(i) << ' ';
+
+                if (std::regex_search(interpreter, std::regex("ruby"))) {
+                    // Hack for Ruby. Ruby also examines the shebang. It tries to
+                    // read the shebang to understand which packages to read from. Since
+                    // this is handled via nix-shell -p, we wrap our ruby script execution
+                    // in ruby -e 'load' which ignores the shebangs.
+
+                    envCommand = (format("exec %1% %2% -e 'load(\"%3%\") -- %4%") % execArgs % interpreter % script % joined.str()).str();
+                } else {
+                    envCommand = (format("exec %1% %2% %3% %4%") % execArgs % interpreter % script % joined.str()).str();
+                }
+            }
+
+            else if (!arg.empty() && arg[0] == '-') {
+                buildArgs.push_back(arg);
+            }
+
+            else if (arg == "-Q" || arg == "--no-build-output") {
+                buildArgs.push_back(arg);
+                instArgs.push_back(arg);
+            }
+
+            else {
+                exprs.push_back(arg);
+            }
+        }
+
+        if (packages && fromArgs) {
+            throw UsageError("‘-p’ and ‘-E’ are mutually exclusive");
+        }
+
+        if (packages) {
+            instArgs.push_back("--expr");
+            auto joined = stringstream{};
+            joined << "with import <nixpkgs> { }; runCommand \"shell\" { buildInputs = [ ";
+            for (const auto & i : exprs)
+                joined << '(' << i << ") ";
+            joined << "]; } \"\"";
+            exprs = Strings{joined.str()};
+        } else if (!fromArgs) {
+            if (exprs.empty() && runEnv && access("shell.nix", F_OK) == 0)
+                exprs.push_back("shell.nix");
+            if (exprs.empty())
+                exprs.push_back("default.nix");
+        }
+
+        if (runEnv)
+            setenv("IN_NIX_SHELL", "1", 1);
+
+        for (auto & expr : exprs) {
+            // Instantiate.
+            auto drvPaths = std::vector<string>{};
+            if (!std::regex_match(expr, std::regex("^/.*\\.drv$"))) {
+                // If we're in a #! script, interpret filenames relative to the
+                // script.
+                if (inShebang && !packages)
+                    expr = absPath(expr, dirOf(script));
+
+                auto instantiateArgs = Strings{"--add-root", drvLink, "--indirect"};
+                for (const auto & arg : instArgs)
+                    instantiateArgs.push_back(arg);
+                instantiateArgs.push_back(expr);
+                auto instOutput = runProgram(settings.nixBinDir + "/nix-instantiate", false, instantiateArgs);
+                drvPaths = tokenizeString<std::vector<string>>(instOutput);
+            } else {
+                drvPaths.push_back(expr);
+            }
+
+            if (runEnv) {
+                if (drvPaths.size() != 1)
+                    throw UsageError("a single derivation is required");
+                auto drvPath = drvPaths[0];
+                drvPath = drvPath.substr(0, drvPath.find_first_of('!'));
+                if (isLink(drvPath))
+                    drvPath = readLink(drvPath);
+                auto drv = store->derivationFromPath(drvPath);
+
+                // Build or fetch all dependencies of the derivation.
+                auto nixStoreArgs = Strings{"-r", "--no-output", "--no-gc-warning"};
+                for (const auto & arg : buildArgs)
+                    nixStoreArgs.push_back(arg);
+                for (const auto & input : drv.inputDrvs)
+                    if (std::all_of(envExclude.cbegin(), envExclude.cend(), [&](const string & exclude) { return !std::regex_search(input.first, std::regex(exclude)); }))
+                        nixStoreArgs.push_back(input.first);
+                for (const auto & src : drv.inputSrcs)
+                    nixStoreArgs.push_back(src);
+                runProgram(settings.nixBinDir + "/nix-store", false, nixStoreArgs);
+
+                // Set the environment.
+                auto tmp = getEnv("TMPDIR", getEnv("XDG_RUNTIME_DIR", "/tmp"));
+                if (pure) {
+                    auto skippedEnv = std::vector<string>{"HOME", "USER", "LOGNAME", "DISPLAY", "PATH", "TERM", "IN_NIX_SHELL", "TZ", "PAGER", "NIX_BUILD_SHELL"};
+                    auto removed = std::vector<string>{};
+                    for (auto i = size_t{0}; environ[i]; ++i) {
+                        auto eq = strchr(environ[i], '=');
+                        if (!eq)
+                            // invalid env, just keep going
+                            continue;
+                        auto name = string(environ[i], eq);
+                        if (find(skippedEnv.begin(), skippedEnv.end(), name) == skippedEnv.end())
+                            removed.emplace_back(std::move(name));
+                    }
+                    for (const auto & name : removed)
+                        unsetenv(name.c_str());
+                    // NixOS hack: prevent /etc/bashrc from sourcing /etc/profile.
+                    setenv("__ETC_PROFILE_SOURCED", "1", 1);
+                }
+                setenv("NIX_BUILD_TOP", tmp.c_str(), 1);
+                setenv("TMPDIR", tmp.c_str(), 1);
+                setenv("TEMPDIR", tmp.c_str(), 1);
+                setenv("TMP", tmp.c_str(), 1);
+                setenv("TEMP", tmp.c_str(), 1);
+                setenv("NIX_STORE", store->storeDir.c_str(), 1);
+                for (const auto & env : drv.env)
+                    setenv(env.first.c_str(), env.second.c_str(), 1);
+
+                // Run a shell using the derivation's environment.  For
+                // convenience, source $stdenv/setup to setup additional
+                // environment variables and shell functions.  Also don't lose
+                // the current $PATH directories.
+                auto rcfile = (Path) tmpDir + "/rc";
+                writeFile(rcfile, (format(
+                        "rm -rf '%1%'; "
+                        "unset BASH_ENV; "
+                        "[ -n \"$PS1\" ] && [ -e ~/.bashrc ] && source ~/.bashrc; "
+                        "%2%"
+                        "dontAddDisableDepTrack=1; "
+                        "[ -e $stdenv/setup ] && source $stdenv/setup; "
+                        "%3%"
+                        "set +e; "
+                        "[ -n \"$PS1\" ] && PS1=\"\\n\\[\\033[1;32m\\][nix-shell:\\w]$\\[\\033[0m\\] \"; "
+                        "if [ \"$(type -t runHook)\" = function ]; then runHook shellHook; fi; "
+                        "unset NIX_ENFORCE_PURITY; "
+                        "unset NIX_INDENT_MAKE; "
+                        "shopt -u nullglob; "
+                        "unset TZ; %4%"
+                        "%5%"
+                        ) % (Path) tmpDir % (pure ? "" : "p=$PATH") % (pure ? "" : "PATH=$PATH:$p; unset p; ") % (getenv("TZ") ? (string("export TZ='") + getenv("TZ") + "'; ") : "") % envCommand).str());
+                setenv("BASH_ENV", rcfile.c_str(), 1);
+                if (interactive)
+                    execlp(getEnv("NIX_BUILD_SHELL", "bash").c_str(), "bash", "--rcfile", rcfile.c_str(), NULL);
+                else
+                    execlp(getEnv("NIX_BUILD_SHELL", "bash").c_str(), "bash", rcfile.c_str(), NULL);
+                throw SysError("executing shell");
+            }
+
+            // Ugly hackery to make "nix-build -A foo.all" produce symlinks
+            // ./result, ./result-dev, and so on, rather than ./result,
+            // ./result-2-dev, and so on.  This combines multiple derivation
+            // paths into one "/nix/store/drv-path!out1,out2,..." argument.
+            auto prevDrvPath = string{};
+            auto drvPaths2 = Strings{};
+            for (const auto & drvPath : drvPaths) {
+                auto p = drvPath;
+                auto output = string{"out"};
+                std::smatch match;
+                if (std::regex_match(drvPath, match, std::regex("(.*)!(.*)"))) {
+                    p = match[1].str();
+                    output = match[2].str();
+                }
+                auto target = readLink(p);
+                if (verbose)
+                    std::cerr << "derivation is " << target << '\n';
+                if (target == prevDrvPath) {
+                    auto last = drvPaths2.back();
+                    drvPaths2.pop_back();
+                    drvPaths2.push_back(last + "," + output);
+                } else {
+                    drvPaths2.push_back(target + "!" + output);
+                    prevDrvPath = target;
+                }
+            }
+            // Build.
+            auto outPaths = Strings{};
+            auto nixStoreArgs = Strings{"--add-root", outLink, "--indirect", "-r"};
+            for (const auto & arg : buildArgs)
+                nixStoreArgs.push_back(arg);
+            for (const auto & path : drvPaths2)
+                nixStoreArgs.push_back(path);
+            auto nixStoreRes = runProgram(settings.nixBinDir + "/nix-store", false, nixStoreArgs);
+            for (const auto & outpath : tokenizeString<std::vector<string>>(nixStoreRes)) {
+                outPaths.push_back(chomp(outpath));
+            }
+
+            if (dryRun)
+                continue;
+            for (const auto & outPath : outPaths) {
+                auto target = readLink(outPath);
+                std::cout << target << '\n';
+            }
+        }
+        return;
+    });
+}
+