Timestamps Change on File Copy

Timestamps Change on File Copy | Generated by AI

Home 2026.04

Question: When copying a file to another file, if the content is the same, will mtime stay unchanged while ctime changes?

Answer:

Short answer: No — both mtime and ctime usually change when you use cp.

Let’s break it down carefully.

1. What `cp` actually does

When you run:

cp source.txt dest.txt

cp does not compare content. It simply:

Opens/creates dest.txt
Writes data into it (even if identical)
Updates metadata

2. What happens to timestamps

Case A: Destination file is overwritten

Even if the content is identical:

mtime (modification time) → ✅ changes Because the file is rewritten
ctime (change time) → ✅ changes Because metadata (write operation) changed

👉 So your assumption is incorrect:

❌ mtime does NOT stay the same ✅ It will update

Case B: Destination file does NOT exist

A new file is created:

mtime → time of creation (write)
ctime → time of creation

Case C: Using `cp -p` (preserve timestamps)

cp -p source.txt dest.txt

Now behavior changes:

mtime → ✅ preserved (same as source)
ctime → ❗ still changes

Why?

ctime is controlled by the filesystem
It updates whenever metadata changes (including setting timestamps)

👉 Important rule:

You cannot manually preserve ctime

3. Key Insight

Even if file content is identical:

cp does not check equality
Writing = modification → updates mtime
Any metadata change → updates ctime

4. When would `mtime` NOT change?

Only if:

You don’t rewrite the file at all

Example tools that avoid rewriting:

rsync --ignore-existing
rsync --size-only
rsync --checksum

These can skip copying if files are identical.

5. Summary

Scenario	mtime	ctime
`cp` overwrite	✅ changes	✅ changes
`cp` new file	✅ new time	✅ new time
`cp -p`	❌ preserved	✅ changes
No write happens	❌ unchanged	❌ unchanged

6. Practical Tip

If you want to avoid unnecessary timestamp changes, use:

rsync -a --ignore-existing

rsync -a --checksum

If you want, I can explain why ctime is fundamentally unmodifiable at the filesystem level (this is a subtle but important Unix design decision).

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