Is there a Linux function to check if a filename with differing case exists?

Is there a Linux function to check if a filename with differing case exists? e.g. to find the difference between Hello.txt and hello.txt

For example if I wanted to open Hello.txt I would like to do:

• try to open a file ( try Hello.txt)

if that fails:

• check if there is a file with different case in the folder (find file hello.txt)

No, there isn't. You'll need to iterate the files in the directory and do the check manually. There are functions to help with that though, e.g. scandir. Since C++17 you have std::filesystem::directory_iterator to iterate through the directory and generic algorithms in the standard library to help with the implementation if needed.

Also, some file systems are case-insensitive and will consider these two equivalent file names anyway. But in general Linux considers file names as just a sequence of byte values. So you will also need to decide in what encoding you want to interpret the file name if there are non-ASCII value in there. If you assume a Unicode encoding, figuring out what exactly "differing case" means is also not trivial.

If such function were to exist, there will be 2ⁿ combinations to check where n is the length of the file name.

For hello.txt, if you include the extension, there will be 256 ways to check (2⁸ = 256). As with exponential time complexity, it becomes an unmanageable problem very fast as n grows.

This is hugely inefficient and so the best way to do case-insensitive filename match is to convert everything to lower (or upper) case and then compare them like @mark-setchell points out.

Since you tagged c++ and linux, I have included a solution which works on linux, written in c++. Remember the function strcasecmp is only present on linux systems.

Also, if we keep track of the index of the match, we need not go through the 2ⁿ iterations if we need the exact case sensitive file name.

#include <iostream>
#include <filesystem>
#include <vector>
#include <cstring>
#include <algorithm>

int main() {
  std::string path = "./";
  std::string to_find = "hello.txt";
  bool exists = false;
  int found_index = 0;
  std::vector<std::string> arr;
  for (const auto &entry: std::filesystem::directory_iterator(path)) {
    arr.push_back(entry.path());
  }
  std::sort(arr.begin(), arr.end()); //std::filesytem::directory_iterator's 
                                //order cannot be determined, so the
                                //vector is sorted. This step is optional though
  for (int i=0; i<arr.size(); i++) {
    std::string a = arr[i].substr(path.size()); // Remove prefix path
    if (strcasecmp(a.c_str(), to_find.c_str()) == 0) {
      exists = true;
      found_index = i;
      break;
    }
  } 
  if (exists) {
    std::cout << "File found and the exact file name is \"" << arr[found_index] << "\"" << std::endl;
  } else {
    std::cout << "File not found" << std::endl;
  }
}

If match is found the above program prints out the exact file name too.