diff --git a/src/daemon.rs b/src/daemon.rs
index 9492f04..df477e5 100644
--- a/src/daemon.rs
+++ b/src/daemon.rs
@@ -1,9 +1,9 @@
 use std::{
-    env, io,
+    env, io, mem,
     ops::Deref,
     os::fd::{AsFd, BorrowedFd, IntoRawFd, OwnedFd, RawFd},
     sync::{
-        LazyLock,
+        Arc, LazyLock,
         atomic::{AtomicUsize, Ordering},
     },
     time::Duration,
@@ -19,6 +19,7 @@ use serde::{Deserialize, Serialize};
 use serde_json::StreamDeserializer;
 
 use crate::{
+    SourceFile, SourceLine,
     daemon_tokfd::{FdInfo, FdKind},
     prelude::*,
 };
@@ -66,6 +67,17 @@ impl ConvenientAttrPath {
         let boxed = iter.map(|s| Box::from(s));
         Self::Split(Box::from_iter(boxed))
     }
+
+    pub fn to_nix_decl(&self) -> Box<str> {
+        use ConvenientAttrPath::*;
+        match self {
+            Dotted(s) => s.clone(),
+            Split(path) => {
+                // FIXME: quote if necessary
+                path.join(".").into_boxed_str()
+            },
+        }
+    }
 }
 
 impl<'i> FromIterator<&'i str> for ConvenientAttrPath {
@@ -89,6 +101,7 @@ impl FromIterator<Box<str>> for ConvenientAttrPath {
 #[derive(Debug, Clone, PartialEq)]
 #[derive(serde::Deserialize, serde::Serialize)]
 #[serde(tag = "action", content = "args", rename_all = "snake_case")]
+// FIXME: rename to not confuse with the clap argument type.
 pub enum DaemonCmd {
     Append {
         name: ConvenientAttrPath,
@@ -112,6 +125,7 @@ fn next_token() -> Token {
 
 #[derive(Debug)]
 pub struct Daemon {
+    config_path: Arc<Path>,
     fd: OwnedFdWithFlags,
     path: Option<Box<Path>>,
 
@@ -197,6 +211,8 @@ impl Daemon {
             IoError::last_os_error().raw_os_error(),
             Some(Errno::BADF.raw_os_error()),
         );
+
+        self.tokfd.remove2(&fd).unwrap_or_else(|| todo!());
     }
 
     fn fd_for_token(&self, token: Token) -> Option<RawFd> {
@@ -215,7 +231,12 @@ impl Daemon {
 }
 
 impl Daemon {
-    pub fn new(fd: OwnedFd, kind: FdKind, name: Option<Box<OsStr>>) -> Self {
+    pub fn new(
+        config_path: Arc<Path>,
+        fd: OwnedFd,
+        kind: FdKind,
+        name: Option<Box<OsStr>>,
+    ) -> Self {
         let mut fd_info: IdOrdMap<FdInfo> = Default::default();
 
         // Supposedly, the only possible ways this can fail are EMFILE, ENFILE, and ENOMEM.
@@ -240,6 +261,7 @@ impl Daemon {
         };
 
         Self {
+            config_path,
             fd,
             path,
             poller,
@@ -250,7 +272,7 @@ impl Daemon {
         }
     }
 
-    pub fn from_unix_socket_path(path: &Path) -> Result<Self, IoError> {
+    pub fn from_unix_socket_path(config_path: Arc<Path>, path: &Path) -> Result<Self, IoError> {
         // We unconditionally unlink() `path` before binding, but completely ignore the result.
         let _ = rustix::fs::unlink(path);
         let listener = UnixListener::bind(path)
@@ -260,13 +282,18 @@ impl Daemon {
         rustix::net::sockopt::set_socket_keepalive(&listener_owned_fd, true).unwrap();
         let path: Box<OsStr> = path.to_path_buf().into_boxed_path().into_boxed_os_str();
 
-        Ok(Self::new(listener_owned_fd, FdKind::Socket, Some(path)))
+        Ok(Self::new(
+            config_path,
+            listener_owned_fd,
+            FdKind::Socket,
+            Some(path),
+        ))
     }
 
-    pub fn open_default_socket() -> Result<Self, IoError> {
+    pub fn open_default_socket(config_path: Arc<Path>) -> Result<Self, IoError> {
         use IoErrorKind::*;
         let preferred = USER_SOCKET_DIR.join("dynix.sock").into_boxed_path();
-        let constructed = match Self::from_unix_socket_path(&preferred) {
+        let constructed = match Self::from_unix_socket_path(config_path.clone(), &preferred) {
             Ok(v) => v,
             Err(e) if e.kind() == Unsupported => {
                 //
@@ -279,7 +306,7 @@ impl Daemon {
                 );
 
                 let fallback = TMPDIR.join("dynix.sock").into_boxed_path();
-                Self::from_unix_socket_path(&fallback).tap_err(|e| {
+                Self::from_unix_socket_path(config_path, &fallback).tap_err(|e| {
                     error!(
                         "failed binding AF_UNIX socket at {}: {e}",
                         fallback.display(),
@@ -294,14 +321,14 @@ impl Daemon {
     /// This panics if stdin cannot be opened.
     ///
     /// If you want to handle that error, use [`Daemon::from_raw_parts()`].
-    pub fn from_stdin() -> Self {
+    pub fn from_stdin(config_path: Arc<Path>) -> Self {
         let stdin = io::stdin();
         let fd = stdin
             .as_fd()
             .try_clone_to_owned()
             .expect("dynix daemon could not open stdin; try a Unix socket?");
 
-        Self::new(fd, FdKind::File, None)
+        Self::new(config_path, fd, FdKind::File, None)
     }
 
     //pub unsafe fn from_raw_parts(fd: OwnedFd) -> Self {
@@ -325,8 +352,11 @@ impl Daemon {
         let _count = rustix::io::read(fd, spare_capacity(&mut self.cmd_buffer))
             .tap_err(|e| error!("read() on daemon fd {fd:?} failed: {e}"))?;
 
+        // So that the loop doesn't borrow from `self`.
+        let mut cmd_buffer = mem::take(&mut self.cmd_buffer);
+
         // The buffer might have existing data from the last read.
-        let deserializer = serde_json::Deserializer::from_slice(&self.cmd_buffer);
+        let deserializer = serde_json::Deserializer::from_slice(&cmd_buffer);
         let stream: StreamDeserializer<_, DaemonCmd> = deserializer.into_iter();
         for cmd in stream {
             let cmd = match cmd {
@@ -334,12 +364,14 @@ impl Daemon {
                 Err(e) if e.is_eof() => {
                     self.next_timeout = Some(Duration::from_secs(4));
                     warn!("Didn't get a valid daemon command; giving the other side 4 seconds...");
+                    let _ = mem::replace(&mut self.cmd_buffer, cmd_buffer);
                     return Ok(());
                 },
                 Err(e) => {
                     warn!("error deserializing command: {e}");
-                    debug!("command buffer was: {:?}", self.cmd_buffer.as_bstr());
-                    self.cmd_buffer.clear();
+                    debug!("command buffer was: {:?}", cmd_buffer.as_bstr());
+                    cmd_buffer.clear();
+                    let _ = mem::replace(&mut self.cmd_buffer, cmd_buffer);
                     // Don't propagate the error unless we have too many.
                     self.fd_error_push(fd.as_raw_fd(), e.into()).tap_err(|e| {
                         error!("Accumulated too many errors for daemon fd {fd:?}: {e}")
@@ -350,9 +382,51 @@ impl Daemon {
             debug!("got cmd: {cmd:?}");
             let _ = rustix::io::write(fd, b"");
             info!("dispatching command");
+            self.dispatch_cmd(cmd).unwrap_or_else(|e| todo!("{e}"));
         }
 
-        self.cmd_buffer.clear();
+        cmd_buffer.clear();
+        let _ = mem::replace(&mut self.cmd_buffer, cmd_buffer);
+
+        Ok(())
+    }
+
+    fn dispatch_cmd(&mut self, cmd: DaemonCmd) -> Result<(), IoError> {
+        let (name, value) = match cmd {
+            DaemonCmd::Append { name, value } => (name, value),
+        };
+        let mut opts = File::options();
+        opts.read(true)
+            .write(true)
+            .create(false)
+            .custom_flags(libc::O_CLOEXEC);
+        let source_file = SourceFile::open_from(self.config_path.clone(), opts)?;
+        let pri = crate::get_where(source_file.clone()).unwrap_or_else(|e| todo!("{e}"));
+        let new_pri = pri - 1;
+        //let new_pri_line =
+        //    crate::get_next_prio_line(source_file.clone(), Arc::from(name), Arc::from(value));
+        // Get next priority line.
+        let source_lines = source_file.lines()?;
+        let penultimate = source_lines.get(source_lines.len() - 2);
+        // FIXME: don't rely on whitespace lol
+        debug_assert_eq!(penultimate.map(SourceLine::text).as_deref(), Some("  ];"));
+        let penultimate = penultimate.unwrap();
+        let new_generation = 0 - new_pri;
+        let new_line = SourceLine {
+            line: penultimate.line,
+            path: source_file.path(),
+            text: Arc::from(format!(
+                "      {} = lib.mkOverride ({}) ({}); # DYNIX GENERATION {}",
+                name.to_nix_decl(),
+                new_pri,
+                value,
+                new_generation,
+            )),
+        };
+
+        drop(source_lines);
+
+        crate::write_next_prio(source_file, new_line).unwrap_or_else(|e| todo!("{e}"));
 
         Ok(())
     }
diff --git a/src/lib.rs b/src/lib.rs
index 426fc44..40f983b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -35,6 +35,8 @@ pub(crate) mod prelude {
 
     pub use bstr::ByteSlice;
 
+    pub use itertools::Itertools;
+
     pub use rustix::io::Errno;
 
     pub use tap::{Pipe, Tap, TapFallible, TapOptional};
@@ -133,17 +135,27 @@ pub fn do_append(args: Arc<Args>, append_args: AppendCmd) -> Result<(), BoxDynEr
 }
 
 //#[tracing::instrument(level = "debug")]
-pub fn do_daemon(_args: Arc<Args>, daemon_args: DaemonCmd) -> Result<(), BoxDynError> {
+pub fn do_daemon(args: Arc<Args>, daemon_args: DaemonCmd) -> Result<(), BoxDynError> {
+    let config_file = Path::new(&args.file);
+    let config_file: PathBuf = if config_file.is_relative() && !config_file.starts_with("./") {
+        iter::once(OsStr::new("./"))
+            .chain(config_file.iter())
+            .collect()
+    } else {
+        config_file.to_path_buf()
+    };
+    let config_file: Arc<Path> = Arc::from(config_file);
+
     // FIXME: make configurable?
     let _ = rustix::process::umask(Mode::from_bits_retain(0o600).complement());
 
     let mut daemon = match daemon_args {
-        DaemonCmd { stdin: true, .. } => Daemon::from_stdin(),
-        DaemonCmd { socket: None, .. } => Daemon::open_default_socket()?,
+        DaemonCmd { stdin: true, .. } => Daemon::from_stdin(config_file),
+        DaemonCmd { socket: None, .. } => Daemon::open_default_socket(config_file)?,
         DaemonCmd {
             socket: Some(socket),
             ..
-        } => Daemon::from_unix_socket_path(&socket)?,
+        } => Daemon::from_unix_socket_path(config_file, &socket)?,
     };
 
     daemon.enter_loop().unwrap();