Update command handling instructions in course-definition.yml

Enhance the command execution logic by specifying behavior for commands with and without execute permissions. The updated instructions clarify that commands without execute permissions will be skipped, while those with permissions will print their path. This aims to improve user understanding of command execution outcomes.

Author: andy1li

course-definition.yml

@@ -314,7 +314,13 @@ stages:
 
       [PATH](https://en.wikipedia.org/wiki/PATH_(variable)) is an environment variable that specifies a set of directories where executable programs are located. When a command is received, the program should search for the command in the directories listed in the PATH environment variable.
 
-      If the command is found, the program should print the path to the command. If the command is not found, the program should print `<command>: not found`.
+      For example, PATH like `/dir1:/dir2:/dir3` means that the program should search for the command in `/dir1`, then `/dir2`, and finally `/dir3`, in that order.
+
+      Caveat: Not all commands found are necessarily executable.
+
+      - If the command is found (but it does not have execute permissions), the program should skip it and continue searching. 
+      - If the command is found (and it has execute permissions), the program should print the path to the command. 
+      - If the command is not found, the program should print `<command>: not found`.
 
       ### Tests
 
@@ -329,8 +335,14 @@ stages:
       ```bash
       $ type ls
       ls is /usr/bin/ls
+      ```
+
+      ```bash
       $ type valid_command
       valid_command is /usr/local/bin/valid_command
+      ```
+
+      ```bash
       $ type invalid_command
       invalid_command: not found
       $
@@ -341,8 +353,9 @@ stages:
       ### Notes
 
       - The actual value of the `PATH` environment variable will be random for each test case.
-      - `PATH` can contain multiple directories separated by colons (`:`), your program should search for programs in each directory in order and return the first match which has executable permissions.
       - Some commands, such as `echo`, can exist as both builtin commands and executable files. In such cases, the `type` command should identify them as builtins.
+      - PATH can include directories that don’t exist on disk, so your code should handle such cases gracefully.
+
     marketing_md: |-
       In this stage, you'll implement the `type` builtin command for your shell.
 
@@ -1120,8 +1133,11 @@ stages:
       1.  **Input:** `custom`<TAB>
           *   The tester types "custom" and presses `<TAB>`. The tester expects that the prompt line changes to <code>custom_executable  </code>.
 
-      The tester will verify that your shell correctly completes the command to the external executable file name.
-      Note the space at the end of the completion.
+      The tester will verify that your shell correctly completes the command to the external executable file name. Note the space at the end of the completion.
+
+      ### Notes
+
+      - PATH can include directories that don't exist on disk, so your code should handle such cases gracefully.
 
     marketing_md: |-
       In this stage, you'll implement support for autocompleting external executables.