How to Use the Command 'Chat' (with Examples)

The ‘chat’ command serves as a scripting tool that automates conversations between a computer and a modem or other serial device. Its primary use is to establish PPP (Point-to-Point Protocol) connections, facilitating communication over telephone lines and serial connections. By scripting interactions, it reduces repetitive manual inputs and ensures error-free connections.

Use Case 1: Execute a Chat Script Directly from the Command Line

Code:

chat 'expect_send_pairs'

Motivation:

Executing a chat script directly from the command line is convenient for temporary or quick configurations where a full script file isn’t necessary. It allows system administrators to swiftly input a command to automate a connection when a persistent script file is not desired.

Explanation:

• chat: Invokes the chat program to start the communication automation process.
• 'expect_send_pairs': A pair of strings where ’expect’ is what the chat is looking out for as input from the device, and ‘send’ is the response that is sent back. This mechanism automates the bidirectional communication flow.

Example Output:

Imagine the chat interaction is like a protocol handshake where the system expects a “READY” input from the modem and responds with “ATZ”. The output displayed would indicate the strings sent and received without complications or intervention.

Use Case 2: Execute a Chat Script from a File

Code:

chat -f 'path/to/chat_script'

Motivation:

Using a file-based chat script is beneficial when dealing with complex scripts that contain multiple expect-send pairs. This method makes it easier to manage, modify, and reuse scripts, ensuring consistency across different environments.

Explanation:

• chat: Launches the chat interaction tool.
• -f: Specifies that a file will be used as the source of the script, allowing for more organized and maintainable scripting.
• 'path/to/chat_script': The path to the script file containing the expect-send pairs. This file dictates the flow of communication.

Example Output:

The interaction follows the instructions in the provided chat script file. It logs the expected and sent pairs, showing a successful sequence handled in a structured and pre-defined manner.

Use Case 3: Set a Custom Timeout for Expecting a Response

Code:

chat -t timeout_in_seconds 'expect_send_pairs'

Motivation:

Custom timeouts are crucial in environments where network delays or unresponsive devices might cause the standard timeout to be too short, leading to failed interactions. By adjusting the timeout, administrators can account for specific network conditions or device latency.

Explanation:

• chat: Starts the chat session tool for automated interaction.
• -t timeout_in_seconds: Sets the period to wait for an expected input before considering the operation failed.
• 'expect_send_pairs': The interaction instructions provided as pairs of expected input and responses.

Example Output:

If the response time between your server and a device is longer than average due to network congestion, setting this option prevents timeout errors and maintains smooth communication.

Use Case 4: Enable Verbose Output to Log the Conversation to Syslog

Code:

chat -v 'expect_send_pairs'

Motivation:

Verbose output is essential for diagnosing issues in automated connections by providing a detailed log of the chat session. It helps in debugging by showing exactly what interactions occurred and which expected inputs were not met.

Explanation:

• chat: Initiates the automated communication script.
• -v: Activates verbose logging which records the entire communication process in the system log, crucial for transparency and troubleshooting.
• 'expect_send_pairs': The pair instructions constituting the dialog sequence between the system and the device.

Example Output:

The system logs interactions showing a series of expect-send outcomes, helpful for administrators to verify or investigate the connection.

Use Case 5: Use a Report File to Log Specific Strings Received During the Conversation

Code:

chat -r path/to/report_file 'expect_send_pairs'

Motivation:

A report file targets specific outputs in the chat, capturing important strings for later analysis rather than sifting through verbose logs. It’s valuable for tracking specific handshake issues or alerting systems to critical error messages.

Explanation:

• chat: Commences the automation of communication execution.
• -r path/to/report_file: Directs specific strings received during communication to a separate report file for review.
• 'expect_send_pairs': Dictates the automated interaction pairs.

Example Output:

The report file contains only the key strings identified during the session, filtering out noise while preserving critical interaction details.

Use Case 6: Dial a Phone Number Using a Variable

Code:

chat -T 'phone_number' '"ATDT\T CONNECT"'

Motivation:

In scenarios where phone numbers change frequently (such as telephony systems handling multiple dial-out numbers), using a variable enhances flexibility, allowing dynamic phone number input without altering the underlying script logic.

Explanation:

• chat: Starts the chat tool for modem scripting.
• -T 'phone_number': Substitutes the variable \T in the script with the specified phone number.
• '"ATDT\T CONNECT"': Represents the dial command and subsequent connectivity expectation, with \T dynamically replaced with the input number.

Example Output:

The command dynamically dials the specified phone number, showing a successful dial and CONNECT indication in the console.

Use Case 7: Include an Abort Condition If a Specific String is Received

Code:

chat 'ABORT "error_string" expect_send_pairs'

Motivation:

Abort conditions are vital to prevent further action when a known error string is encountered. This mechanism avoids unnecessary processing or potentially harmful actions based on undesirable responses during connection attempts.

Explanation:

• chat: Initiates the automated interaction.
• 'ABORT "error_string" expect_send_pairs': Establishes an abort condition that terminates the script if “error_string” is received. ’expect_send_pairs’ continues unless the abort condition is met.

Example Output:

If the error string is detected during interaction, the script halts immediately, preserving system resources and maintaining operational integrity.

Conclusion:

The ‘chat’ command offers versatile automation for managing modem and serial communication by leveraging a variety of useful options. From handling complex scripts to customizing interactions with timeouts, logging, and error management, these use cases demonstrate the wide applicability of the chat command for seamless and efficient device communications.