diff --git a/updater/Android.mk b/updater/Android.mk
index 46c56f4..ac9aecb 100644
--- a/updater/Android.mk
+++ b/updater/Android.mk
@@ -35,6 +35,7 @@
     libfec \
     libfec_rs \
     libfs_mgr \
+    libgtest_prod \
     liblog \
     libselinux \
     libsparse \
diff --git a/updater/commands.cpp b/updater/commands.cpp
index f798c6a..6d4b531 100644
--- a/updater/commands.cpp
+++ b/updater/commands.cpp
@@ -16,28 +16,253 @@
 
 #include "private/commands.h"
 
+#include <ostream>
 #include <string>
+#include <vector>
 
 #include <android-base/logging.h>
+#include <android-base/parseint.h>
+#include <android-base/stringprintf.h>
+#include <android-base/strings.h>
+
+#include "otautil/rangeset.h"
+
+using namespace std::string_literals;
 
 Command::Type Command::ParseType(const std::string& type_str) {
-  if (type_str == "zero") {
-    return Type::ZERO;
-  } else if (type_str == "new") {
-    return Type::NEW;
+  if (type_str == "bsdiff") {
+    return Type::BSDIFF;
   } else if (type_str == "erase") {
     return Type::ERASE;
-  } else if (type_str == "move") {
-    return Type::MOVE;
-  } else if (type_str == "bsdiff") {
-    return Type::BSDIFF;
-  } else if (type_str == "imgdiff") {
-    return Type::IMGDIFF;
-  } else if (type_str == "stash") {
-    return Type::STASH;
   } else if (type_str == "free") {
     return Type::FREE;
+  } else if (type_str == "imgdiff") {
+    return Type::IMGDIFF;
+  } else if (type_str == "move") {
+    return Type::MOVE;
+  } else if (type_str == "new") {
+    return Type::NEW;
+  } else if (type_str == "stash") {
+    return Type::STASH;
+  } else if (type_str == "zero") {
+    return Type::ZERO;
   }
-  LOG(ERROR) << "Invalid type: " << type_str;
   return Type::LAST;
 };
+
+bool Command::ParseTargetInfoAndSourceInfo(const std::vector<std::string>& tokens,
+                                           const std::string& tgt_hash, TargetInfo* target,
+                                           const std::string& src_hash, SourceInfo* source,
+                                           std::string* err) {
+  // We expect the given tokens parameter in one of the following formats.
+  //
+  //    <tgt_ranges> <src_block_count> - <[stash_id:location] ...>
+  //        (loads data from stashes only)
+  //
+  //    <tgt_ranges> <src_block_count> <src_ranges>
+  //        (loads data from source image only)
+  //
+  //    <tgt_ranges> <src_block_count> <src_ranges> <src_ranges_location> <[stash_id:location] ...>
+  //        (loads data from both of source image and stashes)
+
+  // At least it needs to provide three parameters: <tgt_ranges>, <src_block_count> and
+  // "-"/<src_ranges>.
+  if (tokens.size() < 3) {
+    *err = "invalid number of parameters";
+    return false;
+  }
+
+  size_t pos = 0;
+  RangeSet tgt_ranges = RangeSet::Parse(tokens[pos++]);
+  if (!tgt_ranges) {
+    *err = "invalid target ranges";
+    return false;
+  }
+  *target = TargetInfo(tgt_hash, tgt_ranges);
+
+  // <src_block_count>
+  const std::string& token = tokens[pos++];
+  size_t src_blocks;
+  if (!android::base::ParseUint(token, &src_blocks)) {
+    *err = "invalid src_block_count \""s + token + "\"";
+    return false;
+  }
+
+  RangeSet src_ranges;
+  RangeSet src_ranges_location;
+  // "-" or <src_ranges> [<src_ranges_location>]
+  if (tokens[pos] == "-") {
+    // no source ranges, only stashes
+    pos++;
+  } else {
+    src_ranges = RangeSet::Parse(tokens[pos++]);
+    if (!src_ranges) {
+      *err = "invalid source ranges";
+      return false;
+    }
+
+    if (pos >= tokens.size()) {
+      // No stashes, only source ranges.
+      SourceInfo result(src_hash, src_ranges, {}, {});
+
+      // Sanity check the block count.
+      if (result.blocks() != src_blocks) {
+        *err =
+            android::base::StringPrintf("mismatching block count: %zu (%s) vs %zu", result.blocks(),
+                                        src_ranges.ToString().c_str(), src_blocks);
+        return false;
+      }
+
+      *source = result;
+      return true;
+    }
+
+    src_ranges_location = RangeSet::Parse(tokens[pos++]);
+    if (!src_ranges_location) {
+      *err = "invalid source ranges location";
+      return false;
+    }
+  }
+
+  // <[stash_id:stash_location]>
+  std::vector<StashInfo> stashes;
+  while (pos < tokens.size()) {
+    // Each word is a an index into the stash table, a colon, and then a RangeSet describing where
+    // in the source block that stashed data should go.
+    std::vector<std::string> pairs = android::base::Split(tokens[pos++], ":");
+    if (pairs.size() != 2) {
+      *err = "invalid stash info";
+      return false;
+    }
+    RangeSet stash_location = RangeSet::Parse(pairs[1]);
+    if (!stash_location) {
+      *err = "invalid stash location";
+      return false;
+    }
+    stashes.emplace_back(pairs[0], stash_location);
+  }
+
+  SourceInfo result(src_hash, src_ranges, src_ranges_location, stashes);
+  if (src_blocks != result.blocks()) {
+    *err = android::base::StringPrintf("mismatching block count: %zu (%s) vs %zu", result.blocks(),
+                                       src_ranges.ToString().c_str(), src_blocks);
+    return false;
+  }
+
+  *source = result;
+  return true;
+}
+
+Command Command::Parse(const std::string& line, size_t index, std::string* err) {
+  std::vector<std::string> tokens = android::base::Split(line, " ");
+  size_t pos = 0;
+  // tokens.size() will be 1 at least.
+  Type op = ParseType(tokens[pos++]);
+  if (op == Type::LAST) {
+    *err = "invalid type";
+    return {};
+  }
+
+  PatchInfo patch_info;
+  TargetInfo target_info;
+  SourceInfo source_info;
+  StashInfo stash_info;
+
+  if (op == Type::ZERO || op == Type::NEW || op == Type::ERASE) {
+    // zero/new/erase <rangeset>
+    RangeSet tgt_ranges = RangeSet::Parse(tokens[pos++]);
+    if (!tgt_ranges) {
+      return {};
+    }
+    static const std::string kUnknownHash{ "unknown-hash" };
+    target_info = TargetInfo(kUnknownHash, tgt_ranges);
+  } else if (op == Type::STASH) {
+    // stash <stash_id> <src_ranges>
+    if (pos + 2 > tokens.size()) {
+      *err = "missing stash id and/or source ranges";
+      return {};
+    }
+    const std::string& id = tokens[pos++];
+    RangeSet src_ranges = RangeSet::Parse(tokens[pos++]);
+    if (!src_ranges) {
+      *err = "invalid token";
+      return {};
+    }
+    stash_info = StashInfo(id, src_ranges);
+  } else if (op == Type::FREE) {
+    // free <stash_id>
+    if (pos + 1 > tokens.size()) {
+      *err = "missing stash id in free command";
+      return {};
+    }
+    stash_info = StashInfo(tokens[pos++], {});
+  } else if (op == Type::MOVE) {
+    // <hash>
+    if (pos + 1 > tokens.size()) {
+      *err = "missing hash";
+      return {};
+    }
+    std::string hash = tokens[pos++];
+    if (!ParseTargetInfoAndSourceInfo(
+            std::vector<std::string>(tokens.cbegin() + pos, tokens.cend()), hash, &target_info,
+            hash, &source_info, err)) {
+      return {};
+    }
+  } else if (op == Type::BSDIFF || op == Type::IMGDIFF) {
+    // <offset> <length> <srchash> <dsthash>
+    if (pos + 4 > tokens.size()) {
+      *err = "invalid number of tokens";
+      return {};
+    }
+    size_t offset;
+    size_t length;
+    if (!android::base::ParseUint(tokens[pos++], &offset) ||
+        !android::base::ParseUint(tokens[pos++], &length)) {
+      *err = "invalid patch offset/length";
+      return {};
+    }
+    patch_info = PatchInfo(offset, length);
+
+    std::string src_hash = tokens[pos++];
+    std::string dst_hash = tokens[pos++];
+    if (!ParseTargetInfoAndSourceInfo(
+            std::vector<std::string>(tokens.cbegin() + pos, tokens.cend()), dst_hash, &target_info,
+            src_hash, &source_info, err)) {
+      return {};
+    }
+  } else {
+    *err = "invalid op";
+    return {};
+  }
+
+  return Command(op, index, line, patch_info, target_info, source_info, stash_info);
+}
+
+std::ostream& operator<<(std::ostream& os, const Command& command) {
+  os << command.index() << ": " << command.cmdline();
+  return os;
+}
+
+std::ostream& operator<<(std::ostream& os, const TargetInfo& target) {
+  os << target.blocks() << " blocks (" << target.hash_ << "): " << target.ranges_.ToString();
+  return os;
+}
+
+std::ostream& operator<<(std::ostream& os, const StashInfo& stash) {
+  os << stash.blocks() << " blocks (" << stash.id_ << "): " << stash.ranges_.ToString();
+  return os;
+}
+
+std::ostream& operator<<(std::ostream& os, const SourceInfo& source) {
+  os << source.blocks_ << " blocks (" << source.hash_ << "): ";
+  if (source.ranges_) {
+    os << source.ranges_.ToString();
+    if (source.location_) {
+      os << " (location: " << source.location_.ToString() << ")";
+    }
+  }
+  if (!source.stashes_.empty()) {
+    os << " " << source.stashes_.size() << " stash(es)";
+  }
+  return os;
+}
diff --git a/updater/include/private/commands.h b/updater/include/private/commands.h
index b360000..784892f 100644
--- a/updater/include/private/commands.h
+++ b/updater/include/private/commands.h
@@ -16,20 +16,298 @@
 
 #pragma once
 
+#include <ostream>
 #include <string>
+#include <vector>
 
-struct Command {
+#include <gtest/gtest_prod.h>  // FRIEND_TEST
+
+#include "otautil/rangeset.h"
+
+// Represents the target info used in a Command. TargetInfo contains the ranges of the blocks and
+// the expected hash.
+class TargetInfo {
+ public:
+  TargetInfo() = default;
+
+  TargetInfo(std::string hash, RangeSet ranges)
+      : hash_(std::move(hash)), ranges_(std::move(ranges)) {}
+
+  const std::string& hash() const {
+    return hash_;
+  }
+
+  const RangeSet& ranges() const {
+    return ranges_;
+  }
+
+  size_t blocks() const {
+    return ranges_.blocks();
+  }
+
+  bool operator==(const TargetInfo& other) const {
+    return hash_ == other.hash_ && ranges_ == other.ranges_;
+  }
+
+ private:
+  friend std::ostream& operator<<(std::ostream& os, const TargetInfo& source);
+
+  // The hash of the data represented by the object.
+  std::string hash_;
+  // The block ranges that the data should be written to.
+  RangeSet ranges_;
+};
+
+std::ostream& operator<<(std::ostream& os, const TargetInfo& source);
+
+// Represents the stash info used in a Command.
+class StashInfo {
+ public:
+  StashInfo() = default;
+
+  StashInfo(std::string id, RangeSet ranges) : id_(std::move(id)), ranges_(std::move(ranges)) {}
+
+  size_t blocks() const {
+    return ranges_.blocks();
+  }
+
+  const std::string& id() const {
+    return id_;
+  }
+
+  const RangeSet& ranges() const {
+    return ranges_;
+  }
+
+  bool operator==(const StashInfo& other) const {
+    return id_ == other.id_ && ranges_ == other.ranges_;
+  }
+
+ private:
+  friend std::ostream& operator<<(std::ostream& os, const StashInfo& stash);
+
+  // The id (i.e. hash) of the stash.
+  std::string id_;
+  // The matching location of the stash.
+  RangeSet ranges_;
+};
+
+std::ostream& operator<<(std::ostream& os, const StashInfo& stash);
+
+// Represents the source info in a Command, whose data could come from source image, stashed blocks,
+// or both.
+class SourceInfo {
+ public:
+  SourceInfo() = default;
+
+  SourceInfo(std::string hash, RangeSet ranges, RangeSet location, std::vector<StashInfo> stashes)
+      : hash_(std::move(hash)),
+        ranges_(std::move(ranges)),
+        location_(std::move(location)),
+        stashes_(std::move(stashes)) {
+    blocks_ = ranges_.blocks();
+    for (const auto& stash : stashes_) {
+      blocks_ += stash.ranges().blocks();
+    }
+  }
+
+  const std::string& hash() const {
+    return hash_;
+  }
+
+  size_t blocks() const {
+    return blocks_;
+  }
+
+  bool operator==(const SourceInfo& other) const {
+    return hash_ == other.hash_ && ranges_ == other.ranges_ && location_ == other.location_ &&
+           stashes_ == other.stashes_;
+  }
+
+ private:
+  friend std::ostream& operator<<(std::ostream& os, const SourceInfo& source);
+
+  // The hash of the data represented by the object.
+  std::string hash_;
+  // The block ranges from the source image to read data from. This could be a subset of all the
+  // blocks represented by the object, or empty if all the data should be loaded from stash.
+  RangeSet ranges_;
+  // The location in the buffer to load ranges_ into. Empty if ranges_ alone covers all the blocks
+  // (i.e. nothing needs to be loaded from stash).
+  RangeSet location_;
+  // The info for the stashed blocks that are part of the source. Empty if there's none.
+  std::vector<StashInfo> stashes_;
+  // Total number of blocks represented by the object.
+  size_t blocks_{ 0 };
+};
+
+std::ostream& operator<<(std::ostream& os, const SourceInfo& source);
+
+class PatchInfo {
+ public:
+  PatchInfo() = default;
+
+  PatchInfo(size_t offset, size_t length) : offset_(offset), length_(length) {}
+
+  size_t offset() const {
+    return offset_;
+  }
+
+  size_t length() const {
+    return length_;
+  }
+
+  bool operator==(const PatchInfo& other) const {
+    return offset_ == other.offset_ && length_ == other.length_;
+  }
+
+ private:
+  size_t offset_{ 0 };
+  size_t length_{ 0 };
+};
+
+// Command class holds the info for an update command that performs block-based OTA (BBOTA). Each
+// command consists of one or several args, namely TargetInfo, SourceInfo, StashInfo and PatchInfo.
+// The currently used BBOTA version is v4.
+//
+//    zero <tgt_ranges>
+//      - Fill the indicated blocks with zeros.
+//      - Meaningful args: TargetInfo
+//
+//    new <tgt_ranges>
+//      - Fill the blocks with data read from the new_data file.
+//      - Meaningful args: TargetInfo
+//
+//    erase <tgt_ranges>
+//      - Mark the given blocks as empty.
+//      - Meaningful args: TargetInfo
+//
+//    move <hash> <...>
+//      - Read the source blocks, write result to target blocks.
+//      - Meaningful args: TargetInfo, SourceInfo
+//
+//      See the note below for <...>.
+//
+//    bsdiff <patchstart> <patchlen> <srchash> <dsthash> <...>
+//    imgdiff <patchstart> <patchlen> <srchash> <dsthash> <...>
+//      - Read the source blocks, apply a patch, and write result to target blocks.
+//      - Meaningful args: PatchInfo, TargetInfo, SourceInfo
+//
+//      It expects <...> in one of the following formats:
+//
+//        <tgt_ranges> <src_block_count> - <[stash_id:stash_location] ...>
+//          (loads data from stashes only)
+//
+//        <tgt_ranges> <src_block_count> <src_ranges>
+//          (loads data from source image only)
+//
+//        <tgt_ranges> <src_block_count> <src_ranges> <src_ranges_location>
+//                                       <[stash_id:stash_location] ...>
+//          (loads data from both of source image and stashes)
+//
+//    stash <stash_id> <src_ranges>
+//      - Load the given source blocks and stash the data in the given slot of the stash table.
+//      - Meaningful args: StashInfo
+//
+//    free <stash_id>
+//      - Free the given stash data.
+//      - Meaningful args: StashInfo
+//
+class Command {
+ public:
   enum class Type {
-    ZERO,
-    NEW,
-    ERASE,
-    MOVE,
     BSDIFF,
-    IMGDIFF,
-    STASH,
+    ERASE,
     FREE,
+    IMGDIFF,
+    MOVE,
+    NEW,
+    STASH,
+    ZERO,
     LAST,  // Not a valid type.
   };
 
+  Command() = default;
+
+  Command(Type type, size_t index, std::string cmdline, PatchInfo patch, TargetInfo target,
+          SourceInfo source, StashInfo stash)
+      : type_(type),
+        index_(index),
+        cmdline_(std::move(cmdline)),
+        patch_(std::move(patch)),
+        target_(std::move(target)),
+        source_(std::move(source)),
+        stash_(std::move(stash)) {}
+
+  // Parses the given command 'line' into a Command object and returns it. The 'index' is specified
+  // by the caller to index the object. On parsing error, it returns an empty Command object that
+  // evaluates to false, and the specific error message will be set in 'err'.
+  static Command Parse(const std::string& line, size_t index, std::string* err);
+
+  // Parses the command type from the given string.
   static Type ParseType(const std::string& type_str);
+
+  Type type() const {
+    return type_;
+  }
+
+  size_t index() const {
+    return index_;
+  }
+
+  const std::string& cmdline() const {
+    return cmdline_;
+  }
+
+  const PatchInfo& patch() const {
+    return patch_;
+  }
+
+  const TargetInfo& target() const {
+    return target_;
+  }
+
+  const SourceInfo& source() const {
+    return source_;
+  }
+
+  const StashInfo& stash() const {
+    return stash_;
+  }
+
+  constexpr explicit operator bool() const {
+    return type_ != Type::LAST;
+  }
+
+ private:
+  FRIEND_TEST(CommandsTest, ParseTargetInfoAndSourceInfo_InvalidInput);
+  FRIEND_TEST(CommandsTest, ParseTargetInfoAndSourceInfo_StashesOnly);
+  FRIEND_TEST(CommandsTest, ParseTargetInfoAndSourceInfo_SourceBlocksAndStashes);
+  FRIEND_TEST(CommandsTest, ParseTargetInfoAndSourceInfo_SourceBlocksOnly);
+
+  // Parses the target and source info from the given 'tokens' vector. Saves the parsed info into
+  // 'target' and 'source' objects. Returns the parsing result. Error message will be set in 'err'
+  // on parsing error, and the contents in 'target' and 'source' will be undefined.
+  static bool ParseTargetInfoAndSourceInfo(const std::vector<std::string>& tokens,
+                                           const std::string& tgt_hash, TargetInfo* target,
+                                           const std::string& src_hash, SourceInfo* source,
+                                           std::string* err);
+
+  // The type of the command.
+  Type type_{ Type::LAST };
+  // The index of the Command object, which is specified by the caller.
+  size_t index_{ 0 };
+  // The input string that the Command object is parsed from.
+  std::string cmdline_;
+  // The patch info. Only meaningful for BSDIFF and IMGDIFF commands.
+  PatchInfo patch_;
+  // The target info, where the command should be written to.
+  TargetInfo target_;
+  // The source info to load the source blocks for the command.
+  SourceInfo source_;
+  // The stash info. Only meaningful for STASH and FREE commands. Note that although SourceInfo may
+  // also load data from stash, such info will be owned and managed by SourceInfo (i.e. in source_).
+  StashInfo stash_;
 };
+
+std::ostream& operator<<(std::ostream& os, const Command& command);