Silencing SSH Output: Efficiently Logging Only Errors from Remote Commands #

This comprehensive guide details effective strategies for suppressing unwanted output from SSH commands while selectively logging only error messages. We’ll explore various techniques, focusing on practical implementation and troubleshooting common issues. The goal is to streamline your remote execution workflows and improve the clarity of your logs.

Understanding the Problem: Unwanted SSH Output #

When executing commands remotely via SSH, extraneous messages can often clutter the output, obscuring critical error information. Common offenders include connection messages (“Connection to… established”), authorization banners ("!!! AUTHORIZED USE ONLY !!!"), and other informational messages that are irrelevant for logging purposes. These messages hinder efficient error tracking and log analysis. This article addresses this challenge by offering several solutions to filter and redirect output, focusing on preserving only error messages.

The Standard SSH Behavior and its Limitations #

Standard SSH operation inherently produces various output streams – standard output (stdout), and standard error (stderr). By default, both are displayed. While redirecting stderr (2>&1) merges it with stdout, this still leaves you with a stream containing both wanted and unwanted information. Simple grep commands, while appearing as a quick solution, can sometimes inadvertently filter out genuine error messages containing words they also target for removal.

Method 1: Leveraging SSH’s -q (Quiet) Option #

The simplest approach to minimize SSH output is to employ the -q or --quiet option. This flag instructs SSH to suppress most connection and authentication messages, reducing the overall volume of information displayed.

Implementation: #

ssh -q -o StrictHostKeyChecking=no user@${IPADDRESS} "
  $(cat);
  IFERROR=$(checkscript);
  echo "$IFERROR"
" > ${SSHTEMPFILE} 2>&1

This modified command utilizes -q to significantly reduce the volume of output. However, it may not completely eliminate all informational messages, and crucially, it won’t prevent errors from being displayed alongside other output. The redirection to ${SSHTEMPFILE} remains essential for capturing the combined stdout and stderr.

Method 2: Advanced Filtering with grep and Regular Expressions #

This method offers more precise control over filtering. Instead of broadly removing words like “AUTHORIZED,” we’ll employ regular expressions to target specific patterns associated with error messages.

Identifying Error Patterns #

The first step involves analyzing the output of your checkscript to pinpoint patterns uniquely associated with error conditions. This typically involves searching for error codes, specific keywords, or even the structure of error messages themselves. These patterns become the foundation of your grep filtering.

Example: Regex for Error Filtering #

Let’s assume that error messages from your checkscript consistently begin with “ERROR:”. A suitable regular expression to capture these would be ^ERROR:. The caret (^) anchors the match to the beginning of the line.

Implementation: #

typeset -f | sshpass -e ssh -o StrictHostKeyChecking=no user@${IPADDRESS} "
  $(cat);
  IFERROR=$(checkscript);
  echo "$IFERROR"
" 2>&1 | grep -E '^ERROR:' > ${SSHTEMPFILE}

This command pipes the combined output to grep -E '^ERROR:', only preserving lines that match the specified regular expression. The -E flag enables extended regular expressions, providing greater flexibility. Remember to tailor the regular expression to the specific error message formats generated by your checkscript.

Method 3: Redirecting and Filtering Error Streams Separately #

For maximum control, redirect stdout and stderr separately, then process only the stderr stream. This isolates error messages and prevents unwanted output from interfering with your filtering.

Separating Standard Output and Standard Error #

The key here is to redirect stdout to /dev/null (discarding it) and capturing only stderr.

Implementation: #

typeset -f | sshpass -e ssh -o StrictHostKeyChecking=no user@${IPADDRESS} "
  $(cat);
  IFERROR=$(checkscript);
  echo "$IFERROR" 
" > /dev/null 2>${SSHTEMPFILE}

This command directs stdout to /dev/null effectively silencing all normal output. stderr, containing error messages, is redirected to ${SSHTEMPFILE} for logging. No further filtering might be necessary, depending on the nature of your error messages. However, you can enhance this approach with an additional grep command if more precise filtering is required.

Method 4: Using a Custom SSH Wrapper Script #

For robust and reusable solutions, consider creating a custom wrapper script that handles SSH execution and output management. This script can encapsulate your preferred filtering logic, offering consistency and simplifying complex workflows.

Creating a Modular and Reusable Solution #

A custom wrapper script allows for more advanced error handling and logging features. It can even incorporate email notifications upon specific error conditions.

Example Wrapper Script (Bash): #

#!/bin/bash

# Function to execute command remotely and capture only errors
execute_remote() {
  local command="$1"
  local ip="$2"
  local user="$3"

  ssh -o StrictHostKeyChecking=no "$user@$ip" "$command" 2> >(grep -E '^ERROR:' >> "${SSHTEMPFILE}")
}

# Example usage
execute_remote "$(cat)" "${IPADDRESS}" "user"

This script defines a function execute_remote that takes the command, IP address, and username as arguments. It executes the command using SSH and redirects only stderr (using process substitution >(...)) to a grep command that filters for errors before appending to ${SSHTEMPFILE}.

Troubleshooting and Best Practices #

• Regular Expression Testing: Thoroughly test your regular expressions to ensure accurate error identification. Online regex testers are invaluable for this process.
• Error Message Consistency: Strive for consistent error message formats in your checkscript to simplify filtering.
• Log Rotation: Implement log rotation to prevent your log files from growing excessively large.
• SSH Configuration: Ensure your SSH configuration (~/.ssh/config) is properly set up to enhance security and streamline connection management.
• Error Handling in checkscript: Robust error handling within your checkscript itself can provide more informative error messages to capture.

By employing these strategies and incorporating best practices, you can effectively eliminate unwanted SSH output while maintaining clear and informative error logs, significantly improving the manageability and analysis of your remote execution processes. The choice of method depends on the complexity of your error messages and the level of control required. Remember to always prioritize secure SSH practices.