Curl uploads empty files via SFTP without errors when the source file path is incorrect, permissions prevent reading, or the SFTP server’s chroot jail restricts write access to the target directory. This behavior often masks underlying permission or path resolution issues, leading to silent failures where Curl reports success but the remote file remains zero bytes. Diagnosing these problems requires verifying local file integrity, checking server-side directory permissions, and ensuring the SFTP subsystem correctly interprets the upload destination.
Verifying Local File Integrity and Permissions
The most common cause of an empty upload is that Curl cannot read the source file. Even if the file exists, restrictive permissions on the local machine can prevent the data from being streamed to the SFTP server. You must ensure that the user running the Curl command has read access to the file. Use the
ls -l command to inspect the permissions of the local file. If the output shows no read permission for the current user, modify it using
chmod. For example,
chmod 644 filename.txt grants read access to the owner and group.
Additionally, verify that the file is not actually empty before attempting the upload. Run
cat filename.txt or
wc -c filename.txt to check the byte count. If the local file is empty, Curl will faithfully upload an empty file because it only transfers what it reads. This step eliminates client-side data issues before investigating network or server-side configurations. A related issue involving SSH authentication can be found in our guide on
Pass pw for remote user in bash line when ssh from linux to windows, which helps clarify credential handling in automated scripts.
Checking SFTP Server Directory Permissions
When Curl connects to an SFTP server, it attempts to open the target file for writing. If the target directory lacks write permissions for the authenticated user, some SFTP implementations may fail silently or create an empty file instead of throwing a standard error. Check the permissions of the remote directory using an SFTP client or shell access. The directory should have write permissions (e.g.,
drwxr-xr-x) for the user. If the directory is owned by root and the user is not in the sudoers file, uploads will fail.
In environments using OpenSSH, you might encounter "Permission denied" errors if the
authorized_keys file is misconfigured. Ensure that the public key is correctly appended to the
~/.ssh/authorized_keys file on the server. Our article on
Fixing Permission denied with OpenSSH: Manually adding authorized_keys provides detailed steps for resolving key-based authentication issues that could indirectly affect SFTP operations. Always test connectivity and basic file creation manually before relying on Curl automation.
Understanding Chroot Jails and Path Resolution
Many secure SFTP setups use a chroot jail to restrict users to a specific directory. In this configuration, the path specified in the Curl command is relative to the chroot directory, not the system root. If you specify an absolute path like
/uploads/file.txt inside a chrooted environment where the root is
/home/user, Curl might look for
/home/user/uploads/file.txt on the actual filesystem, which does not exist. Instead, you should use a relative path or ensure the absolute path aligns with the chroot perspective.
Verify the SFTP subsystem configuration in
/etc/ssh/sshd_config. Look for
Match Group sftpusers blocks that define
ChrootDirectory. Understanding how this restriction works is crucial for troubleshooting path-related empty uploads. For more details on configuring chroot jails, refer to our guide on
SSH User chroot Jail: Restricting SFTP Access to Specific Directory. Misinterpreting these paths is a frequent source of confusion for administrators managing restricted SFTP accounts.
Debugging Curl Output and Exit Codes
Curl provides verbose output that can reveal hidden errors. Use the
-v flag to enable verbose mode, which displays the handshake, authentication, and transfer details. Look for messages indicating successful file opening or any warnings about unsupported features. Additionally, check the exit code immediately after execution. While some older versions or specific configurations might behave unexpectedly, modern Curl typically returns a non-zero exit code on failure. If you see exit code 0 but the file is empty, the issue is likely logical (wrong path or empty source) rather than a transport error.
For cases involving byte remaining issues or partial transfers, consult our troubleshooting guide on
Troubleshooting CURL Exit Code 18 on Amazon Linux 2: SFTP Byte Remaining Issues. Although this focuses on exit code 18, the diagnostic techniques for network interruptions are applicable to broader SFTP stability problems. Always combine verbose logging with exit code analysis to pinpoint whether the failure occurs during authentication, file opening, or data transfer.
Ensuring Correct Curl Syntax and Options
Incorrect usage of Curl flags can lead to unexpected behavior. When uploading via SFTP, the correct syntax involves specifying the source file and the remote URL. The command structure should resemble
curl --upload-file ./localfile sftp://user@host/path/to/remotefile. Avoid mixing HTTP-specific options like
--ssl-no-revoke, which are irrelevant for SFTP and can cause confusion. Ensure that the protocol is explicitly set to
sftp:// and not
ftp:// unless you are using FTP over SSH, which is less common.
If you are scripting this process, consider using
curl -T as a shorthand for
--upload-file. This reduces the chance of syntax errors. Test the command interactively before integrating it into scripts. Verify that the remote path includes the filename, not just the directory. Some users mistakenly provide only the directory path, causing Curl to attempt to upload a file with no name, resulting in an empty or failed operation. Always double-check the full remote URL string for accuracy.
Frequently Asked Questions
Why does curl return exit code 0 for an empty file?
Curl returns exit code 0 if it successfully completes the transfer protocol, even if the source file was empty. It does not validate the content size against expectations unless explicitly configured to do so. This behavior indicates that the connection succeeded but the data stream was empty, pointing to a source file or permission issue rather than a network failure.
How can I check if my SFTP server uses a chroot jail?
You can check the
/etc/ssh/sshd_config file on the server for
ChrootDirectory directives within
Match blocks. Alternatively, ask your system administrator if the account is restricted to a specific home directory. If you cannot access the config file, try creating a file in the root of your expected path; if it fails, you are likely in a chrooted environment.
What is the difference between sftp:// and ftp:// URLs in curl?
uses the SSH File Transfer Protocol, which runs over an encrypted SSH connection and supports key-based authentication.
uses the plain FTP protocol, which is unencrypted and generally less secure. Using the wrong protocol can lead to authentication failures or security warnings, especially in modern environments where FTP is disabled.
Can I use curl to download files from SFTP servers?
Yes, you can use curl to download files from SFTP servers by using the
-O or
-o option followed by the remote URL. The syntax is similar to uploading, but you specify the remote file path and let curl determine the local destination. This allows for bidirectional transfers using the same tool and authentication methods.
How do I fix permission denied errors during SFTP upload?
Ensure that the remote directory has write permissions for the user account used in the Curl command. On the server side, check the ownership of the target directory and adjust it using
chown or
chmod as needed. If using key-based authentication, verify that the
authorized_keys file has the correct permissions (
600) and ownership.