Resolve the remote output path on the server

The output path was made absolute with os.path.abspath and '~' was
expanded to the local home, so relative paths and '~' pointed to the
wrong folder on the server. Resolve the path on the server instead,
where '~' and relative paths refer to the remote user's home, and
document it.

simple_backup/simple_backup.conf

@@ -4,7 +4,7 @@
 # Files and directories to backup. Multiple items can be separated using a comma (','). It is possible to use wildcards (i.e. '*' to match multiple characters and '~' for the user's home directory).
 inputs=/home/user
 
-# Output directory.
+# Output directory. For remote backups, this is a path on the server ('~' and relative paths refer to the home directory on the server).
 backup_dir=/media/Backup
 
 # Files, directories and patterns to exclude from the backup. Multiple items can be separated using a comma.

simple_backup/simple_backup.py

@@ -189,28 +189,48 @@ class Backup:
             if self._ssh is None:
                 return 5
 
-            _, stdout, _ = self._ssh.exec_command(
-                f'if [ -d {shlex.quote(self.output)} ]; then echo "ok"; fi')
+            output = self._resolve_remote_path(self.output)
 
-            output = stdout.read().decode('utf-8').strip()
-
-            if output != 'ok':
+            if output is None:
                 logger.critical('Output path for backup does not exist')
 
                 return 2
+
+            self.output = output
         else:
             if not os.path.isdir(self.output):
                 logger.critical('Output path for backup does not exist')
 
                 return 2
 
-        self.output = os.path.abspath(self.output)
+            self.output = os.path.abspath(self.output)
 
         if self.keep is None:
             self.keep = -1
 
         return 0
 
+    def _resolve_remote_path(self, path: str) -> Optional[str]:
+        """Return the absolute path of a directory on the server, or None if it doesn't exist"""
+
+        assert self._ssh is not None
+
+        # Leave a leading '~' unquoted, so that the server's shell expands it to the remote home.
+        # Relative paths are resolved from the remote home, where the command starts
+        if path == '~' or path.startswith('~/'):
+            target = f'~/{shlex.quote(path[2:])}'
+        else:
+            target = shlex.quote(path)
+
+        _, stdout, _ = self._ssh.exec_command(f'cd -- {target} && pwd')
+
+        output = stdout.read().decode('utf-8').strip().split('\n')[-1]
+
+        if not output.startswith('/'):
+            return None
+
+        return output
+
     # Function to create the actual backup directory
     def define_backup_dir(self) -> None:
         """Define the actual backup dir"""
@@ -768,14 +788,8 @@ def _read_config(config_file, user: Optional[str] = None):
 
     config_args['inputs'] = inputs
 
-    output = config.get(section, 'backup_dir')
-
-    if user is not None:
-        output = _expand_home(output, user)
-    elif user is None and output.startswith('~'):
-        logger.warning('Cannot expand \'~\', no user specified')
-
-    config_args['output'] = output
+    # '~' is expanded later, since for remote backups it refers to the home on the server
+    config_args['output'] = config.get(section, 'backup_dir')
 
     try:
         exclude = config.get(section, 'exclude')
@@ -928,11 +942,25 @@ def simple_backup() -> int:
         return 6
 
     inputs = args.inputs if args.inputs is not None else config_args['inputs']
-    output = args.output if args.output is not None else config_args['output']
     exclude = args.exclude if args.exclude is not None else config_args['exclude']
     keep = args.keep if args.keep is not None else config_args['keep']
     ssh_host = args.ssh_host if args.ssh_host is not None else config_args['ssh_host']
     ssh_user = args.ssh_user if args.ssh_user is not None else config_args['ssh_user']
+
+    if args.output is not None:
+        output = args.output
+    else:
+        output = config_args['output']
+
+        # For remote backups, '~' is expanded on the server
+        remote = ssh_host is not None and ssh_user is not None
+
+        if output is not None and not remote:
+            if user is not None:
+                output = _expand_home(output, user)
+            elif output.startswith('~'):
+                logger.warning('Cannot expand \'~\', no user specified')
+
     ssh_keyfile = args.keyfile if args.keyfile is not None else config_args['ssh_keyfile']
     remote_sudo = args.remote_sudo or config_args['remote_sudo']