ext4文件系统下复制文件至目录时如何保留目标目录修改时间戳？

Great question—this is a super common pain point when doing bulk file copies on ext4. The issue is that any time you add, remove, or modify files inside a directory, ext4 automatically updates that directory's modification timestamp (mtime)—and tools like cp -a or rsync -a don't have a built-in way to avoid this.

The workaround is straightforward: save the target directory's timestamps before copying, then restore them once the copy is done. Here's how to implement this reliably, both for single directories and bulk scripted operations:

1. Single Directory Workflow #

For a one-off copy, you can run these commands directly in your terminal:

# Define your target directory
TARGET_DIR="/path/to/your/target/dir"

# Save the target's current modification and access timestamps
# We format them to a string `touch` can easily parse
MTIME=$(stat -c "%Y%m%d%H%M.%S" "$TARGET_DIR")
ATIME=$(stat -c "%X%m%d%H%M.%S" "$TARGET_DIR")

# Run your copy operation (choose the tool that fits your needs)
# Example 1: Copy files with rsync, preserving all file attributes
rsync -a /path/to/source/files/* "$TARGET_DIR/"
# Example 2: Copy with cp, preserving permissions, timestamps, etc.
# cp -a /path/to/source/files/* "$TARGET_DIR/"

# Restore the target directory's original timestamps
touch -m -t "$MTIME" "$TARGET_DIR"  # Restore modification time
touch -a -t "$ATIME" "$TARGET_DIR"  # Restore access time (optional but thorough)

2. Bulk Processing Script #

If you need to handle dozens of source-target directory pairs, wrap the logic in a bash script for automation:

#!/bin/bash

# Define your source-target directory pairs (add as many as needed)
# Format: ["/path/to/source"]="/path/to/target"
declare -A COPY_PAIRS=(
    ["/home/user/docs"]="/backup/server/docs"
    ["/home/user/photos"]="/backup/server/photos"
    ["/home/user/projects"]="/backup/server/projects"
)

# Optional: Add trap to restore timestamps if script is interrupted
trap '
    if [ -n "$TARGET" ] && [ -n "$MTIME" ] && [ -n "$ATIME" ]; then
        echo "Script interrupted! Restoring timestamps for $TARGET"
        touch -m -t "$MTIME" "$TARGET"
        touch -a -t "$ATIME" "$TARGET"
    fi
    exit 1
' INT TERM

# Loop through each copy pair
for SOURCE in "${!COPY_PAIRS[@]}"; do
    TARGET="${COPY_PAIRS[$SOURCE]}"
    
    # Skip if source or target doesn't exist
    if [ ! -d "$SOURCE" ]; then
        echo "Warning: Source directory $SOURCE not found - skipping"
        continue
    fi
    if [ ! -d "$TARGET" ]; then
        echo "Warning: Target directory $TARGET not found - skipping"
        continue
    fi
    
    # Save target timestamps
    MTIME=$(stat -c "%Y%m%d%H%M.%S" "$TARGET")
    ATIME=$(stat -c "%X%m%d%H%M.%S" "$TARGET")
    
    # Run bulk copy (--delete removes files in target that aren't in source; remove if not needed)
    rsync -a --delete "$SOURCE/" "$TARGET/"
    
    # Restore timestamps
    touch -m -t "$MTIME" "$TARGET"
    touch -a -t "$ATIME" "$TARGET"
    
    echo "Successfully processed: $SOURCE -> $TARGET (timestamps restored)"
done

Key Notes #

• Permissions: Make sure the user running the script has write access to the target directory—otherwise touch won't be able to restore the timestamps.
• CTime Caveat: You can't manually modify a directory's change timestamp (ctime) on ext4 (it tracks when the directory's metadata was last altered, including timestamp changes). But since you're only concerned with preserving the modification time (mtime), this isn't an issue.
• Tool Flexibility: You can swap rsync for cp or another copy tool—just ensure it preserves the file attributes you need (like permissions, file timestamps).

内容的提问来源于stack exchange，提问作者Robert Murphy