Sample complete callback for linenoise (https://github.com/antirez/linenoise)

callback.m

// Array of all commands available:
NSArray* commandList; 
// Commands that don't take a file as argument (uname, ssh, mosh...):
NSArray* commandsNoFile;

void completion(const char *command, linenoiseCompletions *lc) {
  // autocomplete command for lineNoise
  BOOL isDir;
  NSString* commandString = [NSString stringWithUTF8String:command];
  if ([commandString rangeOfString:@" "].location == NSNotFound) {
    // No spaces. The user is typing a command
    for (NSString* existingCommand in commandList) {
      if ([existingCommand hasPrefix:commandString]) linenoiseAddCompletion(lc, existingCommand.UTF8String);
    }
  } else {
    // the user is typing an argument.
    // Is this one the commands that want a file as an argument?
    NSArray* commandArray = [commandString componentsSeparatedByString:@" "];
    if ([commandsNoFile containsObject:commandArray[0]]) return;

    // Last position of space in the command.
    // Would be better if I could get position of cursor.
    NSString* argument = commandArray.lastObject;
    // which directory?
    BOOL isDir;
    NSString* directory;
    NSString *file;
    if ([[NSFileManager defaultManager] fileExistsAtPath:argument isDirectory:&isDir] && isDir) {
      directory = argument;
      file = @"";
    } else {
      directory = [argument stringByDeletingLastPathComponent]; // can be empty.
      if (directory.length == 0) directory = @".";
      file = [argument lastPathComponent];
    }
    directory = [directory stringByExpandingTildeInPath];
    if ([[NSFileManager defaultManager] fileExistsAtPath:directory isDirectory:&isDir] && isDir) {
      NSArray* filenames = [[NSFileManager defaultManager] contentsOfDirectoryAtPath:directory error:Nil];
      for (NSString *fileName in filenames) {
        if ((file.length == 0) || [fileName hasPrefix:file]) {
          NSString* addition = [fileName substringFromIndex:[file length]];
          NSString * newCommand = [commandString stringByAppendingString:addition];
          linenoiseAddCompletion(lc,[newCommand UTF8String]);
        }
      }
    }
  }
}


// use with: 
- (int)main:(int)argc argv:(char **)argv
{
  // ...
  linenoiseSetCompletionCallback(completion);
  // ...
}