Renaming and Removing Files in OS

 

Renaming Files in OS

📌 System Call: rename(old, new)

• Used to change the name of a file

• Takes:

  • old → current file name

  • new → new file name

🔑 Key Concept: Atomic Operation

• The rename operation is atomic:

  • After a crash → file is either:

    • completely renamed, OR

    • still has the old name

  • ❌ No partial or inconsistent state

💡 Why is this important?

It enables safe file updates, especially in applications like editors or databases.

📖 Example Workflow (Safe Update)

open("foo.txt.tmp")
write(new data)
fsync(fd)
close(fd)
rename("foo.txt.tmp", "foo.txt")

👉 Steps:

1. Write new data to a temporary file

2. Force it to disk using fsync()

3. Atomically replace the old file using rename()

✔ Ensures:

• No data corruption

• Crash-safe updates

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Removing Files in OS

📌 System Call: unlink(filename)

• Used to delete a file

• Removes the file name from directory

⚠️ Important Insight

👉 unlink() does NOT immediately erase file data

Instead:

• Removes the directory entry

• Actual data is deleted only when no references remain

🔑 Why called “unlink”?

Because it:

• Unlinks the filename from its inode

• The file disappears from the directory

📖 Example

rm foo

Internally:

unlink("foo");

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Getting File Status (Metadata)

📌 System Calls:

• stat(pathname)

• fstat(fd)

📊 Purpose:

Retrieve metadata about a file

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📦 Important Metadata Fields

Field	Meaning
st_ino	Inode number (unique file ID)
st_size	File size (bytes)
st_mode	Permissions
st_uid, st_gid	Owner and group
st_nlink	Number of links
st_atime	Last access time
st_mtime	Last modification time
st_ctime	Metadata change time

---

💡 Example Command

stat file

Output shows:

• Size

• Permissions

• Owner

• Timestamps

• Inode number

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🔷 Key Concept: Inode

• Each file is represented internally by an inode

• Stores:

  • Metadata

  • Disk block locations

• Directory maps:

  filename → inode number
  

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🔷 Summary 

• rename()

  • Atomic file renaming

  • Used for safe updates

• unlink()

  • Removes file name (not immediate data deletion)

  • Deletes file when no references remain

• stat() / fstat()

  • Retrieves file metadata

  • Uses inode structure internally