How to best capture and log scp output?

Mastering SCP Output: Capture and Log Transfers Effectively

Learn various techniques to capture and log the output of scp commands, ensuring reliable record-keeping and troubleshooting for your file transfers.

Secure Copy Protocol (scp) is a widely used command-line utility for securely transferring files between local and remote hosts, or between two remote hosts. While scp provides a progress indicator during transfers, capturing its full output, including success messages, errors, and transfer details, is crucial for auditing, debugging, and automation. This article explores several robust methods to effectively capture and log scp output.

Understanding SCP Output Streams

Before diving into logging methods, it's important to understand how scp (and most Unix commands) handles output. Commands typically produce two main types of output streams:

• Standard Output (stdout): This stream is used for normal, expected output, such as file transfer progress, successful completion messages, or directory listings.
• Standard Error (stderr): This stream is reserved for error messages, warnings, and diagnostic information.

To effectively capture all scp output, you need to redirect both of these streams. The following diagram illustrates this concept.

flowchart TD
    A[SCP Command] --> B{Output Streams}
    B --> C[stdout (Progress, Success)]
    B --> D[stderr (Errors, Warnings)]
    C --> E[Terminal / Log File]
    D --> E

SCP Command Output Streams

Method 1: Redirecting Output to a File

The most straightforward way to capture scp output is by redirecting its standard output and standard error streams to a file. This ensures that all messages, whether successful or erroneous, are recorded.

scp /path/to/local/file user@remote:/path/to/remote/directory > scp_output.log 2>&1

Redirecting all scp output to a log file.

Let's break down the redirection:

• > scp_output.log: This redirects stdout (file descriptor 1) to the file scp_output.log. If the file doesn't exist, it's created; if it does, its content is overwritten.
• 2>&1: This redirects stderr (file descriptor 2) to the same location as stdout (file descriptor 1). This ensures that both normal output and error messages go into scp_output.log.

To append to an existing log file instead of overwriting it, use >>:

scp /path/to/local/file user@remote:/path/to/remote/directory >> scp_output.log 2>&1

scp /path/to/local/file user@remote:/path/to/remote/directory 2>&1 | tee scp_output.log

Using tee to log output while displaying it on the terminal.

Method 2: Using the script Command for Session Logging

The script command is a powerful utility that records everything printed to your terminal, including command inputs and outputs, into a typescript file. This is particularly useful for capturing interactive sessions or when you want a complete record of a series of commands, not just a single scp execution.

1. Start the script session

Open your terminal and type script my_scp_session.log. This will start recording all subsequent terminal activity into the file my_scp_session.log.

2. Execute your scp command(s)

Perform your scp operations as usual. All progress, success messages, and any errors will be captured by the script command.

3. Exit the script session

Once you're done, type exit to stop recording. The my_scp_session.log file will now contain a complete transcript of your session.

script my_scp_session.log
scp -r /local/dir user@remote:/remote/dir
exit

Example of using script to log an interactive scp session.

Method 3: Capturing Exit Status for Scripting

Beyond just logging the textual output, it's often critical in scripts to know if an scp command succeeded or failed. The exit status (or return code) of a command provides this information. A zero exit status typically indicates success, while a non-zero status indicates an error.

scp /path/to/local/file user@remote:/path/to/remote/directory > scp_output.log 2>&1

if [ $? -eq 0 ]; then
    echo "SCP transfer successful." >> scp_status.log
else
    echo "SCP transfer failed with exit code $?." >> scp_status.log
    # You can also log the content of scp_output.log here for debugging
    cat scp_output.log >> scp_status.log
fi

Checking scp exit status and logging the result.

In this example:

• $? holds the exit status of the most recently executed command (scp in this case).
• The if statement checks if the exit status is 0 (success) or non-zero (failure).
• Appropriate messages are appended to scp_status.log.