feat: accept chat ids, usernames and links, and keep exports per chat

-c already passed through to tdl, which resolves a numeric argument as an
MTProto id and anything else through gotd's resolver (@name, name, t.me and
tg:// links). Two forms needed handling: a Bot API '-100' prefixed id, which
MTProto does not use and which tdl never strips, is now converted; and a
message link, which names a message rather than a chat, is rejected with a
clear error instead of a resolver failure.

The export JSON now defaults to export-<chat>.json when -c is given, so
exporting a second chat from the same directory cannot silently download the
first chat again from a stale export.json. An existing file is still reused,
but the script now says which file it is using.

README.md

@@ -62,13 +62,43 @@ and warnings go to stderr; press Ctrl-C at any point and it stops cleanly.
 | Flag | Meaning |
 |------|---------|
 | `-r REMOTE:PATH` | **Required.** rclone destination, e.g. `gdrive:telegram/media`, `s3:bucket/tg`, `dav:tg-export` |
-| `-c CHAT` | Chat to export when the JSON does not exist yet: `@username` or a numeric chat id |
-| `-f FILE` | Export JSON to download from (default `export.json`) |
+| `-c CHAT` | Chat to export when the JSON does not exist yet — id, username, or link (see below) |
+| `-f FILE` | Export JSON to download from (default `export-<chat>.json` with `-c`, else `export.json`) |
 | `-d DIR` | Staging directory (default `./staging`) |
 | `-i SECONDS` | Seconds between rclone sweeps (default `60`) |
 | `-a AGE` | rclone `--min-age`, a second guard against moving files still being written (default `2m`) |
 | `-h` | Help |
 
+### Identifying the chat
+
+`-c` accepts every form tdl understands, plus one it doesn't:
+
+| Form | Example |
+|------|---------|
+| Numeric id, as printed by `tdl chat ls` | `-c 1697797156` |
+| Username, with or without `@` | `-c @mygroup` / `-c mygroup` |
+| Public link | `-c https://t.me/mygroup` / `-c t.me/mygroup` |
+| Deep link | `-c 'tg://resolve?domain=mygroup'` |
+| **Bot API id** (converted for you) | `-c -1001697797156` → `1697797156` |
+
+tdl resolves a numeric argument as an MTProto id and anything else through
+gotd's resolver. MTProto has no `-100` prefix, so a Bot API id would otherwise
+fail to resolve; the script strips it and logs the conversion.
+
+A **message** link is rejected — `-c` names a chat, not a message:
+
+```
+$ ./run.sh -r gdrive:tg -c https://t.me/mygroup/123
+error: -c takes a chat, not a message link — pass the chat's username or id
+```
+
+Run `tdl chat ls` to see ids and usernames side by side.
+
+Each chat gets its own export file by default (`export-mygroup.json`,
+`export-1697797156.json`), so exporting a second chat from the same directory
+never reuses the first one's JSON. When the file already exists it is reused and
+the script says so — delete it to re-export.
+
 Anything after `--` is passed straight to `tdl dl`:
 
 ```bash

run.sh

@@ -22,7 +22,7 @@ readonly PROG=${0##*/}
 readonly TEMP_GLOB='*.tmp'
 
 # Defaults
-export_file='export.json'
+export_file=''      # decided after parsing: per-chat when -c is given
 chat=''
 staging='./staging'
 remote=''
@@ -38,8 +38,12 @@ Required:
   -r REMOTE:PATH  rclone destination, e.g. gdrive:telegram/media
 
 Options:
-  -f FILE         tdl export JSON (default: $export_file)
-  -c CHAT         chat to export when FILE does not exist, e.g. @mygroup
+  -f FILE         tdl export JSON (default: export-<chat>.json with -c,
+                  otherwise export.json)
+  -c CHAT         chat to export when FILE does not exist. Accepts a numeric
+                  id as printed by 'tdl chat ls', a username with or without
+                  '@', or a t.me/tg:// link. A Bot API '-100...' id is
+                  converted to the plain id tdl expects.
   -d DIR          staging directory (default: $staging)
   -i SECONDS      seconds between rclone sweeps (default: $interval)
   -a AGE          rclone --min-age, a second guard against moving files still
@@ -77,6 +81,35 @@ tdl_extra=("$@")
 [[ $min_age =~ ^[0-9]+(\.[0-9]+)?(ms|s|m|h|d|w|M|y)?$ ]] \
   || die "-a must be an rclone duration, e.g. 2m"
 
+# tdl resolves a numeric argument as an MTProto id and anything else through
+# gotd's resolver, which handles '@name', 'name' and t.me/tg:// links. Two forms
+# still need help: Bot API ids carry a '-100' prefix that MTProto does not use,
+# and a message link is not a chat.
+if [[ -n $chat ]]; then
+  read -r chat <<<"$chat"        # trim stray whitespace
+  case $chat in
+    '') die "-c requires a chat" ;;
+    *t.me/c/*|*t.me/*/[0-9]*)
+      die "-c takes a chat, not a message link ($chat) — pass the chat's username or id" ;;
+    -100[0-9]*)
+      log "converting Bot API id $chat to MTProto id ${chat#-100}"
+      chat=${chat#-100} ;;
+  esac
+fi
+
+# Keep each chat's export in its own file, so switching -c never silently
+# downloads the previous chat again from a stale export.json.
+if [[ -z $export_file ]]; then
+  if [[ -n $chat ]]; then
+    slug=${chat#@}
+    slug=${slug##*/}
+    slug=$(printf '%s' "$slug" | tr -c 'A-Za-z0-9._-' '_')
+    export_file="export-$slug.json"
+  else
+    export_file='export.json'
+  fi
+fi
+
 for tool in tdl rclone; do
   command -v "$tool" >/dev/null || die "$tool is not installed or not on PATH"
 done
@@ -103,6 +136,8 @@ if [[ ! -f $export_file ]]; then
   [[ -n $chat ]] || die "$export_file not found; pass -c CHAT to export it first"
   log "exporting $chat metadata to $export_file"
   tdl chat export -c "$chat" --all --with-content -o "$export_file"
+else
+  log "using existing $export_file (delete it to re-export)"
 fi
 
 mkdir -p "$staging"