Mastering SFTP Transfers: Ensuring All Files Are Sent with Expect Scripts

In the realm of automated file transfers, especially within scheduled cron jobs, ensuring the integrity and completeness of data transmission is paramount. Many systems rely on robust scripting to handle these critical operations, and when dealing with secure file transfer protocols like SFTP, the nuances can be complex. This article, brought to you by revWhiteShadow, delves deep into a common challenge encountered when using expect scripts to manage SFTP transfers: a scenario where only one of two intended files is successfully uploaded. We will explore the intricacies of the expect command, potential pitfalls in its implementation within shell scripts, and provide comprehensive strategies to guarantee the successful transmission of all associated files, even when dealing with multiple data and control file pairs. Our aim is to offer a definitive guide that surpasses existing content in its detail and actionable solutions, ensuring your automated file transfer processes are both reliable and efficient.

Understanding the SFTP and Expect Script Interaction

The core of our discussion revolves around an expect script designed to automate SFTP operations. Such scripts are invaluable for tasks requiring interactive responses from a remote server, such as providing credentials, navigating directories, and executing commands like put or get. When a shell script, often orchestrated by cron, needs to perform an SFTP transfer, it typically invokes an expect script to manage the interactive session.

The provided scenario highlights a critical issue: the expect script appears to be exiting prematurely, leading to the loss of one file in a paired transfer. This suggests a timing or command execution problem within the expect script itself. Let’s dissect the typical workflow and the potential failure points.

The Anatomy of a Standard Expect SFTP Script

A typical expect script for SFTP will follow a pattern:

Spawn the SFTP process: This initiates the SFTP client and connects to the remote server. The spawn command is fundamental here, launching the specified command in a new process.
```
spawn sftp "$env(LOGINstring):/inbound/ach"
```
In this instance, $env(LOGINstring) is assumed to be an environment variable containing the user and server details, like user@remote-sftp-server.com. The target directory /inbound/ach is also specified.
Set the timeout: SFTP operations, especially file transfers, can take time. Setting an appropriate timeout value is crucial to prevent the script from aborting due to inactivity or slow network conditions. A timeout of 7200 seconds (2 hours) is quite generous, indicating an expectation of potentially large file transfers or a slow connection.
```
set timeout 7200
```
Send commands: The send command is used to send literal strings to the spawned process, mimicking user input. This is where commands like put are sent.
```
send "put $env(dataFILE)\n"
send "put $env(controlFILE)\n"
```
Here, $env(dataFILE) and $env(controlFILE) represent environment variables holding the names of the files to be uploaded. The \n signifies pressing the Enter key.
Exit the SFTP session: After completing the necessary operations, the exit command is sent to gracefully close the SFTP connection.
```
send "exit\n"
```
Interact with the process: The interact command allows the user to take control of the spawned process. In automated scripts, it’s often used to pause execution after a series of send commands, allowing the user to see the output or to transition control back to the calling shell script in a controlled manner.
```
interact
```

The Root Cause: Premature Exit and Incomplete Transfers

The log data provided paints a clear picture:

Calling expect script to transmit the Welfare files...
spawn sftp ouraccount@remote-sftp-server.com:/inbound/ach
Connected to remote-sftp-server.com.
Changing to: /inbound/ach
sftp> put NEI006AHB08659_WELF
Uploading NEI006AHB08659_WELF to /inbound/ach/NEI006AHB08659_WELF
NEI006AHB08659_WELF 0% 0 0.0KB/s --:-- ETA
NEI006AHB08659_WELF 100% 1710 79.6KB/s 00:00
sftp> put NEI007CTB08659_WELF
exit
Uploading NEI007CTB08659_WELF to /inbound/ach/NEI007CTB08659_WELF
Returned from expect script...

We observe that the first put command for NEI006AHB08659_WELF completes successfully. However, the second put command for NEI007CTB08659_WELF is initiated, and then immediately after, the exit command is sent. The crucial observation is that the exit command is executed before the SFTP client has a chance to signal the completion of the put NEI007CTB08659_WELF transfer. The expect script, by default, executes commands sequentially without necessarily waiting for the full completion of an asynchronous operation like a file upload unless specifically instructed to do so.

This is a classic case of the expect script sending commands too rapidly. The send "put ...\n" command tells SFTP to start uploading, but the script then immediately proceeds to the next send "exit\n" without confirming that the upload has finished.

Strategies to Ensure Complete SFTP File Transmission

To overcome this challenge, we need to modify the expect script to explicitly wait for the completion of each put operation before proceeding. expect provides powerful mechanisms for pattern matching and waiting, which are key to resolving this issue.

1. Using `expect` to Wait for SFTP Prompts

The SFTP client typically returns to an sftp> prompt after a command is fully processed. We can leverage this by using expect to wait for this prompt after each put command.

Here’s a refined approach:

#!/usr/bin/expect -f

# Set environment variables (assuming they are passed or set in the calling script)
# set env(LOGINstring) "ouraccount@remote-sftp-server.com"
# set env(dataFILE) "NEI006AHB08659_WELF"
# set env(controlFILE) "NEI007CTB08659_WELF"

# Establish the SFTP connection
spawn sftp "$env(LOGINstring):/inbound/ach"
set timeout 7200

# Wait for the SFTP prompt before sending commands
expect {
    "sftp>" { }
    timeout { send_user "Timeout waiting for initial SFTP prompt\n"; exit 1 }
    eof { send_user "SFTP connection closed prematurely\n"; exit 1 }
}

# Upload the data file and wait for prompt
send "put $env(dataFILE)\n"
expect {
    "sftp>" { }
    timeout { send_user "Timeout waiting for prompt after uploading $env(dataFILE)\n"; exit 1 }
    eof { send_user "SFTP connection closed prematurely after uploading $env(dataFILE)\n"; exit 1 }
}

# Upload the control file and wait for prompt
send "put $env(controlFILE)\n"
expect {
    "sftp>" { }
    timeout { send_user "Timeout waiting for prompt after uploading $env(controlFILE)\n"; exit 1 }
    eof { send_user "SFTP connection closed prematurely after uploading $env(controlFILE)\n"; exit 1 }
}

# Exit the SFTP session
send "exit\n"
expect {
    "sftp>" { } # Sometimes exit might return to sftp> prompt
    eof { }      # Or it might just close the connection (eof)
    timeout { send_user "Timeout waiting for exit confirmation\n"; exit 1 }
}

# No need for 'interact' if we are just scripting the entire session
# interact

Explanation of Changes:

expect { "sftp>" { } ... } blocks: After each send command, we now use an expect block. This block instructs expect to wait until it sees the literal string "sftp>" in the output from the spawned SFTP process. Only when this prompt is received does expect consider the command preceding it to be fully processed and acknowledged by the SFTP client.
Error Handling: Added specific error messages and exit 1 for timeouts or unexpected end-of-file (eof) conditions, providing better diagnostics for failures.
Removed interact: In this fully automated scenario, interact is not necessary. It’s generally used when you want to pass control back to the user. Here, we want the script to complete its defined task.

4.1. Handling Varying SFTP Prompts and Output

While "sftp>" is a common prompt, some SFTP configurations or versions might have slightly different output. It’s crucial to inspect the exact output of your SFTP client if the above doesn’t work. You might need to adjust the pattern to match something else that reliably indicates command completion. Common variations could include:

A newline followed by the prompt: \n sftp>
Messages about file transfer progress: While we want to avoid parsing progress directly, sometimes the prompt appears after a summary line.

How to determine the correct prompt:

The best way to determine the precise prompt is to run the sftp command manually in your terminal and observe the output.

Run sftp youraccount@remote-sftp-server.com:/inbound/ach
Enter your password if prompted.
Manually type put your_file_name.
Observe what appears on the screen after the upload is complete. This will tell you what pattern expect should look for.

2. Using `expect`’s `glob` and `exact` Matching

The expect command is very flexible. We can use glob matching for more general patterns or exact for precise string matching. For SFTP prompts, exact matching is usually sufficient. However, if the output contains dynamic elements, glob might be useful.

For example, if the SFTP client output looked like this after a successful transfer:

Uploading MYFILE.dat to /remote/dir/MYFILE.dat MYFILE.dat 100% 12345 67.89KB/s 00:01 sftp>

The prompt is still "sftp>". However, if the prompt was dynamic, say user@host:/path/sftp>, you might need to adjust.

3. Robust Timeout Management

The set timeout 7200 is a global setting. It applies to the time expect waits for any of the patterns in an expect block. If a single upload is very large, or the network is very slow, expect might time out.

Advanced Timeout Handling:

For even more robustness, you can set timeouts within specific expect blocks if needed, though the initial global timeout is often sufficient. More importantly, understanding why a timeout might occur is key. It’s usually due to:

Very large files: Transfers taking longer than the timeout.
Network congestion or instability: Leading to slow transfer rates.
Server-side issues: The remote SFTP server might be slow to respond or process.

In such cases, increasing the timeout might be a temporary fix, but investigating the underlying cause of slow transfers is recommended.

4. Ensuring File Existence Before Uploading

While not directly related to the premature exit, a robust script should also ensure that the files it intends to upload actually exist in the local directory. This can be done in the calling shell script before invoking the expect script.

Example in the Calling Shell Script:

#!/bin/bash

# ... other script logic ...

DATAFILE="NEI006AHB08659_WELF"
CONTROLFILE="NEI007CTB08659_WELF"

if [ -f "$DATAFILE" ]; then
    export dataFILE="$DATAFILE"
    if [ -f "$CONTROLFILE" ]; then
        export controlFILE="$CONTROLFILE"
        # Now call the expect script
        /path/to/your/sftp_upload.exp
    else
        echo "Error: Control file $CONTROLFILE not found."
        # Handle error: skip, log, notify
    fi
else
    echo "Error: Data file $DATAFILE not found."
    # Handle error: skip, log, notify
fi

This adds a layer of protection, ensuring that the expect script is only called when the necessary files are present locally.

5. Handling Multiple File Pairs

The original problem description mentions that the main script looks for 1 of 6 possible data files and creates a control file for each. This implies a loop in the main shell script. The expect script is likely called within this loop.

How to adapt the expect script for multiple files:

If your expect script is designed to handle a single dataFILE and controlFILE pair, and the main shell script iterates through these pairs, then the expect script as modified above will work correctly within each iteration of the loop.

Example of the calling shell script structure:

#!/bin/bash

# Define the possible file prefixes or patterns
file_patterns=("NEI006AHB" "NEI007CTB" ...) # Example patterns

for pattern in "${file_patterns[@]}"; do
    # Find corresponding data and control files based on the pattern
    # This logic will depend on your naming conventions

    # Example: Assuming files are named like PREFIX_WELF and PREFIX_CTRL
    data_file=$(find . -maxdepth 1 -name "${pattern}_WELF" -print -quit)
    control_file=$(find . -maxdepth 1 -name "${pattern}_CTRL" -print -quit)

    if [ -n "$data_file" ] && [ -n "$control_file" ]; then
        echo "Processing pair: $data_file and $control_file"
        export dataFILE="$data_file"
        export controlFILE="$control_file"
        # Ensure correct path if files are not in the current directory
        # export dataFILE="/path/to/data/$data_file"
        # export controlFILE="/path/to/data/$control_file"

        /path/to/your/sftp_upload.exp
        # Check the exit status of the expect script
        if [ $? -ne 0 ]; then
            echo "SFTP upload failed for $data_file and $control_file"
            # Handle failure: log, retry, alert
        else
            echo "SFTP upload successful for $data_file and $control_file"
            # Optionally move or delete files after successful upload
            # mv "$data_file" processed/
            # mv "$control_file" processed/
        fi
    else
        echo "Skipping pattern $pattern: Missing data or control file."
    fi
done

The key is that each call to the expect script should be configured with the correct dataFILE and controlFILE environment variables for the current pair being processed.

6. Advanced SFTP Scripting with `lftp` or `sftp` Batch Mode

While expect is a powerful tool for interactive sessions, for purely command-driven SFTP transfers, using sftp in batch mode or a more advanced client like lftp might offer cleaner solutions, especially if you don’t need complex interactive logic.

sftp Batch Mode:

You can create a batch file for sftp:

batch_commands.txt:

put NEI006AHB08659_WELF
put NEI007CTB08659_WELF
quit

Then execute it: sftp -b batch_commands.txt ouraccount@remote-sftp-server.com:/inbound/ach

However, sftp batch mode also suffers from the same sequential execution problem without explicit confirmation. You still need a mechanism to ensure each put completes. expect remains a good choice for this granular control.

lftp:

lftp is a sophisticated command-line file transfer program that supports various protocols, including SFTP. It offers more advanced features for scripting and error handling.

Example lftp script:

#!/bin/bash

# Set SFTP connection details
REMOTE_USER="ouraccount"
REMOTE_HOST="remote-sftp-server.com"
REMOTE_DIR="/inbound/ach"
SFTP_LOGIN_STRING="${REMOTE_USER}@${REMOTE_HOST}"

# Assumes password is provided via .netrc or similar secure method
# Or use the -p option for password, but this is less secure
# lftp sftp://${SFTP_LOGIN_STRING}:${REMOTE_PORT}${REMOTE_DIR} -e "set sftp:auto-confirm yes; put \$dataFILE; put \$controlFILE; exit"

# Using lftp with explicit commands and waiting for transfer completion implicitly
# lftp often handles this better by default than expect's simple send.
# The '-e' option executes commands and then exits.
# 'set sftp:auto-confirm yes' can be useful for some operations.
# The core idea is to ensure lftp processes each command fully.

# A more robust lftp approach might involve:
# - Connect
# - cd to directory
# - Mirror or put individual files, checking return codes

# For this specific issue, ensuring each put is complete, lftp often behaves more predictably.
# If your script is calling this, you'd set the env vars as before.

lftp -u "$REMOTE_USER" sftp://$REMOTE_HOST:$REMOTE_PORT$REMOTE_DIR <<EOF
set sftp:auto-confirm yes
set net:timeout 7200
put "$env(dataFILE)"
put "$env(controlFILE)"
exit
EOF

lftp often has better built-in handling for the completion of transfer operations, meaning the put commands might implicitly wait for completion before executing the next command in the sequence, unlike the default behavior of expect’s simple send.

However, given the prompt and the existing expect script, modifying the expect script is the most direct solution.

Best Practices for Robust SFTP Automation

To ensure the highest level of reliability for your automated SFTP processes, consider these best practices:

Secure Credential Management: Avoid hardcoding passwords in scripts. Use SSH keys for passwordless authentication or securely manage credentials using methods like sshpass (with caution) or by leveraging .netrc files for ftp/lftp or SSH agent forwarding for sftp. For expect scripts, storing passwords securely or using key-based auth is essential.
Error Logging: Implement comprehensive logging. Record every step, including successful transfers, failures, timeouts, and any unexpected output. This is invaluable for debugging.
Atomic Operations: If possible, use operations that are atomic or have rollback capabilities. For example, uploading a file and then renaming a corresponding “ready” file on the remote server.
File Integrity Checks: After transfer, consider performing checks like comparing file sizes or MD5 checksums to ensure data integrity. This can be done by downloading the file back or by having the remote system generate a checksum.
Retry Mechanisms: For transient network issues, implement a retry mechanism for failed transfers. This can be done in the calling shell script.
Idempotency: Design your scripts to be idempotent – running them multiple times should have the same effect as running them once. This is particularly important if a cron job might accidentally trigger twice.
Monitoring: Set up monitoring to alert you when transfers fail or when the cron job doesn’t run as expected.

Conclusion: The Power of Precise Expectation

The challenge of a single file upload failing in an expect SFTP script is a common hurdle, often stemming from the script’s failure to wait for the explicit completion of the transfer operation. By incorporating expect blocks that specifically wait for the SFTP prompt after each put command, we can create a much more robust and reliable automation. This granular control ensures that each file is fully transmitted before the script proceeds to the next step, such as sending the exit command.

At revWhiteShadow, we understand the critical nature of these automated processes. Implementing the strategies outlined—focusing on precise pattern matching, robust error handling, and secure credential management—will empower your scripts to outrank any poorly implemented counterparts. By treating each step with the necessary attention to detail, you can build an SFTP automation system that is not only functional but also exceptionally resilient and trustworthy. Remember to always test thoroughly in a development environment before deploying to production. This detailed approach ensures that your data reaches its destination, completely and securely, every single time.

Shell .exp script only sending 1 of 2 files

Mastering SFTP Transfers: Ensuring All Files Are Sent with Expect Scripts #

Understanding the SFTP and Expect Script Interaction #

The Anatomy of a Standard Expect SFTP Script #

The Root Cause: Premature Exit and Incomplete Transfers #

Strategies to Ensure Complete SFTP File Transmission #

1. Using expect to Wait for SFTP Prompts #

4.1. Handling Varying SFTP Prompts and Output #

2. Using expect’s glob and exact Matching #

3. Robust Timeout Management #

4. Ensuring File Existence Before Uploading #

5. Handling Multiple File Pairs #

6. Advanced SFTP Scripting with lftp or sftp Batch Mode #

Best Practices for Robust SFTP Automation #

Conclusion: The Power of Precise Expectation #