diff --git a/README.md b/README.md
index 3d7910a..0f4b63b 100644
--- a/README.md
+++ b/README.md
@@ -95,6 +95,15 @@ is the right default.
 make
 ```
 
+Build just the internal static library artifact:
+
+```sh
+make libatch.a
+```
+
+Note: `libatch.a` is currently an internal build artifact used to modularize
+`atch` internals. It is not yet a documented/stable public API.
+
 ## Usage
 
 ```
diff --git a/atch.c b/atch.c
index 9a516de..3e45f09 100644
--- a/atch.c
+++ b/atch.c
@@ -1,4 +1,9 @@
 #include "atch.h"
+#include "atch_cmd.h"
+#include "atch_cli_opts.h"
+#include "atch_cli_runtime.h"
+#include "atch_paths.h"
+#include "atch_session.h"
 
 /* Env-var name string, computed from progname at startup. */
 const char *session_envvar;
@@ -26,64 +31,6 @@ static void init_envvar_names(void)
 	session_envvar = envname;
 }
 
-/* Returns the basename of the current session socket path. */
-const char *session_shortname(void)
-{
-	const char *p = strrchr(sockname, '/');
-	return p ? p + 1 : sockname;
-}
-
-/* Returns the directory where session sockets are stored. */
-void get_session_dir(char *buf, size_t size)
-{
-	const char *home = getenv("HOME");
-	const char *base = strrchr(progname, '/');
-	struct passwd *pw;
-
-	base = base ? base + 1 : progname;
-
-	/* If $HOME is unset or empty, try the passwd database. */
-	if (!home || !*home) {
-		pw = getpwuid(getuid());
-		if (pw && pw->pw_dir && *pw->pw_dir)
-			home = pw->pw_dir;
-	}
-
-	/* Use $HOME only if it is set and not the root directory. */
-	if (home && *home && strcmp(home, "/") != 0)
-		snprintf(buf, size, "%s/.cache/%s", home, base);
-	else
-		snprintf(buf, size, "/tmp/.%s-%d", base, (int)getuid());
-}
-
-/*
-** Long-path helper for Unix socket operations: saves cwd, chdirs into the
-** directory part of path, calls fn(basename), then restores cwd.
-** Used by connect_socket and create_socket to handle paths > sun_path limit.
-*/
-int socket_with_chdir(char *path, int (*fn)(char *))
-{
-	char *slash = strrchr(path, '/');
-	int dirfd, s;
-
-	if (!slash) {
-		errno = ENAMETOOLONG;
-		return -1;
-	}
-	dirfd = open(".", O_RDONLY);
-	if (dirfd < 0)
-		return -1;
-	*slash = '\0';
-	s = chdir(path) >= 0 ? fn(slash + 1) : -1;
-	*slash = '/';
-	if (s >= 0 && fchdir(dirfd) < 0) {
-		close(s);
-		s = -1;
-	}
-	close(dirfd);
-	return s;
-}
-
 /* argv[0] from the program */
 char *progname;
 /* The name of the passed in socket. */
@@ -108,665 +55,7 @@ int no_ansiterm = 0;
 struct termios orig_term;
 int dont_have_tty;
 
-/* Parse a size string: bare number, or number with k/K (×1024) or m/M (×1048576).
-** Writes result to *out. Returns 0 on success, 1 on error. */
-static int parse_size(const char *s, size_t *out)
-{
-	char *end;
-	unsigned long v;
-
-	if (!s || !*s)
-		return 1;
-	v = strtoul(s, &end, 10);
-	if (end == s)
-		return 1;
-	if (*end == 'k' || *end == 'K') {
-		v *= 1024;
-		end++;
-	} else if (*end == 'm' || *end == 'M') {
-		v *= 1024 * 1024;
-		end++;
-	}
-	if (*end != '\0')
-		return 1;
-	*out = (size_t)v;
-	return 0;
-}
-
-/*
-** Parse option flags from argv/argc. Stops at '--' or a non-option argument.
-** Returns 0 on success, 1 on error (message already printed).
-*/
-static int parse_options(int *argc, char ***argv)
-{
-	while (*argc >= 1 && ***argv == '-') {
-		char *p;
-
-		if (strcmp((*argv)[0], "--") == 0) {
-			++(*argv);
-			--(*argc);
-			break;
-		}
-
-		for (p = (*argv)[0] + 1; *p; ++p) {
-			if (*p == 'E')
-				detach_char = -1;
-			else if (*p == 'z')
-				no_suspend = 1;
-			else if (*p == 'q')
-				quiet = 1;
-			else if (*p == 't')
-				no_ansiterm = 1;
-			else if (*p == 'e') {
-				++(*argv);
-				--(*argc);
-				if (*argc < 1) {
-					printf("%s: No escape character "
-					       "specified.\n", progname);
-					printf("Try '%s --help' for more "
-					       "information.\n", progname);
-					return 1;
-				}
-				if ((*argv)[0][0] == '^' && (*argv)[0][1]) {
-					if ((*argv)[0][1] == '?')
-						detach_char = '\177';
-					else
-						detach_char =
-						    (*argv)[0][1] & 037;
-				} else
-					detach_char = (*argv)[0][0];
-				break;
-			} else if (*p == 'r') {
-				++(*argv);
-				--(*argc);
-				if (*argc < 1) {
-					printf("%s: No redraw method "
-					       "specified.\n", progname);
-					printf("Try '%s --help' for more "
-					       "information.\n", progname);
-					return 1;
-				}
-				if (strcmp((*argv)[0], "none") == 0)
-					redraw_method = REDRAW_NONE;
-				else if (strcmp((*argv)[0], "ctrl_l") == 0)
-					redraw_method = REDRAW_CTRL_L;
-				else if (strcmp((*argv)[0], "winch") == 0)
-					redraw_method = REDRAW_WINCH;
-				else {
-					printf("%s: Invalid redraw method "
-					       "specified.\n", progname);
-					printf("Try '%s --help' for more "
-					       "information.\n", progname);
-					return 1;
-				}
-				break;
-			} else if (*p == 'R') {
-				++(*argv);
-				--(*argc);
-				if (*argc < 1) {
-					printf("%s: No clear method "
-					       "specified.\n", progname);
-					printf("Try '%s --help' for more "
-					       "information.\n", progname);
-					return 1;
-				}
-				if (strcmp((*argv)[0], "none") == 0)
-					clear_method = CLEAR_NONE;
-				else if (strcmp((*argv)[0], "move") == 0)
-					clear_method = CLEAR_MOVE;
-				else {
-					printf("%s: Invalid clear method "
-					       "specified.\n", progname);
-					printf("Try '%s --help' for more "
-					       "information.\n", progname);
-					return 1;
-				}
-				break;
-			} else if (*p == 'C') {
-				++(*argv);
-				--(*argc);
-				if (*argc < 1) {
-					printf("%s: No log size "
-					       "specified.\n", progname);
-					printf("Try '%s --help' for more "
-					       "information.\n", progname);
-					return 1;
-				}
-				if (parse_size((*argv)[0], &log_max_size)) {
-					printf("%s: Invalid log size "
-					       "'%s'.\n", progname, (*argv)[0]);
-					printf("Try '%s --help' for more "
-					       "information.\n", progname);
-					return 1;
-				}
-				break;
-			} else {
-				printf("%s: Invalid option '-%c'\n",
-				       progname, *p);
-				printf("Try '%s --help' for more "
-				       "information.\n", progname);
-				return 1;
-			}
-		}
-		++(*argv);
-		--(*argc);
-	}
-	return 0;
-}
-
-/* Expand a bare session name to its full socket path in-place. */
-static void expand_sockname(void)
-{
-	char dir[512];
-	size_t fulllen;
-	char *full;
-	char *slash;
-
-	if (strchr(sockname, '/') != NULL)
-		return;
-
-	get_session_dir(dir, sizeof(dir));
-	slash = strrchr(dir, '/');
-	if (slash) {
-		*slash = '\0';
-		mkdir(dir, 0700);
-		*slash = '/';
-	}
-	mkdir(dir, 0700);
-	fulllen = strlen(dir) + 1 + strlen(sockname);
-	full = malloc(fulllen + 1);
-	snprintf(full, fulllen + 1, "%s/%s", dir, sockname);
-	sockname = full;
-}
-
-/* Return argv unchanged if argc > 0; otherwise return a {shell, NULL} argv. */
-static char **use_shell_if_no_cmd(int argc, char **argv)
-{
-	static char *shell_argv[2];
-	const char *shell;
-	struct passwd *pw;
-
-	if (argc > 0)
-		return argv;
-	shell = getenv("SHELL");
-	if (!shell || !*shell) {
-		pw = getpwuid(getuid());
-		if (pw && pw->pw_shell && *pw->pw_shell)
-			shell = pw->pw_shell;
-	}
-	if (!shell || !*shell)
-		shell = "/bin/sh";
-	shell_argv[0] = (char *)shell;
-	shell_argv[1] = NULL;
-	return shell_argv;
-}
-
-/* Snapshot terminal settings; sets dont_have_tty if not a tty. */
-static void save_term(void)
-{
-	if (tcgetattr(0, &orig_term) < 0) {
-		memset(&orig_term, 0, sizeof(struct termios));
-		dont_have_tty = 1;
-	}
-}
-
-/* Print error and return 1 if no tty is available. */
-static int require_tty(void)
-{
-	if (dont_have_tty) {
-		printf("%s: attaching to a session requires a terminal.\n",
-		       progname);
-		return 1;
-	}
-	return 0;
-}
-
-/* Consume first arg as session name, expand it, advance argc/argv. */
-static int consume_session(int *argc, char ***argv)
-{
-	if (*argc < 1) {
-		printf("%s: No session was specified.\n", progname);
-		printf("Try '%s --help' for more information.\n", progname);
-		return 1;
-	}
-	sockname = **argv;
-	++(*argv);
-	--(*argc);
-	expand_sockname();
-	return 0;
-}
-
-/* True if arg matches any of the given names (NULL slots are ignored). */
-static int is_cmd(const char *arg, const char *a, const char *b, const char *c)
-{
-	return strcmp(arg, a) == 0 ||
-	    (b && strcmp(arg, b) == 0) || (c && strcmp(arg, c) == 0);
-}
-
-/* atch list [-a] */
-static int cmd_list(int argc, char **argv)
-{
-	int show_all = 0;
-
-	while (argc >= 1 && strcmp(argv[0], "-a") == 0) {
-		show_all = 1;
-		argc--;
-		argv++;
-	}
-	return list_main(show_all);
-}
-
-/* atch current
-** SESSION_ENVVAR holds the colon-separated ancestry chain, outermost first.
-** A single (non-nested) session has no colon. */
-static int cmd_current(void)
-{
-	const char *chain = getenv(SESSION_ENVVAR);
-	char *copy, *seg, *colon;
-	const char *name;
-	int first;
-
-	if (!chain || !*chain)
-		return 1;
-
-	copy = strdup(chain);
-	if (!copy)
-		return 1;
-
-	first = 1;
-	seg = copy;
-	for (;;) {
-		colon = strchr(seg, ':');
-		if (colon)
-			*colon = '\0';
-		name = strrchr(seg, '/');
-		if (!first)
-			printf(" > ");
-		printf("%s", name ? name + 1 : seg);
-		first = 0;
-		if (!colon)
-			break;
-		seg = colon + 1;
-	}
-	printf("\n");
-	free(copy);
-	return 0;
-}
-
-/* atch attach <session> — strict attach, fail if missing */
-static int cmd_attach(int argc, char **argv)
-{
-	if (parse_options(&argc, &argv))
-		return 1;
-	if (consume_session(&argc, &argv))
-		return 1;
-	if (parse_options(&argc, &argv))
-		return 1;
-	if (argc > 0) {
-		printf("%s: Invalid number of arguments.\n", progname);
-		printf("Try '%s --help' for more information.\n", progname);
-		return 1;
-	}
-	save_term();
-	if (require_tty())
-		return 1;
-	return attach_main(0);
-}
-
-/* atch new <session> [cmd...] — create session and attach */
-static int cmd_new(int argc, char **argv)
-{
-	if (parse_options(&argc, &argv))
-		return 1;
-	if (consume_session(&argc, &argv))
-		return 1;
-	if (parse_options(&argc, &argv))
-		return 1;
-	argv = use_shell_if_no_cmd(argc, argv);
-	save_term();
-	if (require_tty())
-		return 1;
-	if (master_main(argv, 1, 0) != 0)
-		return 1;
-	if (!quiet)
-		printf("%s: session '%s' created\n", progname,
-		       session_shortname());
-	return attach_main(0);
-}
-
-/* atch start <session> [cmd...] — create detached */
-static int cmd_start(int argc, char **argv)
-{
-	if (parse_options(&argc, &argv))
-		return 1;
-	if (consume_session(&argc, &argv))
-		return 1;
-	if (parse_options(&argc, &argv))
-		return 1;
-	argv = use_shell_if_no_cmd(argc, argv);
-	save_term();
-	if (master_main(argv, 0, 0) != 0)
-		return 1;
-	if (!quiet)
-		printf("%s: session '%s' started\n", progname,
-		       session_shortname());
-	return 0;
-}
-
-/* atch run <session> [cmd...] — create, master stays in foreground */
-static int cmd_run(int argc, char **argv)
-{
-	if (parse_options(&argc, &argv))
-		return 1;
-	if (consume_session(&argc, &argv))
-		return 1;
-	if (parse_options(&argc, &argv))
-		return 1;
-	argv = use_shell_if_no_cmd(argc, argv);
-	save_term();
-	return master_main(argv, 0, 1);
-}
-
-/* atch push <session> — pipe stdin into session */
-static int cmd_push(int argc, char **argv)
-{
-	if (consume_session(&argc, &argv))
-		return 1;
-	if (argc > 0) {
-		printf("%s: Invalid number of arguments.\n", progname);
-		printf("Try '%s --help' for more information.\n", progname);
-		return 1;
-	}
-	return push_main();
-}
-
-/* atch kill <session> — stop session */
-static int cmd_kill(int argc, char **argv)
-{
-	int force = 0;
-
-	/* Accept -f / --force before or after the session name. */
-	while (argc >= 1 && (strcmp(argv[0], "-f") == 0 ||
-			     strcmp(argv[0], "--force") == 0)) {
-		force = 1;
-		argc--;
-		argv++;
-	}
-	if (consume_session(&argc, &argv))
-		return 1;
-	while (argc >= 1 && (strcmp(argv[0], "-f") == 0 ||
-			     strcmp(argv[0], "--force") == 0)) {
-		force = 1;
-		argc--;
-		argv++;
-	}
-	if (argc > 0) {
-		printf("%s: Invalid number of arguments.\n", progname);
-		printf("Try '%s --help' for more information.\n", progname);
-		return 1;
-	}
-	return kill_main(force);
-}
-
-/* atch clear <session> — truncate the on-disk session log */
-static int cmd_clear(int argc, char **argv)
-{
-	char log_path[600];
-	int fd;
-
-	if (argc > 0) {
-		if (consume_session(&argc, &argv))
-			return 1;
-	} else {
-		const char *chain = getenv(SESSION_ENVVAR);
-		const char *last;
-
-		if (!chain || !*chain) {
-			printf("%s: No session was specified.\n", progname);
-			printf("Try '%s --help' for more information.\n",
-			       progname);
-			return 1;
-		}
-		last = strrchr(chain, ':');
-		sockname = (char *)(last ? last + 1 : chain);
-	}
-	if (argc > 0) {
-		printf("%s: Invalid number of arguments.\n", progname);
-		printf("Try '%s --help' for more information.\n", progname);
-		return 1;
-	}
-	snprintf(log_path, sizeof(log_path), "%s.log", sockname);
-	fd = open(log_path, O_WRONLY | O_TRUNC);
-	if (fd >= 0) {
-		close(fd);
-		if (!quiet)
-			printf("%s: session '%s' log cleared\n",
-			       progname, session_shortname());
-	} else if (errno != ENOENT) {
-		printf("%s: %s: %s\n", progname, log_path, strerror(errno));
-		return 1;
-	}
-	return 0;
-}
-
-/* Scan log fd backward and return the byte offset to seek to before
-** printing the last nlines lines of output. */
-static off_t find_tail_start(int fd, off_t size, int nlines)
-{
-	char buf[BUFSIZE];
-	int count = 0;
-	off_t pos = size;
-
-	while (pos > 0 && count <= nlines) {
-		off_t chunk = pos > (off_t)sizeof(buf) ? (off_t)sizeof(buf) : pos;
-		ssize_t n;
-		ssize_t i;
-
-		pos -= chunk;
-		lseek(fd, pos, SEEK_SET);
-		n = read(fd, buf, (size_t)chunk);
-		if (n <= 0)
-			break;
-		for (i = n - 1; i >= 0; i--) {
-			if (buf[i] == '\n') {
-				if (++count > nlines)
-					return pos + i + 1;
-			}
-		}
-	}
-	return 0;
-}
-
-/* atch tail [-f] [-n N] <session> — print last N lines of session log */
-static int cmd_tail(int argc, char **argv)
-{
-	int follow = 0, nlines = 10;
-	char log_path[600];
-	unsigned char rbuf[BUFSIZE];
-	off_t size, start;
-	ssize_t n;
-	int fd;
-
-	/* Parse -f and -n N (also -nN) before the session name */
-	while (argc >= 1 && argv[0][0] == '-' && argv[0][1] != '\0') {
-		if (strcmp(argv[0], "-f") == 0) {
-			follow = 1;
-			argc--;
-			argv++;
-		} else if (strcmp(argv[0], "-n") == 0) {
-			if (argc < 2) {
-				printf("%s: -n requires an argument\n", progname);
-				printf("Try '%s --help' for more information.\n",
-				       progname);
-				return 1;
-			}
-			nlines = atoi(argv[1]);
-			argc -= 2;
-			argv += 2;
-		} else if (strncmp(argv[0], "-n", 2) == 0 &&
-			   argv[0][2] != '\0') {
-			nlines = atoi(argv[0] + 2);
-			argc--;
-			argv++;
-		} else {
-			printf("%s: Invalid option '%s'\n", progname, argv[0]);
-			printf("Try '%s --help' for more information.\n",
-			       progname);
-			return 1;
-		}
-	}
-	if (nlines < 1)
-		nlines = 1;
-
-	if (consume_session(&argc, &argv))
-		return 1;
-	if (argc > 0) {
-		printf("%s: Invalid number of arguments.\n", progname);
-		printf("Try '%s --help' for more information.\n", progname);
-		return 1;
-	}
-
-	snprintf(log_path, sizeof(log_path), "%s.log", sockname);
-	fd = open(log_path, O_RDONLY);
-	if (fd < 0) {
-		if (errno == ENOENT)
-			printf("%s: no log for session '%s'\n", progname,
-			       session_shortname());
-		else
-			printf("%s: %s: %s\n", progname, log_path,
-			       strerror(errno));
-		return 1;
-	}
-
-	size = lseek(fd, 0, SEEK_END);
-	if (size > 0) {
-		start = find_tail_start(fd, size, nlines);
-		lseek(fd, start, SEEK_SET);
-		while ((n = read(fd, rbuf, sizeof(rbuf))) > 0)
-			write(1, rbuf, (size_t)n);
-	}
-
-	if (follow) {
-		signal(SIGPIPE, SIG_IGN);
-		for (;;) {
-			usleep(250000);
-			while ((n = read(fd, rbuf, sizeof(rbuf))) > 0)
-				write(1, rbuf, (size_t)n);
-		}
-	}
-
-	close(fd);
-	return 0;
-}
-
-static int cmd_rm(int argc, char **argv)
-{
-	int all = 0;
-
-	if (argc >= 1 && strcmp(argv[0], "-a") == 0) {
-		all = 1;
-		argc--;
-		argv++;
-	}
-
-	if (all) {
-		if (argc > 0) {
-			printf("%s: Invalid number of arguments.\n", progname);
-			printf("Try '%s --help' for more information.\n",
-			       progname);
-			return 1;
-		}
-		return rm_main(1);
-	}
-
-	if (consume_session(&argc, &argv))
-		return 1;
-	if (argc > 0) {
-		printf("%s: Invalid number of arguments.\n", progname);
-		printf("Try '%s --help' for more information.\n", progname);
-		return 1;
-	}
-	return rm_main(0);
-}
-
-/* Default: atch <session> [cmd...] — attach-or-create */
-static int cmd_open(char *session, int argc, char **argv)
-{
-	sockname = session;
-	expand_sockname();
-	if (parse_options(&argc, &argv))
-		return 1;
-	argv = use_shell_if_no_cmd(argc, argv);
-	save_term();
-	if (require_tty())
-		return 1;
-	if (attach_main(1) != 0) {
-		if (errno == ECONNREFUSED || errno == ENOENT) {
-			int saved_errno = errno;
-
-			replay_session_log(saved_errno);
-			if (saved_errno == ECONNREFUSED)
-				unlink(sockname);
-			if (master_main(argv, 1, 0) != 0)
-				return 1;
-			if (!quiet)
-				printf("%s: session '%s' created\n", progname,
-				       session_shortname());
-		}
-		return attach_main(0);
-	}
-	return 0;
-}
-
-static void usage(void)
-{
-	printf(PACKAGE_NAME " - version %s, compiled on %s at %s.\n"
-	       "Usage:\n"
-	       "  " PACKAGE_NAME " [<session> [command...]]"
-	       "\t\tAttach to session or create it\n"
-	       "  " PACKAGE_NAME " <command> [options] ...\n"
-	       "\n"
-	       "Commands:\n"
-	       "  attach  <session>"
-	       "\t\t\tStrict attach (fail if session missing)\n"
-	       "  new     <session> [command...]"
-	       "\tCreate session and attach\n"
-	       "  start   <session> [command...]"
-	       "\tCreate session, detached\n"
-	       "  run     <session> [command...]"
-	       "\tCreate session, master in foreground\n"
-	       "  push    <session>"
-	       "\t\t\tPipe stdin into session\n"
-	       "  kill    [-f] <session>"
-	       "\t\tStop session (SIGTERM then SIGKILL)\n"
-	       "    -f, --force\t\t\tSkip grace period, send SIGKILL immediately\n"
-	       "  clear   [<session>]"
-	       "\t\t\tTruncate the session log\n"
-	       "  tail    [-f] [-n N] <session>"
-	       "\tPrint last N lines of session log\n"
-	       "    -f\t\t\t\tFollow log output\n"
-	       "    -n <lines>\t\t\tNumber of lines (default 10)\n"
-	       "  list    [-a]\t\t\t\tList sessions (-a includes exited)\n"
-	       "  rm      [-a] [<session>]"
-	       "\t\tRemove stale/exited session(s)\n"
-	       "    -a\t\t\t\tRemove all stale and exited sessions\n"
-	       "  current\t\t\t\tPrint current session name\n"
-	       "\n"
-	       "Options:\n"
-	       "  -e <char>\tSet detach character (default: ^\\)\n"
-	       "  -E\t\tDisable detach character\n"
-	       "  -r <method>\tRedraw method: none | ctrl_l | winch\n"
-	       "  -R <method>\tClear method:  none | move\n"
-	       "  -z\t\tDisable suspend key\n"
-	       "  -q\t\tSuppress messages\n"
-	       "  -t\t\tDisable VT100 assumptions\n"
-	       "  -C <size>\tLog cap: 0=disable, e.g. 128k, 4m (default 1m)\n"
-	       "\nURL: " PACKAGE_URL "\n\n",
-	       PACKAGE_VERSION, __DATE__, __TIME__);
-	exit(0);
-}
-
-int main(int argc, char **argv)
+int atch_cli_main(int argc, char **argv)
 {
 	const char *cmd;
 
@@ -802,7 +91,7 @@ int main(int argc, char **argv)
 		if (c != 'e' && c != 'E' && c != 'r' && c != 'R' &&
 		    c != 'z' && c != 'q' && c != 't' && c != 'C')
 			break;
-		if (parse_options(&argc, &argv))
+		if (atch_parse_options(&argc, &argv))
 			return 1;
 	}
 	if (argc < 1)
@@ -841,7 +130,8 @@ int main(int argc, char **argv)
 		sockname = *argv;
 		++argv;
 		--argc;
-		expand_sockname();
+		if (expand_sockname())
+		return 1;
 
 		if (mode == 'p') {
 			if (argc > 0) {
@@ -864,7 +154,7 @@ int main(int argc, char **argv)
 			return kill_main(0);
 		}
 
-		if (parse_options(&argc, &argv))
+		if (atch_parse_options(&argc, &argv))
 			return 1;
 		if (mode != 'a')
 			argv = use_shell_if_no_cmd(argc, argv);
@@ -915,29 +205,32 @@ int main(int argc, char **argv)
 	++argv;
 	--argc;
 
-	if (is_cmd(cmd, "list", "l", "ls"))
+	switch (atch_resolve_command(cmd)) {
+	case ATCH_CMD_LIST:
 		return cmd_list(argc, argv);
-	if (is_cmd(cmd, "current", NULL, NULL))
+	case ATCH_CMD_CURRENT:
 		return cmd_current();
-	if (is_cmd(cmd, "attach", "a", NULL))
+	case ATCH_CMD_ATTACH:
 		return cmd_attach(argc, argv);
-	if (is_cmd(cmd, "new", "n", NULL))
+	case ATCH_CMD_NEW:
 		return cmd_new(argc, argv);
-	if (is_cmd(cmd, "start", "s", NULL))
+	case ATCH_CMD_START:
 		return cmd_start(argc, argv);
-	if (is_cmd(cmd, "run", NULL, NULL))
+	case ATCH_CMD_RUN:
 		return cmd_run(argc, argv);
-	if (is_cmd(cmd, "push", "p", NULL))
+	case ATCH_CMD_PUSH:
 		return cmd_push(argc, argv);
-	if (is_cmd(cmd, "kill", "k", NULL))
+	case ATCH_CMD_KILL:
 		return cmd_kill(argc, argv);
-	if (is_cmd(cmd, "clear", NULL, NULL))
+	case ATCH_CMD_CLEAR:
 		return cmd_clear(argc, argv);
-	if (is_cmd(cmd, "tail", NULL, NULL))
+	case ATCH_CMD_TAIL:
 		return cmd_tail(argc, argv);
-	if (is_cmd(cmd, "rm", NULL, NULL))
+	case ATCH_CMD_RM:
 		return cmd_rm(argc, argv);
-
-	/* Smart default: treat first arg as session name → attach-or-create */
-	return cmd_open((char *)cmd, argc, argv);
+	case ATCH_CMD_OPEN:
+	default:
+		/* Smart default: treat first arg as session name → attach-or-create */
+		return cmd_open((char *)cmd, argc, argv);
+	}
 }
diff --git a/atch_cli.h b/atch_cli.h
new file mode 100644
index 0000000..e7ac532
--- /dev/null
+++ b/atch_cli.h
@@ -0,0 +1,6 @@
+#ifndef ATCH_CLI_H
+#define ATCH_CLI_H
+
+int atch_cli_main(int argc, char **argv);
+
+#endif
diff --git a/atch_cli_opts.c b/atch_cli_opts.c
new file mode 100644
index 0000000..d553113
--- /dev/null
+++ b/atch_cli_opts.c
@@ -0,0 +1,140 @@
+#include "atch.h"
+#include "atch_cli_opts.h"
+
+/* Parse a size string: bare number, or number with k/K (×1024) or m/M (×1048576).
+** Writes result to *out. Returns 0 on success, 1 on error. */
+static int parse_size(const char *s, size_t *out)
+{
+	char *end;
+	unsigned long v;
+
+	if (!s || !*s)
+		return 1;
+	v = strtoul(s, &end, 10);
+	if (end == s)
+		return 1;
+	if (*end == 'k' || *end == 'K') {
+		v *= 1024;
+		end++;
+	} else if (*end == 'm' || *end == 'M') {
+		v *= 1024 * 1024;
+		end++;
+	}
+	if (*end != '\0')
+		return 1;
+	*out = (size_t)v;
+	return 0;
+}
+
+/* Parse option flags from argv/argc. Stops at '--' or a non-option argument.
+** Returns 0 on success, 1 on error (message already printed). */
+int atch_parse_options(int *argc, char ***argv)
+{
+	while (*argc >= 1 && ***argv == '-') {
+		char *p;
+
+		if (strcmp((*argv)[0], "--") == 0) {
+			++(*argv);
+			--(*argc);
+			break;
+		}
+
+		for (p = (*argv)[0] + 1; *p; ++p) {
+			if (*p == 'E')
+				detach_char = -1;
+			else if (*p == 'z')
+				no_suspend = 1;
+			else if (*p == 'q')
+				quiet = 1;
+			else if (*p == 't')
+				no_ansiterm = 1;
+			else if (*p == 'e') {
+				++(*argv);
+				--(*argc);
+				if (*argc < 1) {
+					printf("%s: No escape character specified.\n", progname);
+					printf("Try '%s --help' for more information.\n",
+					       progname);
+					return 1;
+				}
+				if ((*argv)[0][0] == '^' && (*argv)[0][1]) {
+					if ((*argv)[0][1] == '?')
+						detach_char = '\177';
+					else
+						detach_char = (*argv)[0][1] & 037;
+				} else
+					detach_char = (*argv)[0][0];
+				break;
+			} else if (*p == 'r') {
+				++(*argv);
+				--(*argc);
+				if (*argc < 1) {
+					printf("%s: No redraw method specified.\n", progname);
+					printf("Try '%s --help' for more information.\n",
+					       progname);
+					return 1;
+				}
+				if (strcmp((*argv)[0], "none") == 0)
+					redraw_method = REDRAW_NONE;
+				else if (strcmp((*argv)[0], "ctrl_l") == 0)
+					redraw_method = REDRAW_CTRL_L;
+				else if (strcmp((*argv)[0], "winch") == 0)
+					redraw_method = REDRAW_WINCH;
+				else {
+					printf("%s: Invalid redraw method specified.\n", progname);
+					printf("Try '%s --help' for more information.\n",
+					       progname);
+					return 1;
+				}
+				break;
+			} else if (*p == 'R') {
+				++(*argv);
+				--(*argc);
+				if (*argc < 1) {
+					printf("%s: No clear method specified.\n", progname);
+					printf("Try '%s --help' for more information.\n",
+					       progname);
+					return 1;
+				}
+				if (strcmp((*argv)[0], "none") == 0)
+					clear_method = CLEAR_NONE;
+				else if (strcmp((*argv)[0], "move") == 0)
+					clear_method = CLEAR_MOVE;
+				else {
+					printf("%s: Invalid clear method specified.\n", progname);
+					printf("Try '%s --help' for more information.\n",
+					       progname);
+					return 1;
+				}
+				break;
+			} else if (*p == 'C') {
+				++(*argv);
+				--(*argc);
+				if (*argc < 1) {
+					printf("%s: No log size specified.\n", progname);
+					printf("Try '%s --help' for more information.\n",
+					       progname);
+					return 1;
+				}
+				if (parse_size((*argv)[0], &log_max_size)) {
+					printf("%s: Invalid log size '%s'.\n", progname,
+					       (*argv)[0]);
+					printf("Try '%s --help' for more information.\n",
+					       progname);
+					return 1;
+				}
+				break;
+			} else {
+				printf("%s: Invalid option '-%c'\n", progname, *p);
+				printf("Try '%s --help' for more information.\n", progname);
+				return 1;
+			}
+
+			if (*argc < 2)
+				break;
+		}
+		++(*argv);
+		--(*argc);
+	}
+	return 0;
+}
diff --git a/atch_cli_opts.h b/atch_cli_opts.h
new file mode 100644
index 0000000..45b481a
--- /dev/null
+++ b/atch_cli_opts.h
@@ -0,0 +1,6 @@
+#ifndef ATCH_CLI_OPTS_H
+#define ATCH_CLI_OPTS_H
+
+int atch_parse_options(int *argc, char ***argv);
+
+#endif
diff --git a/atch_cli_runtime.c b/atch_cli_runtime.c
new file mode 100644
index 0000000..fa2b5f2
--- /dev/null
+++ b/atch_cli_runtime.c
@@ -0,0 +1,498 @@
+#include "atch.h"
+#include "atch_cli_opts.h"
+#include "atch_session.h"
+
+/* Expand session name to full socket path in-place. */
+int expand_sockname(void)
+{
+	char *full = atch_expand_session_name_dup(sockname);
+	if (!full) {
+		printf("%s: out of memory\n", progname);
+		return 1;
+	}
+	sockname = full;
+	return 0;
+}
+
+/* Return argv unchanged if argc > 0; otherwise return a {shell, NULL} argv. */
+char **use_shell_if_no_cmd(int argc, char **argv)
+{
+	static char *shell_argv[2];
+	const char *shell;
+	struct passwd *pw;
+
+	if (argc > 0)
+		return argv;
+	shell = getenv("SHELL");
+	if (!shell || !*shell) {
+		pw = getpwuid(getuid());
+		if (pw && pw->pw_shell && *pw->pw_shell)
+			shell = pw->pw_shell;
+	}
+	if (!shell || !*shell)
+		shell = "/bin/sh";
+	shell_argv[0] = (char *)shell;
+	shell_argv[1] = NULL;
+	return shell_argv;
+}
+
+/* Snapshot terminal settings; sets dont_have_tty if not a tty. */
+void save_term(void)
+{
+	if (tcgetattr(0, &orig_term) < 0) {
+		memset(&orig_term, 0, sizeof(struct termios));
+		dont_have_tty = 1;
+	}
+}
+
+/* Print error and return 1 if no tty is available. */
+int require_tty(void)
+{
+	if (dont_have_tty) {
+		printf("%s: attaching to a session requires a terminal.\n",
+		       progname);
+		return 1;
+	}
+	return 0;
+}
+
+/* Consume first arg as session name, expand it, advance argc/argv. */
+int consume_session(int *argc, char ***argv)
+{
+	if (*argc < 1) {
+		printf("%s: No session was specified.\n", progname);
+		printf("Try '%s --help' for more information.\n", progname);
+		return 1;
+	}
+	sockname = **argv;
+	++(*argv);
+	--(*argc);
+	if (expand_sockname())
+		return 1;
+	return 0;
+}
+
+/* atch list [-a] */
+int cmd_list(int argc, char **argv)
+{
+	int show_all = 0;
+
+	while (argc >= 1 && strcmp(argv[0], "-a") == 0) {
+		show_all = 1;
+		argc--;
+		argv++;
+	}
+	return list_main(show_all);
+}
+
+/* atch current
+** SESSION_ENVVAR holds the colon-separated ancestry chain, outermost first.
+** A single (non-nested) session has no colon. */
+int cmd_current(void)
+{
+	const char *chain = getenv(SESSION_ENVVAR);
+	char *copy, *seg, *colon;
+	const char *name;
+	int first;
+
+	if (!chain || !*chain)
+		return 1;
+
+	copy = strdup(chain);
+	if (!copy)
+		return 1;
+
+	first = 1;
+	seg = copy;
+	for (;;) {
+		colon = strchr(seg, ':');
+		if (colon)
+			*colon = '\0';
+		name = strrchr(seg, '/');
+		if (!first)
+			printf(" > ");
+		printf("%s", name ? name + 1 : seg);
+		first = 0;
+		if (!colon)
+			break;
+		seg = colon + 1;
+	}
+	printf("\n");
+	free(copy);
+	return 0;
+}
+
+/* atch attach <session> — strict attach, fail if missing */
+int cmd_attach(int argc, char **argv)
+{
+	if (atch_parse_options(&argc, &argv))
+		return 1;
+	if (consume_session(&argc, &argv))
+		return 1;
+	if (atch_parse_options(&argc, &argv))
+		return 1;
+	if (argc > 0) {
+		printf("%s: Invalid number of arguments.\n", progname);
+		printf("Try '%s --help' for more information.\n", progname);
+		return 1;
+	}
+	save_term();
+	if (require_tty())
+		return 1;
+	return attach_main(0);
+}
+
+/* atch new <session> [cmd...] — create session and attach */
+int cmd_new(int argc, char **argv)
+{
+	if (atch_parse_options(&argc, &argv))
+		return 1;
+	if (consume_session(&argc, &argv))
+		return 1;
+	if (atch_parse_options(&argc, &argv))
+		return 1;
+	argv = use_shell_if_no_cmd(argc, argv);
+	save_term();
+	if (require_tty())
+		return 1;
+	if (master_main(argv, 1, 0) != 0)
+		return 1;
+	if (!quiet)
+		printf("%s: session '%s' created\n", progname,
+		       session_shortname());
+	return attach_main(0);
+}
+
+/* atch start <session> [cmd...] — create detached */
+int cmd_start(int argc, char **argv)
+{
+	if (atch_parse_options(&argc, &argv))
+		return 1;
+	if (consume_session(&argc, &argv))
+		return 1;
+	if (atch_parse_options(&argc, &argv))
+		return 1;
+	argv = use_shell_if_no_cmd(argc, argv);
+	save_term();
+	if (master_main(argv, 0, 0) != 0)
+		return 1;
+	if (!quiet)
+		printf("%s: session '%s' started\n", progname,
+		       session_shortname());
+	return 0;
+}
+
+/* atch run <session> [cmd...] — create, master stays in foreground */
+int cmd_run(int argc, char **argv)
+{
+	if (atch_parse_options(&argc, &argv))
+		return 1;
+	if (consume_session(&argc, &argv))
+		return 1;
+	if (atch_parse_options(&argc, &argv))
+		return 1;
+	argv = use_shell_if_no_cmd(argc, argv);
+	save_term();
+	return master_main(argv, 0, 1);
+}
+
+/* atch push <session> — pipe stdin into session */
+int cmd_push(int argc, char **argv)
+{
+	if (consume_session(&argc, &argv))
+		return 1;
+	if (argc > 0) {
+		printf("%s: Invalid number of arguments.\n", progname);
+		printf("Try '%s --help' for more information.\n", progname);
+		return 1;
+	}
+	return push_main();
+}
+
+/* atch kill <session> — stop session */
+int cmd_kill(int argc, char **argv)
+{
+	int force = 0;
+
+	/* Accept -f / --force before or after the session name. */
+	while (argc >= 1 && (strcmp(argv[0], "-f") == 0 ||
+			     strcmp(argv[0], "--force") == 0)) {
+		force = 1;
+		argc--;
+		argv++;
+	}
+	if (consume_session(&argc, &argv))
+		return 1;
+	while (argc >= 1 && (strcmp(argv[0], "-f") == 0 ||
+			     strcmp(argv[0], "--force") == 0)) {
+		force = 1;
+		argc--;
+		argv++;
+	}
+	if (argc > 0) {
+		printf("%s: Invalid number of arguments.\n", progname);
+		printf("Try '%s --help' for more information.\n", progname);
+		return 1;
+	}
+	return kill_main(force);
+}
+
+/* atch clear <session> — truncate the on-disk session log */
+int cmd_clear(int argc, char **argv)
+{
+	char log_path[600];
+	int fd;
+
+	if (argc > 0) {
+		if (consume_session(&argc, &argv))
+			return 1;
+	} else {
+		const char *chain = getenv(SESSION_ENVVAR);
+		const char *last;
+
+		if (!chain || !*chain) {
+			printf("%s: No session was specified.\n", progname);
+			printf("Try '%s --help' for more information.\n",
+			       progname);
+			return 1;
+		}
+		last = strrchr(chain, ':');
+		sockname = (char *)(last ? last + 1 : chain);
+	}
+	if (argc > 0) {
+		printf("%s: Invalid number of arguments.\n", progname);
+		printf("Try '%s --help' for more information.\n", progname);
+		return 1;
+	}
+	snprintf(log_path, sizeof(log_path), "%s.log", sockname);
+	fd = open(log_path, O_WRONLY | O_TRUNC);
+	if (fd >= 0) {
+		close(fd);
+		if (!quiet)
+			printf("%s: session '%s' log cleared\n",
+			       progname, session_shortname());
+	} else if (errno != ENOENT) {
+		printf("%s: %s: %s\n", progname, log_path, strerror(errno));
+		return 1;
+	}
+	return 0;
+}
+
+/* Scan log fd backward and return the byte offset to seek to before
+** printing the last nlines lines of output. */
+static off_t find_tail_start(int fd, off_t size, int nlines)
+{
+	char buf[BUFSIZE];
+	int count = 0;
+	off_t pos = size;
+
+	while (pos > 0 && count <= nlines) {
+		off_t chunk = pos > (off_t)sizeof(buf) ? (off_t)sizeof(buf) : pos;
+		ssize_t n;
+		ssize_t i;
+
+		pos -= chunk;
+		lseek(fd, pos, SEEK_SET);
+		n = read(fd, buf, (size_t)chunk);
+		if (n <= 0)
+			break;
+		for (i = n - 1; i >= 0; i--) {
+			if (buf[i] == '\n') {
+				if (++count > nlines)
+					return pos + i + 1;
+			}
+		}
+	}
+	return 0;
+}
+
+/* atch tail [-f] [-n N] <session> — print last N lines of session log */
+int cmd_tail(int argc, char **argv)
+{
+	int follow = 0, nlines = 10;
+	char log_path[600];
+	unsigned char rbuf[BUFSIZE];
+	off_t size, start;
+	ssize_t n;
+	int fd;
+
+	/* Parse -f and -n N (also -nN) before the session name */
+	while (argc >= 1 && argv[0][0] == '-' && argv[0][1] != '\0') {
+		if (strcmp(argv[0], "-f") == 0) {
+			follow = 1;
+			argc--;
+			argv++;
+		} else if (strcmp(argv[0], "-n") == 0) {
+			if (argc < 2) {
+				printf("%s: -n requires an argument\n", progname);
+				printf("Try '%s --help' for more information.\n",
+				       progname);
+				return 1;
+			}
+			nlines = atoi(argv[1]);
+			argc -= 2;
+			argv += 2;
+		} else if (strncmp(argv[0], "-n", 2) == 0 &&
+			   argv[0][2] != '\0') {
+			nlines = atoi(argv[0] + 2);
+			argc--;
+			argv++;
+		} else {
+			printf("%s: Invalid option '%s'\n", progname, argv[0]);
+			printf("Try '%s --help' for more information.\n",
+			       progname);
+			return 1;
+		}
+	}
+	if (nlines < 1)
+		nlines = 1;
+
+	if (consume_session(&argc, &argv))
+		return 1;
+	if (argc > 0) {
+		printf("%s: Invalid number of arguments.\n", progname);
+		printf("Try '%s --help' for more information.\n", progname);
+		return 1;
+	}
+
+	snprintf(log_path, sizeof(log_path), "%s.log", sockname);
+	fd = open(log_path, O_RDONLY);
+	if (fd < 0) {
+		if (errno == ENOENT)
+			printf("%s: no log for session '%s'\n", progname,
+			       session_shortname());
+		else
+			printf("%s: %s: %s\n", progname, log_path,
+			       strerror(errno));
+		return 1;
+	}
+
+	size = lseek(fd, 0, SEEK_END);
+	if (size > 0) {
+		start = find_tail_start(fd, size, nlines);
+		lseek(fd, start, SEEK_SET);
+		while ((n = read(fd, rbuf, sizeof(rbuf))) > 0)
+			write(1, rbuf, (size_t)n);
+	}
+
+	if (follow) {
+		signal(SIGPIPE, SIG_IGN);
+		for (;;) {
+			usleep(250000);
+			while ((n = read(fd, rbuf, sizeof(rbuf))) > 0)
+				write(1, rbuf, (size_t)n);
+		}
+	}
+
+	close(fd);
+	return 0;
+}
+
+int cmd_rm(int argc, char **argv)
+{
+	int all = 0;
+
+	if (argc >= 1 && strcmp(argv[0], "-a") == 0) {
+		all = 1;
+		argc--;
+		argv++;
+	}
+
+	if (all) {
+		if (argc > 0) {
+			printf("%s: Invalid number of arguments.\n", progname);
+			printf("Try '%s --help' for more information.\n",
+			       progname);
+			return 1;
+		}
+		return rm_main(1);
+	}
+
+	if (consume_session(&argc, &argv))
+		return 1;
+	if (argc > 0) {
+		printf("%s: Invalid number of arguments.\n", progname);
+		printf("Try '%s --help' for more information.\n", progname);
+		return 1;
+	}
+	return rm_main(0);
+}
+
+/* Default: atch <session> [cmd...] — attach-or-create */
+int cmd_open(char *session, int argc, char **argv)
+{
+	sockname = session;
+	if (expand_sockname())
+		return 1;
+	if (atch_parse_options(&argc, &argv))
+		return 1;
+	argv = use_shell_if_no_cmd(argc, argv);
+	save_term();
+	if (require_tty())
+		return 1;
+	if (attach_main(1) != 0) {
+		if (errno == ECONNREFUSED || errno == ENOENT) {
+			int saved_errno = errno;
+
+			replay_session_log(saved_errno);
+			if (saved_errno == ECONNREFUSED)
+				unlink(sockname);
+			if (master_main(argv, 1, 0) != 0)
+				return 1;
+			if (!quiet)
+				printf("%s: session '%s' created\n", progname,
+				       session_shortname());
+		}
+		return attach_main(0);
+	}
+	return 0;
+}
+
+void usage(void)
+{
+	printf(PACKAGE_NAME " - version %s, compiled on %s at %s.\n"
+	       "Usage:\n"
+	       "  " PACKAGE_NAME " [<session> [command...]]"
+	       "\t\tAttach to session or create it\n"
+	       "  " PACKAGE_NAME " <command> [options] ...\n"
+	       "\n"
+	       "Commands:\n"
+	       "  attach  <session>"
+	       "\t\t\tStrict attach (fail if session missing)\n"
+	       "  new     <session> [command...]"
+	       "\tCreate session and attach\n"
+	       "  start   <session> [command...]"
+	       "\tCreate session, detached\n"
+	       "  run     <session> [command...]"
+	       "\tCreate session, master in foreground\n"
+	       "  push    <session>"
+	       "\t\t\tPipe stdin into session\n"
+	       "  kill    [-f] <session>"
+	       "\t\tStop session (SIGTERM then SIGKILL)\n"
+	       "    -f, --force\t\t\tSkip grace period, send SIGKILL immediately\n"
+	       "  clear   [<session>]"
+	       "\t\t\tTruncate the session log\n"
+	       "  tail    [-f] [-n N] <session>"
+	       "\tPrint last N lines of session log\n"
+	       "    -f\t\t\t\tFollow log output\n"
+	       "    -n <lines>\t\t\tNumber of lines (default 10)\n"
+	       "  list    [-a]\t\t\t\tList sessions (-a includes exited)\n"
+	       "  rm      [-a] [<session>]"
+	       "\t\tRemove stale/exited session(s)\n"
+	       "    -a\t\t\t\tRemove all stale and exited sessions\n"
+	       "  current\t\t\t\tPrint current session name\n"
+	       "\n"
+	       "Options:\n"
+	       "  -e <char>\tSet detach character (default: ^\\)\n"
+	       "  -E\t\tDisable detach character\n"
+	       "  -r <method>\tRedraw method: none | ctrl_l | winch\n"
+	       "  -R <method>\tClear method:  none | move\n"
+	       "  -z\t\tDisable suspend key\n"
+	       "  -q\t\tSuppress messages\n"
+	       "  -t\t\tDisable VT100 assumptions\n"
+	       "  -C <size>\tLog cap: 0=disable, e.g. 128k, 4m (default 1m)\n"
+	       "\nURL: " PACKAGE_URL "\n\n",
+	       PACKAGE_VERSION, __DATE__, __TIME__);
+	exit(0);
+}
+
diff --git a/atch_cli_runtime.h b/atch_cli_runtime.h
new file mode 100644
index 0000000..08ef9e7
--- /dev/null
+++ b/atch_cli_runtime.h
@@ -0,0 +1,25 @@
+#ifndef ATCH_CLI_RUNTIME_H
+#define ATCH_CLI_RUNTIME_H
+
+int expand_sockname(void);
+char **use_shell_if_no_cmd(int argc, char **argv);
+void save_term(void);
+int require_tty(void);
+int consume_session(int *argc, char ***argv);
+
+int cmd_list(int argc, char **argv);
+int cmd_current(void);
+int cmd_attach(int argc, char **argv);
+int cmd_new(int argc, char **argv);
+int cmd_start(int argc, char **argv);
+int cmd_run(int argc, char **argv);
+int cmd_push(int argc, char **argv);
+int cmd_kill(int argc, char **argv);
+int cmd_clear(int argc, char **argv);
+int cmd_tail(int argc, char **argv);
+int cmd_rm(int argc, char **argv);
+int cmd_open(char *session, int argc, char **argv);
+
+void usage(void);
+
+#endif
diff --git a/atch_cmd.c b/atch_cmd.c
new file mode 100644
index 0000000..2912907
--- /dev/null
+++ b/atch_cmd.c
@@ -0,0 +1,36 @@
+#include <string.h>
+
+#include "atch_cmd.h"
+
+static int is_cmd(const char *arg, const char *a, const char *b, const char *c)
+{
+	return (a && strcmp(arg, a) == 0) || (b && strcmp(arg, b) == 0) ||
+	    (c && strcmp(arg, c) == 0);
+}
+
+enum atch_command atch_resolve_command(const char *cmd)
+{
+	if (is_cmd(cmd, "list", "l", "ls"))
+		return ATCH_CMD_LIST;
+	if (is_cmd(cmd, "current", NULL, NULL))
+		return ATCH_CMD_CURRENT;
+	if (is_cmd(cmd, "attach", "a", NULL))
+		return ATCH_CMD_ATTACH;
+	if (is_cmd(cmd, "new", "n", NULL))
+		return ATCH_CMD_NEW;
+	if (is_cmd(cmd, "start", "s", NULL))
+		return ATCH_CMD_START;
+	if (is_cmd(cmd, "run", NULL, NULL))
+		return ATCH_CMD_RUN;
+	if (is_cmd(cmd, "push", "p", NULL))
+		return ATCH_CMD_PUSH;
+	if (is_cmd(cmd, "kill", "k", NULL))
+		return ATCH_CMD_KILL;
+	if (is_cmd(cmd, "clear", NULL, NULL))
+		return ATCH_CMD_CLEAR;
+	if (is_cmd(cmd, "tail", NULL, NULL))
+		return ATCH_CMD_TAIL;
+	if (is_cmd(cmd, "rm", NULL, NULL))
+		return ATCH_CMD_RM;
+	return ATCH_CMD_OPEN;
+}
diff --git a/atch_cmd.h b/atch_cmd.h
new file mode 100644
index 0000000..7550b43
--- /dev/null
+++ b/atch_cmd.h
@@ -0,0 +1,21 @@
+#ifndef ATCH_CMD_H
+#define ATCH_CMD_H
+
+enum atch_command {
+	ATCH_CMD_OPEN = 0,
+	ATCH_CMD_LIST,
+	ATCH_CMD_CURRENT,
+	ATCH_CMD_ATTACH,
+	ATCH_CMD_NEW,
+	ATCH_CMD_START,
+	ATCH_CMD_RUN,
+	ATCH_CMD_PUSH,
+	ATCH_CMD_KILL,
+	ATCH_CMD_CLEAR,
+	ATCH_CMD_TAIL,
+	ATCH_CMD_RM,
+};
+
+enum atch_command atch_resolve_command(const char *cmd);
+
+#endif
diff --git a/atch_paths.c b/atch_paths.c
new file mode 100644
index 0000000..7ea2414
--- /dev/null
+++ b/atch_paths.c
@@ -0,0 +1,60 @@
+#include "atch.h"
+#include "atch_paths.h"
+
+/* Returns the basename of the current session socket path. */
+const char *session_shortname(void)
+{
+	const char *p = strrchr(sockname, '/');
+	return p ? p + 1 : sockname;
+}
+
+/* Returns the directory where session sockets are stored. */
+void get_session_dir(char *buf, size_t size)
+{
+	const char *home = getenv("HOME");
+	const char *base = strrchr(progname, '/');
+	struct passwd *pw;
+
+	base = base ? base + 1 : progname;
+
+	/* If $HOME is unset or empty, try the passwd database. */
+	if (!home || !*home) {
+		pw = getpwuid(getuid());
+		if (pw && pw->pw_dir && *pw->pw_dir)
+			home = pw->pw_dir;
+	}
+
+	/* Use $HOME only if it is set and not the root directory. */
+	if (home && *home && strcmp(home, "/") != 0)
+		snprintf(buf, size, "%s/.cache/%s", home, base);
+	else
+		snprintf(buf, size, "/tmp/.%s-%d", base, (int)getuid());
+}
+
+/*
+** Long-path helper for Unix socket operations: saves cwd, chdirs into the
+** directory part of path, calls fn(basename), then restores cwd.
+** Used by connect_socket and create_socket to handle paths > sun_path limit.
+*/
+int socket_with_chdir(char *path, int (*fn)(char *))
+{
+	char *slash = strrchr(path, '/');
+	int dirfd, s;
+
+	if (!slash) {
+		errno = ENAMETOOLONG;
+		return -1;
+	}
+	dirfd = open(".", O_RDONLY);
+	if (dirfd < 0)
+		return -1;
+	*slash = '\0';
+	s = chdir(path) >= 0 ? fn(slash + 1) : -1;
+	*slash = '/';
+	if (s >= 0 && fchdir(dirfd) < 0) {
+		close(s);
+		s = -1;
+	}
+	close(dirfd);
+	return s;
+}
diff --git a/atch_paths.h b/atch_paths.h
new file mode 100644
index 0000000..3a96390
--- /dev/null
+++ b/atch_paths.h
@@ -0,0 +1,8 @@
+#ifndef ATCH_PATHS_H
+#define ATCH_PATHS_H
+
+const char *session_shortname(void);
+void get_session_dir(char *buf, size_t size);
+int socket_with_chdir(char *path, int (*fn)(char *));
+
+#endif
diff --git a/atch_session.c b/atch_session.c
new file mode 100644
index 0000000..073a3e9
--- /dev/null
+++ b/atch_session.c
@@ -0,0 +1,30 @@
+#include "atch.h"
+#include "atch_session.h"
+
+char *atch_expand_session_name_dup(const char *name)
+{
+	char dir[512];
+	size_t fulllen;
+	char *full;
+	char *slash;
+
+	if (!name || !*name)
+		return NULL;
+	if (strchr(name, '/') != NULL)
+		return strdup(name);
+
+	get_session_dir(dir, sizeof(dir));
+	slash = strrchr(dir, '/');
+	if (slash) {
+		*slash = '\0';
+		mkdir(dir, 0700);
+		*slash = '/';
+	}
+	mkdir(dir, 0700);
+	fulllen = strlen(dir) + 1 + strlen(name);
+	full = malloc(fulllen + 1);
+	if (!full)
+		return NULL;
+	snprintf(full, fulllen + 1, "%s/%s", dir, name);
+	return full;
+}
diff --git a/atch_session.h b/atch_session.h
new file mode 100644
index 0000000..749fa68
--- /dev/null
+++ b/atch_session.h
@@ -0,0 +1,11 @@
+#ifndef ATCH_SESSION_H
+#define ATCH_SESSION_H
+
+/* Resolve a user-provided session name to full socket path.
+ * - If name contains '/', returns strdup(name)
+ * - Otherwise returns "<session-dir>/<name>"
+ * Caller owns returned memory.
+ */
+char *atch_expand_session_name_dup(const char *name);
+
+#endif
diff --git a/main.c b/main.c
new file mode 100644
index 0000000..9d3a8ff
--- /dev/null
+++ b/main.c
@@ -0,0 +1,6 @@
+#include "atch_cli.h"
+
+int main(int argc, char **argv)
+{
+	return atch_cli_main(argc, argv);
+}
diff --git a/makefile b/makefile
index eb43a61..4e31459 100644
--- a/makefile
+++ b/makefile
@@ -11,8 +11,9 @@ else
   STATIC_FLAG = -static
 endif
 
-OBJ = attach.o master.o atch.o
-SRC = attach.c master.c atch.c
+LIBOBJ = attach.o master.o atch.o atch_paths.o atch_session.o atch_cmd.o atch_cli_opts.o atch_cli_runtime.o
+OBJ = main.o $(LIBOBJ)
+SRC = main.c attach.c master.c atch.c atch_paths.c atch_session.c atch_cmd.c atch_cli_opts.c atch_cli_runtime.c
 
 IMAGE = atch-builder
 BUILDDIR ?= .
@@ -20,8 +21,13 @@ BUILDDIR ?= .
 archs = amd64 arm64
 arch ?= $(shell arch)
 
-atch: $(OBJ)
-	$(CC) -o $(BUILDDIR)/$@ $(STATIC_FLAG) $(LDFLAGS) $(OBJ) $(LIBS)
+.DEFAULT_GOAL := atch
+
+libatch.a: $(LIBOBJ)
+	ar rcs $@ $(LIBOBJ)
+
+atch: main.o libatch.a
+	$(CC) -o $(BUILDDIR)/$@ $(STATIC_FLAG) $(LDFLAGS) main.o libatch.a $(LIBS)
 
 atch.1.md: README.md scripts/readme2man.sh
 	bash scripts/readme2man.sh $< > $@
@@ -32,7 +38,7 @@ atch.1: atch.1.md
 man: atch.1
 
 clean:
-	rm -f atch $(OBJ) *.1.md *.c~
+	rm -f atch libatch.a $(OBJ) *.1.md *.c~
 
 .PHONY: fmt
 fmt:
@@ -43,9 +49,15 @@ fmt-all:
 	$(MAKE) fmt SRCS="*.c"
 
 
+main.o: ./main.c ./atch_cli.h
 attach.o: ./attach.c ./atch.h config.h
 master.o: ./master.c ./atch.h config.h
-atch.o: ./atch.c ./atch.h config.h
+atch.o: ./atch.c ./atch.h config.h ./atch_cli.h ./atch_paths.h ./atch_session.h ./atch_cmd.h ./atch_cli_opts.h ./atch_cli_runtime.h
+atch_paths.o: ./atch_paths.c ./atch.h config.h ./atch_paths.h
+atch_session.o: ./atch_session.c ./atch.h config.h ./atch_session.h
+atch_cmd.o: ./atch_cmd.c ./atch_cmd.h
+atch_cli_opts.o: ./atch_cli_opts.c ./atch.h config.h ./atch_cli_opts.h
+atch_cli_runtime.o: ./atch_cli_runtime.c ./atch.h config.h ./atch_cli_opts.h ./atch_session.h
 
 .PHONY: build-image
 build-image: