Update kill command note

Author: Yutsuten

notes/tool/linux/package/kill.md

@@ -10,11 +10,11 @@ Kill processes by process ID.
 kill OPTIONS PID
 ```
 
-| Option | Description |
-| --- | --- |
-| `-s` `--signal` | The signal to send. It may be given as a name or a number. |
-| `-l` `--list` | Print a list of signal names, or convert the given signal number to a name. |
-| `-L` `--table` | Pretty print a table of signal names and numbers. |
+| Option          | Description                                                                                   |
+| --------------- | --------------------------------------------------------------------------------------------- |
+| `-s` `--signal` | The signal to send. It may be given as a name or a number. Use 0 to just run the validations. |
+| `-l` `--list`   | Print a list of signal names, or convert the given signal number to a name.                   |
+| `-L` `--table`  | Pretty print a table of signal names and numbers.                                             |
 
 Examples:
 
@@ -24,6 +24,12 @@ kill -9 PID  # Same as `-s 9` (SIGKILL)
 kill -9 %JOBID
 ```
 
+Check if a process is still running.
+
+```shell
+kill -0 PID
+```
+
 ## Pkill
 
 Kill processes by process name (pattern).