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

agetty is an alternative to the widely used getty command in Linux systems. Its primary function is to manage terminal lines, enabling connections to them, and prompt the user for a login name to initiate the login process. This program is typically invoked by the init process but can be used manually for various specific use cases. Some of the key features include setting baud rates for serial connections, managing timeout settings, bypassing login prompts, and more.

Use Case 1: Connect stdin to a Port and Specify a Baud Rate

Code:

agetty tty 115200

Motivation:

When working with legacy systems, embedded devices, or hardware that communicates over serial interfaces, setting an appropriate baud rate is crucial for ensuring reliable data transmission. By default, agetty assumes a baud rate of 9600 bps, but many modern devices require faster communication speeds. Setting the baud rate matches the transmission speed used by the connected device.

Explanation:

• tty: This represents the terminal port you want to connect to, which typically maps to a device file in /dev, such as /dev/ttyS0 for the first serial port.
• 115200: This argument sets the baud rate for the serial connection. A baud rate of 115200 is a common setting for modern hardware devices requiring fast data transfer.

Example Output:

Attempting connection to /dev/ttyS0 at 115200bps… Login prompt appears, ready for user input or further terminal commands.

Use Case 2: Set a Timeout for the Login

Code:

agetty -t 30 -

Motivation:

In environments where security is paramount, such as public kiosks or shared terminals, it is sometimes necessary to ensure that an unattended terminal session does not remain active indefinitely. By specifying a timeout, an administrator can define how long agetty will wait for a login prompt to be completed, thus improving security and resource allocation.

Explanation:

• -t 30: This flag sets a timeout of 30 seconds. It tells agetty to exit if no login attempt is made within this period.
• -: This marks the standard input as already connected to a TTY, meaning no further device specification is needed.

Example Output:

If no user input occurs within the allotted 30 seconds, the session will terminate, reverting the terminal to its initial state.

Use Case 3: Assume the tty is 8-Bit and Override TERM

Code:

agetty -8 - term_var

Motivation:

Some applications require an 8-bit clean data path for processing extended ASCII characters or binary data. Moreover, overriding the TERM variable can address incompatibilities between different terminal types and applications. This setup is particularly useful in software development and debugging environments where terminal compatibility might be an issue.

Explanation:

• -8: This option configures the data path to 8 bits, allowing for unprocessed transmission of all 8-bit characters.
• -: Suggests that standard input is already connected to a terminal.
• term_var: This sets a new terminal type, ignoring the TERM value set by the init process.

Example Output:

The session is prepared for 8-bit data transfer, with the user-specified terminal type influencing display behavior.

Use Case 4: Skip the Login and Use Another Program

Code:

agetty -n -l /usr/bin/custom_login_program tty

Motivation:

In some scenarios, particularly in specialized or constrained environments, bypassing the standard login process might be necessary to provide a seamless user experience. Utilizing a custom login program allows for automated processes or special authentication methods, which can be more suitable for specific high-security applications or integration into kiosk systems.

Explanation:

• -n: Instructs agetty to skip the login prompt entirely.
• -l /usr/bin/custom_login_program: Replaces /bin/login with an alternative login program specified by the user.
• tty: Indicates which terminal to perform the action on.

Example Output:

Direct invocation of the specified custom login program, enabling its own authentication or startup logic.

Use Case 5: Do Not Display the Pre-login File

Code:

agetty -i -

Motivation:

For environments where user experience customization or security concerns preclude displaying sensitive information, bypassing the default pre-login issue file (/etc/issue) provides control over what is presented before user interaction. This can enhance the terminal’s boot sequence for embedded systems or tightly controlled user interactions.

Explanation:

• -i: This flag instructs agetty not to display any pre-login message or issue file.
• -: Notifies agetty to treat the current terminal as already prepared for operation.

Example Output:

A clean login prompt is displayed, effectively hiding any predefined system messages or information.

Use Case 6: Change the Root Directory and Specify a Fake Host

Code:

agetty -r /mnt/newroot -H fake_host -

Motivation:

In environments where system chroot environments are used, such as during certain system recovery processes or virtualization scenarios, changing the root directory can simulate a different system configuration. Additionally, writing a specific fake host for identification purposes can assist in tracking or managing multiple terminal sessions.

Explanation:

• -r /mnt/newroot: Adjusts the root directory to /mnt/newroot, reflecting a chrooted environment.
• -H fake_host: Logs the session under a fabricated hostname for identification in session management databases.
• -: Indicates existing connection to a valid terminal.

Example Output:

A login prompt appears under the context of the new root directory, identified by the fake hostname in session records.

Conclusion:

The agetty command provides a robust set of options for managing terminal sessions, enhancing flexibility and control within complex Linux environments. Understanding these use cases allows system administrators to tailor terminal behavior precisely to meet the demands of their specific hardware environments, security policies, or user interface requirements.