Add tail command to read session logs

Author: arthurpro

README.md

@@ -83,6 +83,7 @@ If no command is given, `$SHELL` is used.
 | `push <session>` | Copy stdin verbatim to the session. |
 | `kill [-f] <session>` | Gracefully stop a session (SIGTERM, then SIGKILL after 5 s if needed). With `-f` / `--force`, skip the grace period and send SIGKILL immediately. |
 | `clear [<session>]` | Truncate the on-disk session log. Defaults to the current session when run inside one. |
+| `tail [-f] [-n N] <session>` | Print the last N lines of the session log (default: 10). With `-f`, follow new output as it is written. |
 | `list` | List all sessions. Shows `[attached]` when a client is connected, `[stale]` for leftover sockets with no running master. Prints `(no sessions)` when the list is empty. |
 | `current` | Print the current session name and exit 0 if inside a session; exit 1 silently if not. |
 
@@ -166,6 +167,16 @@ atch -e '^A' attach work
 atch list
 ```
 
+**Inspect the last 20 lines of a session log:**
+```sh
+atch tail -n 20 work
+```
+
+**Follow a session's output without attaching:**
+```sh
+atch tail -f work
+```
+
 **Kill a session:**
 ```sh
 atch kill work

atch.c

@@ -542,6 +542,116 @@ static int cmd_clear(int argc, char **argv)
 	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;
+}
+
 /* Default: atch <session> [cmd...] — attach-or-create */
 static int cmd_open(char *session, int argc, char **argv)
 {
@@ -595,6 +705,10 @@ static void usage(void)
 	       "    -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\t\t\t\t\tList all sessions\n"
 	       "  current\t\t\t\tPrint current session name\n"
 	       "\n"
@@ -779,6 +893,8 @@ int main(int argc, char **argv)
 		return cmd_kill(argc, argv);
 	if (is_cmd(cmd, "clear", NULL, NULL))
 		return cmd_clear(argc, argv);
+	if (is_cmd(cmd, "tail", NULL, NULL))
+		return cmd_tail(argc, argv);
 
 	/* Smart default: treat first arg as session name → attach-or-create */
 	return cmd_open((char *)cmd, argc, argv);

tests/test.sh

@@ -628,14 +628,69 @@ run "$ATCH" start -C foo C-bad sleep 999
 assert_exit     "-C invalid: exit 1"                 1 "$rc"
 assert_contains "-C invalid: message"                "Invalid log size" "$out"
 
-# ── 21. no-args → usage ──────────────────────────────────────────────────────
+# ── 21. tail command ─────────────────────────────────────────────────────────
+
+# tail with no session
+run "$ATCH" tail
+assert_exit     "tail: no session → exit 1"          1 "$rc"
+assert_contains "tail: no session → message"         "No session was specified" "$out"
+
+# tail on nonexistent session (no log file)
+run "$ATCH" tail s-noexist-tail
+assert_exit     "tail: no log → exit 1"              1 "$rc"
+assert_contains "tail: no log → message"             "no log" "$out"
+
+# tail basic: start session that writes 20 numbered lines then sleeps
+# Use zero-padded numbers so e.g. "line01" is not a substring of "line11"
+"$ATCH" start s-tail sh -c \
+    'i=1; while [ $i -le 20 ]; do printf "line%02d\n" $i; i=$((i+1)); done; sleep 999'
+sleep 0.2
+# default 10 lines: should see line11..line20 but not line01
+run "$ATCH" tail s-tail
+assert_exit         "tail: exits 0"                       0 "$rc"
+assert_contains     "tail: shows recent line"             "line20" "$out"
+assert_not_contains "tail: omits early lines"             "line01" "$out"
+
+# -n 5: should see line16..line20
+run "$ATCH" tail -n 5 s-tail
+assert_exit         "tail -n 5: exits 0"                  0 "$rc"
+assert_contains     "tail -n 5: shows line in range"      "line20" "$out"
+assert_not_contains "tail -n 5: omits earlier lines"      "line10" "$out"
+
+# -n with combined flag style (-n5)
+run "$ATCH" tail -n5 s-tail
+assert_exit         "tail -n5 (compact): exits 0"         0 "$rc"
+assert_contains     "tail -n5 (compact): shows line20"    "line20" "$out"
+
+# extra args rejected
+run "$ATCH" tail s-tail extra
+assert_exit     "tail: extra arg → exit 1"           1 "$rc"
+assert_contains "tail: extra arg → message"          "Invalid number of arguments" "$out"
+
+# invalid option
+run "$ATCH" tail -x s-tail
+assert_exit     "tail: invalid option → exit 1"      1 "$rc"
+assert_contains "tail: invalid option → message"     "Invalid option" "$out"
+
+# -n missing argument
+run "$ATCH" tail -n
+assert_exit     "tail -n missing arg: exit 1"        1 "$rc"
+assert_contains "tail -n missing arg: message"       "-n requires an argument" "$out"
+
+tidy s-tail
+
+# ── 22. no-args → usage ──────────────────────────────────────────────────────
 
 # Invoking with zero arguments calls usage() (exits 0, prints help).
 # We already consumed the binary name in main, so argc < 1 → usage().
 run "$ATCH"
 assert_exit     "no args: exits 0 (usage)"           0 "$rc"
 assert_contains "no args: shows Usage:"              "Usage:" "$out"
 
+# tail command appears in help
+run "$ATCH" --help
+assert_contains "help: shows tail command"           "tail" "$out"
+
 # ── summary ──────────────────────────────────────────────────────────────────
 
 printf "\n1..%d\n" "$T"