Quote paths in commands run on the SSH server

Remote commands were built with f-strings, with paths either unquoted
or in double quotes. Paths with spaces broke ls and find, and $ or
backticks in a path were expanded by the server's shell. Quote every
path with shlex.quote.

simple_backup/simple_backup.py

@@ -190,7 +190,7 @@ class Backup:
                 return 5
 
             _, stdout, _ = self._ssh.exec_command(
-                f'if [ -d "{self.output}" ]; then echo "ok"; fi')
+                f'if [ -d {shlex.quote(self.output)} ]; then echo "ok"; fi')
 
             output = stdout.read().decode('utf-8').strip()
 
@@ -229,7 +229,7 @@ class Backup:
             assert self._ssh is not None
 
             _, stdout, _ = self._ssh.exec_command(
-                f'ls {self.output}/simple_backup')
+                f'ls {shlex.quote(self.output + "/simple_backup")}')
 
             dirs = stdout.read().decode('utf-8').strip().split('\n')
 
@@ -247,10 +247,10 @@ class Backup:
                 for i in range(n_backup - self.keep):
                     if self.remote_sudo:
                         _, _, stderr = self._ssh.exec_command(
-                            f'sudo rm -r "{self.output}/simple_backup/{dirs[i]}"')
+                            f'sudo rm -r {shlex.quote(f"{self.output}/simple_backup/{dirs[i]}")}')
                     else:
                         _, _, stderr = self._ssh.exec_command(
-                            f'rm -r "{self.output}/simple_backup/{dirs[i]}"')
+                            f'rm -r {shlex.quote(f"{self.output}/simple_backup/{dirs[i]}")}')
 
                     err = stderr.read().decode('utf-8').strip().split('\n')[0]
 
@@ -304,7 +304,7 @@ class Backup:
                 sys.exit(5)
 
             _, stdout, _ = self._ssh.exec_command(
-                f'find {self.output}/simple_backup/ -mindepth 1 -maxdepth 1 -type d | sort')
+                f'find {shlex.quote(self.output + "/simple_backup/")} -mindepth 1 -maxdepth 1 -type d | sort')
             output = stdout.read().decode('utf-8').strip().split('\n')
 
             if output[-1] != '':