diff --git a/Cargo.lock b/Cargo.lock
index b5723ae..92e5251 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -364,6 +364,8 @@ dependencies = [
  "fehler",
  "git2",
  "globset",
+ "ignore",
+ "lexiclean",
  "libc",
  "log",
  "pretty_env_logger",
@@ -376,6 +378,7 @@ dependencies = [
  "strum_macros",
  "tempfile",
  "url",
+ "walkdir",
 ]
 
 [[package]]
diff --git a/bin/gen/Cargo.toml b/bin/gen/Cargo.toml
index b8f8647..b4acaa2 100644
--- a/bin/gen/Cargo.toml
+++ b/bin/gen/Cargo.toml
@@ -12,6 +12,7 @@ chrono            = "0.4.11"
 fehler            = "1.0.0"
 git2              = "0.13.1"
 globset           = "0.4.5"
+ignore            = "0.4.14"
 libc              = "0.2.69"
 log               = "0.4.8"
 pretty_env_logger = "0.4.0"
@@ -22,6 +23,8 @@ structopt         = "0.3.12"
 strum             = "0.18.0"
 strum_macros      = "0.18.0"
 tempfile          = "3.1.0"
+walkdir           = "2.3.1"
+lexiclean         = "0.0.1"
 
 [dependencies.serde]
 version  = "1.0.106"
diff --git a/bin/gen/src/common.rs b/bin/gen/src/common.rs
index 7d5a11e..1031cc5 100644
--- a/bin/gen/src/common.rs
+++ b/bin/gen/src/common.rs
@@ -6,7 +6,7 @@ pub(crate) use std::{
   fs::{self, File},
   io,
   ops::Deref,
-  path::{Path, PathBuf},
+  path::{Path, PathBuf, StripPrefixError},
   process::{self, Command, ExitStatus, Stdio},
   str,
 };
@@ -16,6 +16,8 @@ pub(crate) use cargo_toml::Manifest;
 pub(crate) use chrono::{DateTime, NaiveDateTime, Utc};
 pub(crate) use fehler::{throw, throws};
 pub(crate) use git2::{Commit, Oid, Repository};
+pub(crate) use ignore::overrides::OverrideBuilder;
+pub(crate) use lexiclean::Lexiclean;
 pub(crate) use libc::EXIT_FAILURE;
 pub(crate) use log::info;
 pub(crate) use regex::Regex;
@@ -26,6 +28,7 @@ pub(crate) use structopt::StructOpt;
 pub(crate) use strum::VariantNames;
 pub(crate) use strum_macros::{EnumVariantNames, IntoStaticStr};
 pub(crate) use url::Url;
+pub(crate) use walkdir::WalkDir;
 
 // modules
 pub(crate) use crate::error;
diff --git a/bin/gen/src/error.rs b/bin/gen/src/error.rs
index 62dba36..6937def 100644
--- a/bin/gen/src/error.rs
+++ b/bin/gen/src/error.rs
@@ -52,6 +52,12 @@ pub(crate) enum Error {
   },
   #[snafu(display("I/O error at `{}`: {}", path.display(), source))]
   Filesystem { path: PathBuf, source: io::Error },
+  #[snafu(display("I/O error copying `{}` to `{}`: {}", src.display(), dst.display(), source))]
+  FilesystemCopy {
+    src: PathBuf,
+    dst: PathBuf,
+    source: io::Error,
+  },
   #[snafu(display("Git error: {}", source))]
   Git { source: git2::Error },
   #[snafu(display("Regex compilation error: {}", source))]
@@ -67,6 +73,12 @@ pub(crate) enum Error {
   TemplateRender { source: askama::Error },
   #[snafu(display("Failed to get workdir for repo at `{}`", repo.display()))]
   Workdir { repo: PathBuf },
+  #[snafu(display("Failed to build overrides: {}", source))]
+  Ignore { source: ignore::Error },
+  #[snafu(display("Failed to traverse worktree: {}", source))]
+  Walkdir { source: walkdir::Error },
+  #[snafu(display("Failed to strip path prefix: {}", source))]
+  StripPrefix { source: StripPrefixError },
 }
 
 impl From<regex::Error> for Error {
@@ -86,3 +98,15 @@ impl From<cargo_toml::Error> for Error {
     Self::CargoToml { source }
   }
 }
+
+impl From<ignore::Error> for Error {
+  fn from(source: ignore::Error) -> Self {
+    Self::Ignore { source }
+  }
+}
+
+impl From<walkdir::Error> for Error {
+  fn from(source: walkdir::Error) -> Self {
+    Self::Walkdir { source }
+  }
+}
diff --git a/bin/gen/src/opt.rs b/bin/gen/src/opt.rs
index 76d7bd1..55426cd 100644
--- a/bin/gen/src/opt.rs
+++ b/bin/gen/src/opt.rs
@@ -4,6 +4,8 @@ use crate::common::*;
 pub(crate) enum Opt {
   #[structopt(about("Update all generated docs"))]
   All,
+  #[structopt(about("Generate book"))]
+  Book,
   #[structopt(about("Generate the changelog"))]
   Changelog,
   #[structopt(about("Print a commit template to standard output"))]
@@ -12,10 +14,10 @@ pub(crate) enum Opt {
   CommitTypes,
   #[structopt(about("Generate completion scripts"))]
   CompletionScripts,
+  #[structopt(about("Diff generated content between commits"))]
+  Diff,
   #[structopt(about("Generate readme"))]
   Readme,
-  #[structopt(about("Generate book"))]
-  Book,
   #[structopt(about("Generate man pages"))]
   Man,
 }
@@ -70,16 +72,20 @@ impl Opt {
       Self::Readme => Self::readme(&project)?,
       Self::Book => Self::book(&project)?,
       Self::Man => Self::man(&project)?,
-      Self::All => {
-        Self::changelog(&project)?;
-        Self::completion_scripts(&project)?;
-        Self::readme(&project)?;
-        Self::book(&project)?;
-        Self::man(&project)?;
-      }
+      Self::Diff => Self::diff(&project)?,
+      Self::All => Self::all(&project)?,
     }
   }
 
+  #[throws]
+  pub(crate) fn all(project: &Project) {
+    Self::changelog(&project)?;
+    Self::completion_scripts(&project)?;
+    Self::readme(&project)?;
+    Self::book(&project)?;
+    Self::man(&project)?;
+  }
+
   #[throws]
   pub(crate) fn changelog(project: &Project) {
     info!("Generating changelog…");
@@ -109,6 +115,91 @@ impl Opt {
     .status_into_result()?
   }
 
+  #[throws]
+  pub(crate) fn diff(project: &Project) {
+    let tmp = tempfile::tempdir().context(error::Tempdir)?;
+
+    let generated = &[
+      "/CHANGELOG.md",
+      "/README.md",
+      "/book/src/SUMMARY.md",
+      "/book/src/bittorrent.md",
+      "/book/src/commands.md",
+      "/book/src/commands/*",
+      "/book/src/faq.md",
+      "/book/src/introduction.md",
+      "/book/src/references.md",
+      "/book/src/references/*",
+      "/completions/*",
+      "/man/*",
+    ];
+
+    let gen = |name: &str| -> Result<(), Error> {
+      cmd!("cargo", "run", "--package", "gen", "all").status_into_result()?;
+
+      let dir = tmp.path().join(name);
+
+      fs::create_dir(&dir).context(error::Filesystem { path: &dir })?;
+
+      let mut builder = OverrideBuilder::new(&project.root);
+
+      for pattern in generated {
+        builder.add(pattern)?;
+      }
+
+      let overrides = builder.build()?;
+
+      for result in WalkDir::new(&project.root) {
+        let entry = result?;
+
+        let src = entry.path();
+
+        if src.is_dir() {
+          continue;
+        }
+
+        if !overrides.matched(&src, false).is_whitelist() {
+          continue;
+        }
+
+        let relative = src
+          .strip_prefix(&project.root)
+          .context(error::StripPrefix)?;
+
+        let dst = dir.join(relative);
+
+        let dst_dir = dst.join("..").lexiclean();
+
+        fs::create_dir_all(&dst_dir).context(error::Filesystem { path: dst_dir })?;
+
+        fs::copy(&src, &dst).context(error::FilesystemCopy { src, dst })?;
+      }
+
+      Ok(())
+    };
+
+    const HEAD: &str = "HEAD";
+
+    gen(HEAD)?;
+
+    let head = project.repo.head()?.peel_to_commit()?;
+
+    let parent = head.parent(0)?;
+
+    let parent_hash = parent.id().to_string();
+
+    cmd!("git", "checkout", &parent_hash).status_into_result()?;
+
+    gen(&parent_hash)?;
+
+    cmd!("diff", "-r", parent_hash, HEAD)
+      .current_dir(tmp.path())
+      .status_into_result()
+      .ok();
+
+    cmd!("git", "checkout", &head.id().to_string()).status_into_result()?;
+  }
+
   #[throws]
   pub(crate) fn readme(project: &Project) {
     info!("Generating readme…");