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Identify read-only files from permission records

Jump TradingSoftware Engineer · Online Assessment
MediumSoftware EngineerOnline AssessmentCoding & Algorithms

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This problem tests your ability to parse Unix-style permission strings and implement filtering logic based on bitmask-like constraints.

Problem Statement

You are given a list of file metadata entries, where each entry is a string formatted as "permissions filename". The permissions portion is a 9-character string representing Unix-style file permissions (e.g., "rwxr-xr-x").

The permission string is divided into three segments of three characters each:

  1. User (the first 3 characters)
  2. Group (the middle 3 characters)
  3. Others (the last 3 characters)

In each segment, the character at index 0 represents 'read' (r), index 1 represents 'write' (w), and index 2 represents 'execute' (x). If a permission is missing, it is represented by a hyphen (-).

A file is considered read-only if it has no 'write' (w) permissions for the user, the group, OR others. In other words, for a file to be read-only, all three write-position characters must be hyphens ('-').

Implement the function find_read_only_files(file_list) that returns a list of filenames that are read-only, sorted alphabetically.

Constraints

  • file_list contains between 1 and 1000 strings.
  • Each filename consists of alphanumeric characters.
  • Each permission string is exactly 9 characters long.

Examples

Example 1: Input: ["r-xr-xr-x file1", "rw-r--r-- file2", "r--r--r-- file3"] Output: ["file1", "file3"] Explanation:

  • file1 (r-xr-xr-x): Write positions are all '-', so it is read-only.
  • file2 (rw-r--r--): User has write access, so it is not read-only.
  • file3 (r--r--r--): Write positions are all '-', so it is read-only.

Function Signature

def find_read_only_files(file_list: list[str]) -> list[str]:
    pass

Hints

Hint 1

The write permissions are located at indices 1, 4, and 7 of the 9-character permission string.

Hint 2

A file is read-only if the characters at indices 1, 4, and 7 are all equal to the hyphen character '-'.

Hint 3

Split each string by the space character to separate the permissions from the filename, then apply the condition.

Follow-up questions

  • How would you modify your solution to identify files where ONLY the owner has write permission?
  • If the input was a stream of millions of files, how would you optimize the memory usage?
  • How would you handle potential malformed input strings where the permission segment is not exactly 9 characters?