Program.cs

using System.Diagnostics;
using System.Text;

// defaults
var checkTime = false;
var dryRun = false;

var usageMessage = "Usage: checknew.exe [--dry-run] [--check-time] [local] host:remote";

int nextArg;
for (nextArg = 0; nextArg < args.Length && args[nextArg].StartsWith("--"); nextArg++) {
    switch (args[nextArg]) {
        case "--check-time":
            checkTime = true;
            break;
        case "--dry-run":
            dryRun = true;
            break;
        case "--help":
            Console.WriteLine(usageMessage);
            return 0;
        default:
            Console.WriteLine("Unknown option: {0}", args[nextArg]);
            Console.WriteLine(usageMessage);
            return 1;
    }
}

args = args[nextArg..];

string[] remote = Array.Empty<string>();
string localDir = "";
if (args.Length == 2) {
    localDir = args[0];
    remote = args[1].Split(':', 2);
} else if (args.Length == 1) {
    localDir = Directory.GetCurrentDirectory();
    remote = args[0].Split(':', 2);
} else {
    Console.WriteLine("Usage: checknew.exe [local] host:remote");
    return 1;
}

if (remote.Length != 2) {
    Debug.Assert(remote.Length == 1);
    Console.WriteLine($"Wrong remote format: {remote[0]}");
    Console.WriteLine(usageMessage);
    return 1;
}

var remoteHost = remote[0];
var remoteDir = remote[1];

using var findCmd = new Process() {
    StartInfo = {
        FileName = "ssh",
        RedirectStandardOutput = true,
        ArgumentList = {
            remoteHost, "find", ShellQuote(remoteDir), "-type", "f", "-printf '%T@ %s %P\\n'"
        },
        // This is needed because `ssh` command returns utf-8 encoded text.
        StandardOutputEncoding = Encoding.UTF8
    }
};
findCmd.Start();

var commands = Path.GetTempFileName();
bool newFilesFound = false;
using (var writer = new StreamWriter(commands)) {
    string? line;
    while ((line = findCmd.StandardOutput.ReadLine()) != null) {
        var remoteFile = ParseFindString(line);
        var filePath = remoteFile.Path.Replace('/', '\\');
        var localPath = Path.Join(localDir, filePath);
        var fileinfo = new FileInfo(localPath);

        if (!fileinfo.Exists) {
            Console.WriteLine($"File {filePath} doesn't exist locally!");
        } else {
            long localSize = fileinfo.Length;
            DateTime localModified = fileinfo.LastWriteTime;
            if (localSize != remoteFile.Size) {
                Console.WriteLine($"File {filePath} has a different size. {ToHumanSize(localSize)} (local) <- {ToHumanSize(remoteFile.Size)} (remote).");
            } else if (localModified < remoteFile.Modified) {
                if (!checkTime) {
                    // ignore the difference in modified time
                    continue;
                }
                Console.WriteLine($"File {filePath} is outdated. {localModified} (local) <- {remoteFile.Modified} (remote).");
            } else {
                continue;
            }
        }

        try {
            Directory.CreateDirectory(fileinfo.DirectoryName!);
        } catch (IOException) {
            Console.WriteLine($"Can't create a local directory for a file: {localPath}. Skipping it.");
            continue;
        }

        newFilesFound = true;
        var remotePath = remoteDir.TrimEnd('/') + '/' + remoteFile.Path;
        var command = $"get \"{remotePath}\" \"{localPath}\"";
        writer.WriteLine(command);
    }
}

findCmd.WaitForExit();
if (findCmd.ExitCode != 0) {
    Console.WriteLine("Find command exited with non-zero code!");
    return 1;
}

if (!newFilesFound) {
    Console.WriteLine("Done! No new files found.");
    return 0;
}

Console.WriteLine($"Commands to sftp: {commands}");

if (dryRun) {
    Console.WriteLine("Dry run. No copying will be performed!");
    return 0;
}

using var sftpCmd = new Process() {
    StartInfo = {
        FileName = "sftp",
        RedirectStandardOutput = true,
        RedirectStandardError = true,
        Arguments = $"-b {commands} {remoteHost}"
    }
};
sftpCmd.Start();

var result = await Task.WhenAll(sftpCmd.StandardOutput.ReadToEndAsync(), sftpCmd.StandardError.ReadToEndAsync());

sftpCmd.WaitForExit();
if (sftpCmd.ExitCode != 0) {
    Console.WriteLine("Sftp command exited with non-zero code!");
    Console.WriteLine(result[1]);
    return 1;
} else {
    Console.WriteLine("Done!");
    return 0;
}

string ShellQuote(string str) {
    return "'" + str.Replace("'", "'\\''") + "'";
}

// example:
// 1739519415.0753701630 38694 .bash_history
(DateTime Modified, long Size, string Path) ParseFindString(string s) {
    var data = s.Split(' ', 3);
    Debug.Assert(data.Length == 3);
    var date = TimestampToDateTime(data[0]);
    return (date, long.Parse(data[1]), data[2]);
}

DateTime TimestampToDateTime(string unixTimestamp) {
    unixTimestamp = unixTimestamp.Split(".")[0];
    DateTimeOffset dateTimeOffset = DateTimeOffset.FromUnixTimeSeconds(long.Parse(unixTimestamp));
    DateTime dateTime = dateTimeOffset.DateTime.ToLocalTime();
    return dateTime;
}

string ToHumanSize(long bytes) {
    long KB = 1024;
    long MB = KB * 1024;
    long GB = MB * 1024;
    long TB = GB * 1024;
    double size = bytes;
    if (bytes >= TB) {
        size = Math.Round((double)bytes / TB, 2);
        return $"{size} TB";
    } else if (bytes >= GB) {
        size = Math.Round((double)bytes / GB, 2);
        return $"{size} GB";
    } else if (bytes >= MB) {
        size = Math.Round((double)bytes / MB, 2);
        return $"{size} MB";
    } else if (bytes >= KB) {
        size = Math.Round((double)bytes / KB, 2);
        return $"{size} KB";
    } else {
        return $"{size} Bytes";
    }
}