ssh option to enable tty

To enable TTY (teletypewriter) allocation in an SSH (Secure Shell) session, you have a couple of options that cater to different scenarios involving interactive sessions on remote servers. Understanding how to manage TTY allocation is essential for executing commands that require user interaction, such as running sudo.

Understanding TTY Allocation in SSH

TTY allocation essentially allows your SSH client to request a pseudo-terminal (PTY) from the remote server. This is particularly important when running interactive applications or scripts that depend on terminal capabilities.

TTY Allocation Scenarios

Interactive Shell Access: When you log into a remote system via SSH, TTY allocation allows you to interact with that session as if you were using a local terminal.
Executing Commands Needing Interactive Features: Certain commands, especially those that use sudo, require a TTY to function correctly.

SSH Options for TTY Allocation

Using the `-t` Option

The most common way to enable TTY allocation is by using the -t flag:

ssh -t user@remote_host

This command forces SSH to allocate a pseudo-terminal for the session.

Forcing TTY Allocation with `-tt`

In some situations, particularly when your local terminal setup doesn't provide a TTY (for example, running SSH from a script), you might need to use the -tt option, which forces TTY allocation even if no TTY is present locally:

ssh -tt user@remote_host

Using -tt is particularly useful in automation or scripting environments where "stdin" is not associated with a terminal, allowing you to execute commands that require a terminal interface.

Global Configuration with `RequestTTY`

If you often connect to a server and require TTY allocation, you can configure this in your SSH client configuration file (typically located at ~/.ssh/config). Here is how you can enforce TTY allocation for specific hosts or globally:

Host remote_host
    RequestTTY force

This configuration ensures that every SSH session initiated with this host will request a TTY automatically.

Practical Examples

Running a Command with Sudo

When you need to run a command that requires sudo privileges, TTY is often needed:

ssh -t user@remote_host "sudo apt update"

Scripting with SSH

If running a script that interacts with the remote server in a loop, you might need to force TTY allocation:

ssh -tt user@remote_host "bash -s" < local_script.sh

Conclusion

Using the SSH options -t or -tt to enable TTY allocation is a straightforward yet essential practice when working with interactive remote sessions or commands requiring terminal-like interaction. Properly configuring TTY settings can facilitate smoother operations, especially in scripts and automated tasks. For regular usage, consider setting defaults in your SSH config file to eliminate the need to specify these flags repeatedly.

For more detailed instructions, you can refer to the Linux man pages which elucidate on SSH command-line options and their implications.