diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index daa9885..4807c4b 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -1,24 +1,24 @@
 name: CI
-on: push
+on:
+  push:
+  pull_request:
+
 jobs:
   test:
-    strategy:
-      matrix:
-        os: [ubuntu-latest, macos-latest]
-    runs-on: ${{matrix.os}}
+    runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2.3.4
         with:
           submodules: recursive
-      - uses: goto-bus-stop/setup-zig@v1.0.0
+      - uses: goto-bus-stop/setup-zig@v1.2.5
         with:
           version: 0.7.0
       - run: zig build
   lint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1
-      - uses: goto-bus-stop/setup-zig@v1.0.0
+      - uses: actions/checkout@v2.3.4
+      - uses: goto-bus-stop/setup-zig@v1.2.5
         with:
           version: 0.7.0
       - run: zig fmt --check .
diff --git a/clap/args.zig b/clap/args.zig
index 52626fc..2868050 100644
--- a/clap/args.zig
+++ b/clap/args.zig
@@ -5,6 +5,7 @@ const debug = std.debug;
 const heap = std.heap;
 const mem = std.mem;
 const process = std.process;
+const testing = std.testing;
 
 /// An example of what methods should be implemented on an arg iterator.
 pub const ExampleArgIterator = struct {
@@ -77,3 +78,264 @@ pub const OsIterator = struct {
         }
     }
 };
+
+/// An argument iterator that takes a string and parses it into arguments, simulating
+/// how shells split arguments.
+pub const ShellIterator = struct {
+    const Error = error{
+        DanglingEscape,
+        QuoteNotClosed,
+    } || mem.Allocator.Error;
+
+    arena: heap.ArenaAllocator,
+    str: []const u8,
+
+    pub fn init(allocator: *mem.Allocator, str: []const u8) ShellIterator {
+        return .{
+            .arena = heap.ArenaAllocator.init(allocator),
+            .str = str,
+        };
+    }
+
+    pub fn deinit(iter: *ShellIterator) void {
+        iter.arena.deinit();
+    }
+
+    pub fn next(iter: *ShellIterator) Error!?[]const u8 {
+        // Whenever possible, this iterator will return slices into `str` instead of
+        // allocating. Sometimes this is not possible, for example, escaped characters
+        // have be be unescape, so we need to allocate in this case.
+        var list = std.ArrayList(u8).init(&iter.arena.allocator);
+        var start: usize = 0;
+        var state: enum {
+            skip_whitespace,
+            no_quote,
+            no_quote_escape,
+            single_quote,
+            double_quote,
+            double_quote_escape,
+            after_quote,
+        } = .skip_whitespace;
+
+        for (iter.str) |c, i| {
+            switch (state) {
+                // The state that skips the initial whitespace.
+                .skip_whitespace => switch (c) {
+                    ' ', '\t', '\n' => {},
+                    '\'' => {
+                        start = i + 1;
+                        state = .single_quote;
+                    },
+                    '"' => {
+                        start = i + 1;
+                        state = .double_quote;
+                    },
+                    '\\' => {
+                        start = i + 1;
+                        state = .no_quote_escape;
+                    },
+                    else => {
+                        start = i;
+                        state = .no_quote;
+                    },
+                },
+
+                // The state that parses the none quoted part of a argument.
+                .no_quote => switch (c) {
+                    // We are doing parsing a none quoted argument when we hit a
+                    // whitespace.
+                    ' ', '\t', '\n' => {
+                        defer iter.str = iter.str[i..];
+                        return iter.result(start, i, &list);
+                    },
+
+                    // Slicing is not possible if a quote starts while parsing none
+                    // quoted args.
+                    // Example:
+                    // ab'cd' -> abcd
+                    '\'' => {
+                        try list.appendSlice(iter.str[start..i]);
+                        start = i + 1;
+                        state = .single_quote;
+                    },
+                    '"' => {
+                        try list.appendSlice(iter.str[start..i]);
+                        start = i + 1;
+                        state = .double_quote;
+                    },
+
+                    // Slicing is not possible if we need to escape a character.
+                    // Example:
+                    // ab\"d -> ab"d
+                    '\\' => {
+                        try list.appendSlice(iter.str[start..i]);
+                        start = i + 1;
+                        state = .no_quote_escape;
+                    },
+                    else => {},
+                },
+
+                // We're in this state after having parsed the quoted part of an
+                // argument. This state works mostly the same as .no_quote, but
+                // is aware, that the last character seen was a quote, which should
+                // not be part of the argument. This is why you will see `i - 1` here
+                // instead of just `i` when `iter.str` is sliced.
+                .after_quote => switch (c) {
+                    ' ', '\t', '\n' => {
+                        defer iter.str = iter.str[i..];
+                        return iter.result(start, i - 1, &list);
+                    },
+                    '\'' => {
+                        try list.appendSlice(iter.str[start .. i - 1]);
+                        start = i + 1;
+                        state = .single_quote;
+                    },
+                    '"' => {
+                        try list.appendSlice(iter.str[start .. i - 1]);
+                        start = i + 1;
+                        state = .double_quote;
+                    },
+                    '\\' => {
+                        try list.appendSlice(iter.str[start .. i - 1]);
+                        start = i + 1;
+                        state = .no_quote_escape;
+                    },
+                    else => {
+                        try list.appendSlice(iter.str[start .. i - 1]);
+                        start = i;
+                        state = .no_quote;
+                    },
+                },
+
+                // The states that parse the quoted part of arguments. The only differnece
+                // between single and double quoted arguments is that single quoted
+                // arguments ignore escape sequences, while double quoted arguments
+                // does escaping.
+                .single_quote => switch (c) {
+                    '\'' => state = .after_quote,
+                    else => {},
+                },
+                .double_quote => switch (c) {
+                    '"' => state = .after_quote,
+                    '\\' => {
+                        try list.appendSlice(iter.str[start..i]);
+                        start = i + 1;
+                        state = .double_quote_escape;
+                    },
+                    else => {},
+                },
+
+                // The state we end up when after the escape character (`\`). All these
+                // states do is transition back into the previous state.
+                // TODO: Are there any escape sequences that does transform the second
+                //       character into something else? For example, in Zig, `\n` is
+                //       transformed into the line feed ascii character.
+                .no_quote_escape => switch (c) {
+                    else => state = .no_quote,
+                },
+                .double_quote_escape => switch (c) {
+                    else => state = .double_quote,
+                },
+            }
+        }
+
+        defer iter.str = iter.str[iter.str.len..];
+        switch (state) {
+            .skip_whitespace => return null,
+            .no_quote => return iter.result(start, iter.str.len, &list),
+            .after_quote => return iter.result(start, iter.str.len - 1, &list),
+            .no_quote_escape => return Error.DanglingEscape,
+            .single_quote,
+            .double_quote,
+            .double_quote_escape,
+            => return Error.QuoteNotClosed,
+        }
+    }
+
+    fn result(iter: *ShellIterator, start: usize, end: usize, list: *std.ArrayList(u8)) Error!?[]const u8 {
+        const res = iter.str[start..end];
+
+        // If we already have something in `list` that means that we could not
+        // parse the argument without allocation. We therefor need to just append
+        // the rest we have to the list and return that.
+        if (list.items.len != 0) {
+            try list.appendSlice(res);
+            return list.toOwnedSlice();
+        }
+        return res;
+    }
+};
+
+fn testShellIteratorOk(str: []const u8, allocations: usize, expect: []const []const u8) void {
+    var allocator = testing.FailingAllocator.init(testing.allocator, allocations);
+    var it = ShellIterator.init(&allocator.allocator, str);
+    defer it.deinit();
+
+    for (expect) |e| {
+        if (it.next()) |actual| {
+            testing.expect(actual != null);
+            testing.expectEqualStrings(e, actual.?);
+        } else |err| testing.expectEqual(@as(anyerror![]const u8, e), err);
+    }
+
+    if (it.next()) |actual| {
+        testing.expectEqual(@as(?[]const u8, null), actual);
+        testing.expectEqual(allocations, allocator.allocations);
+    } else |err| testing.expectEqual(@as(anyerror!void, {}), err);
+}
+
+fn testShellIteratorErr(str: []const u8, expect: anyerror) void {
+    var it = ShellIterator.init(testing.allocator, str);
+    defer it.deinit();
+
+    while (it.next() catch |err| {
+        testing.expectError(expect, @as(anyerror!void, err));
+        return;
+    }) |_| {}
+
+    testing.expectError(expect, @as(anyerror!void, {}));
+}
+
+test "ShellIterator" {
+    testShellIteratorOk("a", 0, &[_][]const u8{"a"});
+    testShellIteratorOk("'a'", 0, &[_][]const u8{"a"});
+    testShellIteratorOk("\"a\"", 0, &[_][]const u8{"a"});
+    testShellIteratorOk("a b", 0, &[_][]const u8{ "a", "b" });
+    testShellIteratorOk("'a' b", 0, &[_][]const u8{ "a", "b" });
+    testShellIteratorOk("\"a\" b", 0, &[_][]const u8{ "a", "b" });
+    testShellIteratorOk("a 'b'", 0, &[_][]const u8{ "a", "b" });
+    testShellIteratorOk("a \"b\"", 0, &[_][]const u8{ "a", "b" });
+    testShellIteratorOk("'a b'", 0, &[_][]const u8{"a b"});
+    testShellIteratorOk("\"a b\"", 0, &[_][]const u8{"a b"});
+    testShellIteratorOk("\"a\"\"b\"", 1, &[_][]const u8{"ab"});
+    testShellIteratorOk("'a''b'", 1, &[_][]const u8{"ab"});
+    testShellIteratorOk("'a'b", 1, &[_][]const u8{"ab"});
+    testShellIteratorOk("a'b'", 1, &[_][]const u8{"ab"});
+    testShellIteratorOk("a\\ b", 1, &[_][]const u8{"a b"});
+    testShellIteratorOk("\"a\\ b\"", 1, &[_][]const u8{"a b"});
+    testShellIteratorOk("'a\\ b'", 0, &[_][]const u8{"a\\ b"});
+    testShellIteratorOk("   a     b      ", 0, &[_][]const u8{ "a", "b" });
+    testShellIteratorOk("\\  \\ ", 0, &[_][]const u8{ " ", " " });
+
+    testShellIteratorOk(
+        \\printf 'run\nuninstall\n'
+    , 0, &[_][]const u8{ "printf", "run\\nuninstall\\n" });
+    testShellIteratorOk(
+        \\setsid -f steam "steam://$action/$id"
+    , 0, &[_][]const u8{ "setsid", "-f", "steam", "steam://$action/$id" });
+    testShellIteratorOk(
+        \\xargs -I% rg --no-heading --no-line-number --only-matching
+        \\    --case-sensitive --multiline --text --byte-offset '(?-u)%' $@
+        \\
+    , 0, &[_][]const u8{
+        "xargs",            "-I%",             "rg",               "--no-heading",
+        "--no-line-number", "--only-matching", "--case-sensitive", "--multiline",
+        "--text",           "--byte-offset",   "(?-u)%",           "$@",
+    });
+
+    testShellIteratorErr("'a", error.QuoteNotClosed);
+    testShellIteratorErr("'a\\", error.QuoteNotClosed);
+    testShellIteratorErr("\"a", error.QuoteNotClosed);
+    testShellIteratorErr("\"a\\", error.QuoteNotClosed);
+    testShellIteratorErr("a\\", error.DanglingEscape);
+}