diff --git a/src/libmain/common-args.cc b/src/libmain/common-args.cc
new file mode 100644
index 000000000..54dbb1da3
--- /dev/null
+++ b/src/libmain/common-args.cc
@@ -0,0 +1,37 @@
+#include "common-args.hh"
+#include "globals.hh"
+
+namespace nix {
+
+MixCommonArgs::MixCommonArgs(const string & programName)
+{
+    mkFlag('v', "verbose", "increase verbosity level", []() {
+        verbosity = (Verbosity) (verbosity + 1);
+    });
+
+    mkFlag(0, "quiet", "decrease verbosity level", []() {
+        verbosity = verbosity > lvlError ? (Verbosity) (verbosity - 1) : lvlError;
+    });
+
+    mkFlag(0, "debug", "enable debug output", []() {
+        verbosity = lvlDebug;
+    });
+
+    mkFlag1(0, "log-type", "type", "set logging format ('pretty', 'flat', 'systemd')",
+        [](std::string s) {
+            if (s == "pretty") logType = ltPretty;
+            else if (s == "escapes") logType = ltEscapes;
+            else if (s == "flat") logType = ltFlat;
+            else if (s == "systemd") logType = ltSystemd;
+            else throw UsageError("unknown log type");
+        });
+
+    mkFlag(0, "option", {"name", "value"}, "set a Nix configuration option (overriding nix.conf)", 2,
+        [](Strings ss) {
+            auto name = ss.front(); ss.pop_front();
+            auto value = ss.front();
+            settings.set(name, value);
+        });
+}
+
+}
diff --git a/src/libmain/common-args.hh b/src/libmain/common-args.hh
new file mode 100644
index 000000000..62657e0d0
--- /dev/null
+++ b/src/libmain/common-args.hh
@@ -0,0 +1,22 @@
+#pragma once
+
+#include "args.hh"
+
+namespace nix {
+
+struct MixCommonArgs : virtual Args
+{
+    MixCommonArgs(const string & programName);
+};
+
+struct MixDryRun : virtual Args
+{
+    bool dryRun;
+
+    MixDryRun()
+    {
+        mkFlag(0, "dry-run", "show what this command would do without doing it", &dryRun);
+    }
+};
+
+}
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 88ed52497..bf6bf5b71 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -1,7 +1,8 @@
 #include "config.h"
 
-#include "shared.hh"
+#include "common-args.hh"
 #include "globals.hh"
+#include "shared.hh"
 #include "store-api.hh"
 #include "util.hh"
 
@@ -84,16 +85,6 @@ void printMissing(ref<Store> store, const PathSet & willBuild,
 }
 
 
-static void setLogType(string lt)
-{
-    if (lt == "pretty") logType = ltPretty;
-    else if (lt == "escapes") logType = ltEscapes;
-    else if (lt == "flat") logType = ltFlat;
-    else if (lt == "systemd") logType = ltSystemd;
-    else throw UsageError("unknown log type");
-}
-
-
 string getArg(const string & opt,
     Strings::iterator & i, const Strings::iterator & end)
 {
@@ -164,77 +155,80 @@ void initNix()
 }
 
 
+struct LegacyArgs : public MixCommonArgs
+{
+    std::function<bool(Strings::iterator & arg, const Strings::iterator & end)> parseArg;
+
+    LegacyArgs(const std::string & programName,
+        std::function<bool(Strings::iterator & arg, const Strings::iterator & end)> parseArg)
+        : MixCommonArgs(programName), parseArg(parseArg)
+    {
+        mkFlag('Q', "no-build-output", "do not show build output",
+            &settings.buildVerbosity, lvlVomit);
+
+        mkFlag(0, "print-build-trace", "emit special build trace message",
+            &settings.printBuildTrace);
+
+        mkFlag('K', "keep-failed", "keep temporary directories of failed builds",
+            &settings.keepFailed);
+
+        mkFlag('k', "keep-going", "keep going after a build fails",
+            &settings.keepGoing);
+
+        mkFlag(0, "fallback", "build from source if substitution fails", []() {
+            settings.set("build-fallback", "true");
+        });
+
+        auto intSettingAlias = [&](char shortName, const std::string & longName,
+            const std::string & description, const std::string & dest) {
+            mkFlag<unsigned int>(shortName, longName, description, [=](unsigned int n) {
+                settings.set(dest, std::to_string(n));
+            });
+        };
+
+        intSettingAlias('j', "max-jobs", "maximum number of parallel builds", "build-max-jobs");
+        intSettingAlias(0, "cores", "maximum number of CPU cores to use inside a build", "build-cores");
+        intSettingAlias(0, "max-silent-time", "number of seconds of silence before a build is killed", "build-max-silent-time");
+        intSettingAlias(0, "timeout", "number of seconds before a build is killed", "build-timeout");
+
+        mkFlag(0, "readonly-mode", "do not write to the Nix store",
+            &settings.readOnlyMode);
+
+        mkFlag(0, "no-build-hook", "disable use of the build hook mechanism",
+            &settings.useBuildHook, false);
+
+        mkFlag(0, "show-trace", "show Nix expression stack trace in evaluation errors",
+            &settings.showTrace);
+
+        mkFlag(0, "no-gc-warning", "disable warning about not using ‘--add-root’",
+            &gcWarning, false);
+    }
+
+    bool processFlag(Strings::iterator & pos, Strings::iterator end) override
+    {
+        if (MixCommonArgs::processFlag(pos, end)) return true;
+        bool res = parseArg(pos, end);
+        if (res) ++pos;
+        return res;
+    }
+
+    bool processArgs(const Strings & args, bool finish) override
+    {
+        if (args.empty()) return true;
+        assert(args.size() == 1);
+        Strings ss(args);
+        auto pos = ss.begin();
+        if (!parseArg(pos, ss.end()))
+            throw UsageError(format("unexpected argument ‘%1%’") % args.front());
+        return true;
+    }
+};
+
+
 void parseCmdLine(int argc, char * * argv,
     std::function<bool(Strings::iterator & arg, const Strings::iterator & end)> parseArg)
 {
-    /* Put the arguments in a vector. */
-    Strings args;
-    argc--; argv++;
-    while (argc--) args.push_back(*argv++);
-
-    /* Process default options. */
-    for (Strings::iterator i = args.begin(); i != args.end(); ++i) {
-        string arg = *i;
-
-        /* Expand compound dash options (i.e., `-qlf' -> `-q -l -f'). */
-        if (arg.length() > 2 && arg[0] == '-' && arg[1] != '-' && isalpha(arg[1])) {
-            *i = (string) "-" + arg[1];
-            auto next = i; ++next;
-            for (unsigned int j = 2; j < arg.length(); j++)
-                if (isalpha(arg[j]))
-                    args.insert(next, (string) "-" + arg[j]);
-                else {
-                    args.insert(next, string(arg, j));
-                    break;
-                }
-            arg = *i;
-        }
-
-        if (arg == "--verbose" || arg == "-v") verbosity = (Verbosity) (verbosity + 1);
-        else if (arg == "--quiet") verbosity = verbosity > lvlError ? (Verbosity) (verbosity - 1) : lvlError;
-        else if (arg == "--log-type") {
-            string s = getArg(arg, i, args.end());
-            setLogType(s);
-        }
-        else if (arg == "--no-build-output" || arg == "-Q")
-            settings.buildVerbosity = lvlVomit;
-        else if (arg == "--print-build-trace")
-            settings.printBuildTrace = true;
-        else if (arg == "--keep-failed" || arg == "-K")
-            settings.keepFailed = true;
-        else if (arg == "--keep-going" || arg == "-k")
-            settings.keepGoing = true;
-        else if (arg == "--fallback")
-            settings.set("build-fallback", "true");
-        else if (arg == "--max-jobs" || arg == "-j")
-            settings.set("build-max-jobs", getArg(arg, i, args.end()));
-        else if (arg == "--cores")
-            settings.set("build-cores", getArg(arg, i, args.end()));
-        else if (arg == "--readonly-mode")
-            settings.readOnlyMode = true;
-        else if (arg == "--max-silent-time")
-            settings.set("build-max-silent-time", getArg(arg, i, args.end()));
-        else if (arg == "--timeout")
-            settings.set("build-timeout", getArg(arg, i, args.end()));
-        else if (arg == "--no-build-hook")
-            settings.useBuildHook = false;
-        else if (arg == "--show-trace")
-            settings.showTrace = true;
-        else if (arg == "--no-gc-warning")
-            gcWarning = false;
-        else if (arg == "--option") {
-            ++i; if (i == args.end()) throw UsageError("‘--option’ requires two arguments");
-            string name = *i;
-            ++i; if (i == args.end()) throw UsageError("‘--option’ requires two arguments");
-            string value = *i;
-            settings.set(name, value);
-        }
-        else {
-            if (!parseArg(i, args.end()))
-                throw UsageError(format("unrecognised option ‘%1%’") % *i);
-        }
-    }
-
+    LegacyArgs(baseNameOf(argv[0]), parseArg).parseCmdline(argvToStrings(argc, argv));
     settings.update();
 }
 
diff --git a/src/libmain/shared.hh b/src/libmain/shared.hh
index e4b984f06..20744be0a 100644
--- a/src/libmain/shared.hh
+++ b/src/libmain/shared.hh
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "util.hh"
+#include "args.hh"
 
 #include <signal.h>
 
@@ -9,8 +10,6 @@
 
 namespace nix {
 
-MakeError(UsageError, nix::Error);
-
 class Exit : public std::exception
 {
 public:
diff --git a/src/libutil/args.cc b/src/libutil/args.cc
new file mode 100644
index 000000000..6e4b82a27
--- /dev/null
+++ b/src/libutil/args.cc
@@ -0,0 +1,179 @@
+#include "args.hh"
+#include "hash.hh"
+
+namespace nix {
+
+void Args::parseCmdline(const Strings & _cmdline)
+{
+    Strings pendingArgs;
+    bool dashDash = false;
+
+    Strings cmdline(_cmdline);
+
+    for (auto pos = cmdline.begin(); pos != cmdline.end(); ) {
+
+        auto arg = *pos;
+
+        /* Expand compound dash options (i.e., `-qlf' -> `-q -l -f',
+           `-j3` -> `-j 3`). */
+        if (!dashDash && arg.length() > 2 && arg[0] == '-' && arg[1] != '-' && isalpha(arg[1])) {
+            *pos = (string) "-" + arg[1];
+            auto next = pos; ++next;
+            for (unsigned int j = 2; j < arg.length(); j++)
+                if (isalpha(arg[j]))
+                    cmdline.insert(next, (string) "-" + arg[j]);
+                else {
+                    cmdline.insert(next, string(arg, j));
+                    break;
+                }
+            arg = *pos;
+        }
+
+        if (!dashDash && arg == "--") {
+            dashDash = true;
+            ++pos;
+        }
+        else if (!dashDash && std::string(arg, 0, 1) == "-") {
+            if (!processFlag(pos, cmdline.end()))
+                throw UsageError(format("unrecognised flag ‘%1%’") % arg);
+        }
+        else {
+            pendingArgs.push_back(*pos++);
+            if (processArgs(pendingArgs, false))
+                pendingArgs.clear();
+        }
+    }
+
+    processArgs(pendingArgs, true);
+}
+
+void Args::printHelp(const string & programName, std::ostream & out)
+{
+    std::cout << "Usage: " << programName << " <FLAGS>...";
+    for (auto & exp : expectedArgs) {
+        std::cout << renderLabels({exp.label});
+        // FIXME: handle arity > 1
+        if (exp.arity == 0) std::cout << "...";
+    }
+    std::cout << "\n";
+
+    auto s = description();
+    if (s != "")
+        std::cout << "\nSummary: " << s << ".\n";
+
+    if (longFlags.size()) {
+        std::cout << "\n";
+        std::cout << "Flags:\n";
+        printFlags(out);
+    }
+}
+
+void Args::printFlags(std::ostream & out)
+{
+    Table2 table;
+    for (auto & flags : longFlags)
+        table.push_back(std::make_pair(
+                "--" + flags.first + renderLabels(flags.second.labels),
+                flags.second.description));
+    printTable(out, table);
+}
+
+bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
+{
+    assert(pos != end);
+
+    auto process = [&](const std::string & name, const Flag & flag) -> bool {
+        ++pos;
+        Strings args;
+        for (size_t n = 0 ; n < flag.arity; ++n) {
+            if (pos == end)
+                throw UsageError(format("flag ‘%1%’ requires %2% argument(s)")
+                    % name % flag.arity);
+            args.push_back(*pos++);
+        }
+        flag.handler(args);
+        return true;
+    };
+
+    if (string(*pos, 0, 2) == "--") {
+        auto i = longFlags.find(string(*pos, 2));
+        if (i == longFlags.end()) return false;
+        return process("--" + i->first, i->second);
+    }
+
+    if (string(*pos, 0, 1) == "-" && pos->size() == 2) {
+        auto c = (*pos)[1];
+        auto i = shortFlags.find(c);
+        if (i == shortFlags.end()) return false;
+        return process(std::string("-") + c, i->second);
+    }
+
+    return false;
+}
+
+bool Args::processArgs(const Strings & args, bool finish)
+{
+    if (expectedArgs.empty()) {
+        if (!args.empty())
+            throw UsageError(format("unexpected argument ‘%1%’") % args.front());
+        return true;
+    }
+
+    auto & exp = expectedArgs.front();
+
+    bool res = false;
+
+    if ((exp.arity == 0 && finish) ||
+        (exp.arity > 0 && args.size() == exp.arity))
+    {
+        exp.handler(args);
+        expectedArgs.pop_front();
+        res = true;
+    }
+
+    if (finish && !expectedArgs.empty())
+        throw UsageError("more arguments are required");
+
+    return res;
+}
+
+void Args::mkHashTypeFlag(const std::string & name, HashType * ht)
+{
+    mkFlag1(0, name, "TYPE", "hash algorithm (‘md5’, ‘sha1’, ‘sha256’, or ‘sha512’)", [=](std::string s) {
+        *ht = parseHashType(s);
+        if (*ht == htUnknown)
+            throw UsageError(format("unknown hash type ‘%1%’") % s);
+    });
+}
+
+Strings argvToStrings(int argc, char * * argv)
+{
+    Strings args;
+    argc--; argv++;
+    while (argc--) args.push_back(*argv++);
+    return args;
+}
+
+std::string renderLabels(const Strings & labels)
+{
+    std::string res;
+    for (auto label : labels) {
+        for (auto & c : label) c = std::toupper(c);
+        res += " <" + label + ">";
+    }
+    return res;
+}
+
+void printTable(std::ostream & out, const Table2 & table)
+{
+    size_t max = 0;
+    for (auto & row : table)
+        max = std::max(max, row.first.size());
+    for (auto & row : table) {
+        out << "  " << row.first
+            << std::string(max - row.first.size() + 2, ' ')
+            << row.second << "\n";
+    }
+}
+
+}
diff --git a/src/libutil/args.hh b/src/libutil/args.hh
new file mode 100644
index 000000000..c61a2b02a
--- /dev/null
+++ b/src/libutil/args.hh
@@ -0,0 +1,160 @@
+#pragma once
+
+#include <iostream>
+#include <map>
+#include <memory>
+
+#include "util.hh"
+
+namespace nix {
+
+MakeError(UsageError, nix::Error);
+
+enum HashType : char;
+
+class Args
+{
+public:
+
+    /* Parse the command line, throwing a UsageError if something goes
+       wrong. */
+    void parseCmdline(const Strings & cmdline);
+
+    virtual void printHelp(const string & programName, std::ostream & out);
+
+    virtual std::string description() { return ""; }
+
+protected:
+
+    /* Flags. */
+    struct Flag
+    {
+        std::string description;
+        Strings labels;
+        size_t arity;
+        std::function<void(Strings)> handler;
+    };
+
+    std::map<std::string, Flag> longFlags;
+    std::map<char, Flag> shortFlags;
+
+    virtual bool processFlag(Strings::iterator & pos, Strings::iterator end);
+
+    void printFlags(std::ostream & out);
+
+    /* Positional arguments. */
+    struct ExpectedArg
+    {
+        std::string label;
+        size_t arity; // 0 = any
+        std::function<void(Strings)> handler;
+    };
+
+    std::list<ExpectedArg> expectedArgs;
+
+    virtual bool processArgs(const Strings & args, bool finish);
+
+public:
+
+    /* Helper functions for constructing flags / positional
+       arguments. */
+
+    void mkFlag(char shortName, const std::string & longName,
+        const Strings & labels, const std::string & description,
+        size_t arity, std::function<void(Strings)> handler)
+    {
+        auto flag = Flag{description, labels, arity, handler};
+        if (shortName) shortFlags[shortName] = flag;
+        longFlags[longName] = flag;
+    }
+
+    void mkFlag(char shortName, const std::string & longName,
+        const std::string & description, std::function<void()> fun)
+    {
+        mkFlag(shortName, longName, {}, description, 0, std::bind(fun));
+    }
+
+    void mkFlag1(char shortName, const std::string & longName,
+        const std::string & label, const std::string & description,
+        std::function<void(std::string)> fun)
+    {
+        mkFlag(shortName, longName, {label}, description, 1, [=](Strings ss) {
+            fun(ss.front());
+        });
+    }
+
+    void mkFlag(char shortName, const std::string & name,
+        const std::string & description, bool * dest)
+    {
+        mkFlag(0, name, description, dest, true);
+    }
+
+    void mkFlag(char shortName, const std::string & longName,
+        const std::string & label, const std::string & description,
+        string * dest)
+    {
+        mkFlag1(shortName, longName, label, description, [=](std::string s) {
+            *dest = s;
+        });
+    }
+
+    void mkHashTypeFlag(const std::string & name, HashType * ht);
+
+    template<class T>
+    void mkFlag(char shortName, const std::string & longName, const std::string & description,
+        T * dest, const T & value)
+    {
+        mkFlag(shortName, longName, {}, description, 0, [=](Strings ss) {
+            *dest = value;
+        });
+    }
+
+    template<class I>
+    void mkIntFlag(char shortName, const std::string & longName,
+        const std::string & description, I * dest)
+    {
+        mkFlag<I>(shortName, longName, description, [=](I n) {
+            *dest = n;
+        });
+    }
+
+    template<class I>
+    void mkFlag(char shortName, const std::string & longName,
+        const std::string & description, std::function<void(I)> fun)
+    {
+        mkFlag(shortName, longName, {"N"}, description, 1, [=](Strings ss) {
+            I n;
+            if (!string2Int(ss.front(), n))
+                throw UsageError(format("flag ‘--%1%’ requires a integer argument") % longName);
+            fun(n);
+        });
+    }
+
+    /* Expect a string argument. */
+    void expectArg(const std::string & label, string * dest)
+    {
+        expectedArgs.push_back(ExpectedArg{label, 1, [=](Strings ss) {
+            *dest = ss.front();
+        }});
+    }
+
+    /* Expect 0 or more arguments. */
+    void expectArgs(const std::string & label, Strings * dest)
+    {
+        expectedArgs.push_back(ExpectedArg{label, 0, [=](Strings ss) {
+            *dest = ss;
+        }});
+    }
+};
+
+Strings argvToStrings(int argc, char * * argv);
+
+/* Helper function for rendering argument labels. */
+std::string renderLabels(const Strings & labels);
+
+/* Helper function for printing 2-column tables. */
+typedef std::vector<std::pair<std::string, std::string>> Table2;
+
+void printTable(std::ostream & out, const Table2 & table);
+
+}