How to Resume an Interrupted SCP File Transfer in Linux

How to Resume an Interrupted SCP File Transfer in Linux

SCP (Secure Copy Protocol) is a reliable and secure method for transferring files between machines over a network. However, sometimes your file transfer can be interrupted due to network issues, server timeouts, or simply because the connection was dropped. When transferring large files, starting from scratch can be frustrating and time-consuming. Fortunately, there are ways to resume an interrupted SCP file transfer without having to re-transfer the entire file. In this guide, we’ll cover practical solutions to help you efficiently handle interruptions and resume file transfers on Linux.

Understanding the Limitations of SCP

SCP is widely used because of its simplicity and security. It utilizes SSH (Secure Shell) to encrypt and authenticate file transfers. The command syntax is straightforward, making it a favorite among sysadmins and developers:

scp user@remote:/path/to/file /local/destination

Despite its advantages, SCP lacks built-in functionality to resume file transfers. If a transfer is interrupted, SCP does not automatically allow you to pick up where it left off. This limitation means that, by default, you have to start the entire transfer again, which is inefficient for large files or slow connections.

Alternative Methods to Resume Interrupted SCP Transfers

Since SCP itself doesn’t support resuming transfers, we have to use alternative methods. Below are several techniques you can use to achieve this.

1. Using rsync to Resume Transfers

rsync is a powerful file transfer tool that offers a way to resume interrupted transfers seamlessly. It works similarly to SCP, using SSH for secure connections, but it has the added benefit of supporting incremental transfers. This makes it perfect for resuming large file transfers.

Step-by-Step Guide to Using rsync

Suppose your SCP transfer was interrupted while copying a large file from a remote server. To resume using rsync, follow these steps:

1. Install rsync on your system if it’s not already installed:
   sudo apt-get install rsync # On Debian/Ubuntu
sudo yum install rsync # On CentOS/RHEL
2. Use the rsync command with the -P or --partial --progress flags to resume the transfer: rsync -P user@remote:/path/to/largefile /local/destination
   • The -P flag combines --partial (which allows resuming partial transfers) and --progress (which shows the progress of the transfer).
3. Example of resuming a large file transfer: rsync -P [email protected]:/home/user/bigfile.iso /home/localuser/bigfile.iso
4. Using a Different SSH Port: If your SSH server is running on a non-standard port (e.g., 23), you can specify the port using the -e option:
   rsync -P -e "ssh -p 23" user@remote:/path/to/largefile /local/destination Example:
   rsync -P -e "ssh -p 23" user@remote:/path/to/largefile /local/destination This command tells rsync to use SSH on port 23 to connect to the remote server.

Why Use rsync?

• Efficiency: Only the parts of the file that were not transferred will be copied.
• Progress Display: The --progress flag shows real-time transfer progress.
• Flexibility: Works over SSH just like SCP, maintaining security.

2. Using wget for HTTP/HTTPS Transfers

If you are downloading a file via HTTP or HTTPS and the transfer was interrupted, wget can resume the download effortlessly.

Resuming a Download with wget

1. Download the file using wget with the -c (continue) option:
   wget -c https://example.com/largefile.zip
2. If the download is interrupted, run the same command again, and wget will resume where it left off.

Why Use wget?

• Simple Command: Easy to remember and use.
• Built-In Resume Feature: The -c flag makes resuming downloads effortless.
• Versatility: Supports downloads over HTTP, HTTPS, and FTP.

3. Using lftp for FTP/SFTP Transfers

For FTP or SFTP file transfers, lftp is an excellent command-line utility that supports resuming interrupted downloads.

Resuming Transfers with lftp

1. Install lftp if it’s not already on your system:
   sudo apt-get install lftp # On Debian/Ubuntu
sudo yum install lftp # On CentOS/RHEL
2. Connect to the remote server using lftp: lftp [email protected]
3. Navigate to the file’s location and start the download with pget (parallel get), which supports resuming: pget -c largefile.zip
4. If the download is interrupted, simply reconnect and run the same pget -c command to resume.

Why Use lftp?

• Parallel Downloads: Speeds up transfers by downloading parts of the file simultaneously.
• Resume Support: The -c option ensures interrupted downloads can be resumed.
• Interactive Shell: Similar to a traditional FTP client, making it easy to use.

Tips for Avoiding Interrupted Transfers

While knowing how to resume transfers is helpful, preventing interruptions in the first place is even better. Here are some tips to ensure smooth file transfers:

1. Use a Stable Network Connection: Ensure you’re connected to a reliable network to avoid dropouts.
2. Increase SSH Timeout Settings: Modify SSH settings to prevent timeouts: Add the following to /etc/ssh/sshd_config on the server: ClientAliveInterval 60 ClientAliveCountMax 100 This keeps connections alive longer, reducing the risk of timeouts.
3. Use Screen or Tmux: Run your transfers inside a screen or tmux session so that even if your terminal disconnects, the transfer continues in the background.
4. Split Large Files: If possible, split large files into smaller parts using the split command and transfer them individually:
   split -b 500M largefile.tar.gz part_

Conclusion

While SCP doesn’t support resuming interrupted file transfers, tools like rsync, wget, and lftp provide excellent alternatives to handle this efficiently. rsync is particularly versatile because it works over SSH and resumes large file transfers effortlessly. By understanding these tools and applying best practices for stable connections, you can make file transfers in Linux more reliable and less frustrating.

Whether you’re a sysadmin managing remote servers or a developer handling large datasets, these techniques will save you time and ensure your file transfers are completed smoothly. Keep these commands in your toolkit, and you’ll never have to worry about restarting a huge transfer from scratch again.