Cygwin Bash Script: File Fails Silently with rsync and diff

Cygwin's rsync and diff behave differently than native Windows tools due to line ending conversions. The silent failure occurs because diff reports differences in CRLF vs LF, but WinMerge ignores them by default. To fix this, configure diff to ignore whitespace and line endings, or use rsync with the --checksum flag to force a binary comparison of file contents rather than relying on timestamps and sizes.

Understanding the Cygwin Environment

Cygwin provides a Linux-compatible environment on Windows, translating POSIX system calls into Windows NT API calls. This translation layer introduces subtle differences when handling text files. Specifically, Cygwin often converts between Unix-style line endings (LF) and Windows-style line endings (CRLF). When you run diff inside a Bash script, it may detect these line ending changes as significant differences, even if the actual content is identical. However, graphical tools like WinMerge might be configured to ignore such whitespace differences, leading to the discrepancy where your script says files differ, but the visual tool says they are the same. This mismatch is the primary cause of the "silent failure" you are experiencing. Your script likely exits with a non-zero status code from diff, triggering an error path or skipping the backup, while you visually confirm no changes exist. To resolve this, we must align the textual comparison logic of diff with the expectations of your workflow.

Configuring Diff for Text Consistency

The most common solution is to adjust the flags passed to diff. By default, diff is strict about whitespace and line endings. You can instruct it to ignore these differences using the -w (ignore all whitespace) and --ignore-cr-at-eol flags. If you are comparing directory trees, you should also use the -r flag for recursive comparison. Here is how you can modify your comparison step:

diff -rqw --ignore-cr-at-eol /path/to/source /path/to/destination

This command tells diff to compare files recursively, ignoring any whitespace differences, including carriage returns at the end of lines. If this command returns no output, the files are considered identical by Cygwin standards, matching what WinMerge would show if it were configured similarly.

Using Rsync for Accurate Comparison

If you are using rsync primarily to check for differences before copying, relying on its default behavior might still lead to issues. rsync uses modification times and file sizes by default to determine if a file needs updating. In Cygwin, file timestamps can sometimes have different resolutions or interpretations compared to native Windows applications, leading to false positives. To ensure rsync performs a true content-based comparison, use the --checksum (-c) option. This forces rsync to calculate checksums for all files, bypassing the timestamp heuristic. While slower, this method guarantees that only files with truly different content are transferred.

rsync -avc --dry-run /source/ /destination/

The --dry-run flag allows you to see exactly which files would be copied without actually modifying anything. This is crucial for debugging. If the dry run shows no files to transfer, then rsync considers the directories identical. For more information on ensuring idempotency in your backups, you might want to review Is rsync idempotent?.

Dry Run Diagnostics

Before running your full backup script, always test with a dry run. This prevents accidental data loss or overwrites during debugging. The verbose output (-v) combined with --dry-run will list every file that differs. If the list is empty, your synchronization logic is sound. If you need to verify that you have enough space for the destination drive before proceeding, consider checking Determining If Enough Space is Available in Destination Drive for rsync Dry Run to avoid runtime failures.

Handling File Attributes and Permissions

Another potential source of silent failures is the handling of file attributes. Cygwin maps Windows file attributes (like Read-Only, Hidden) to POSIX permissions. If your script does not preserve these attributes correctly, rsync might perceive a file as changed simply because its permission bits differ, even if the content is identical. Use the --perms and --no-perms flags depending on your needs. If you are syncing between two Cygwin environments, preserving permissions is usually desired. If you are syncing to a native Windows directory, you might want to strip POSIX-specific permissions to avoid cluttering the destination with irrelevant metadata. Additionally, ensure that your script handles symlinks correctly. Use the -l or --links flag in rsync if you intend to preserve symbolic links. Without this, rsync will dereference symlinks and copy the target files, which can drastically change the structure of your backup.

Debugging the Script Output

When a script fails silently, adding diagnostic logging is essential. Instead of suppressing errors, redirect both standard output and standard error to a log file. This allows you to inspect the exact exit codes returned by diff and rsync.

#!/bin/bash
set -e  # Exit immediately if a command exits with a non-zero status

log_file="backup_debug.log"
exec >"$log_file" 2>&1

echo "Starting comparison..."
diff -rqw /source /dest
if [ $? -eq 0 ]; then
    echo "Files are identical."
else
    echo "Differences found. Initiating sync..."
    rsync -av /source/ /dest/
fi

The set -e directive ensures that the script stops if any command fails, making it easier to pinpoint where the process breaks. The exec command redirects all subsequent output to the log file, giving you a complete trail of execution.

Common Pitfalls in Cygwin Backups

One frequent mistake is mixing forward slashes and backslashes in paths. Cygwin prefers forward slashes. Using mixed delimiters can cause path resolution errors that appear as silent failures. Always normalize your paths using cygpath if you are interacting with native Windows commands from within Bash. Furthermore, ensure that your Cygwin installation includes the necessary packages. The diffutils package provides the diff command, and rsync is provided by the rsync package. If either is missing, commands may fail silently or return unexpected results. Check your installation via the Cygwin setup executable to verify these components are present. For scenarios where you need to merge directories rather than just sync them, understanding how rsync handles overlapping files is critical. See Using rsync to Merge Two Source Directories for advanced strategies on handling complex directory structures.

Frequently Asked Questions

Why does rsync think files are different when they look the same?

Rsync often relies on file size and modification time. If timestamps have sub-second precision differences or if file sizes vary due to metadata, rsync will deem them different. Using the --checksum flag forces a content-based comparison, resolving this issue.

How do I make diff ignore case differences?

Add the -i flag to your diff command. This makes the comparison case-insensitive. Combine it with -w to ignore whitespace as well, ensuring that only substantive content changes trigger a difference report.

Can I use WinMerge directly from the command line?

Yes, WinMerge supports command-line arguments. You can invoke it with wm.exe /u /dl "Source" /dr "Dest" /s /x to open a unified view. However, integrating it into a Bash script requires careful handling of window focus and exit codes, which can complicate automation.

What causes rsync to delete files unexpectedly?

The --delete flag in rsync removes files from the destination that do not exist in the source. If you are not using this flag explicitly, rsync will not delete files. Always double-check your flags to ensure you are not inadvertently invoking deletion modes.

Is it safe to run rsync on a mounted network drive?

Running rsync on network drives can be slow and prone to timeout errors. It is generally safer to sync locally first, then move the data. If you must sync directly, use the --timeout flag to handle latency issues and ensure you have a stable connection.