Chilkat SSH – SendReqExec vs SendReqShell

Chilkat’s SSH class provides the ability to start a shell session by calling SendReqShell, or by calling QuickShell.  You can alternatively run a single remote command by calling SendReqExec.

SendReqExec does the “exec” command, whereas SendReqShell (or QuickShell) starts an interactive shell session where bytes sent to the SSH server via methods such as ChannelSendString are the equivalent of keystrokes on a keyboard to the session, and the stdout of the session is consumed by calling methods such as ChannelReceiveUntilMatch, ChannelPoll, ChannelRead, etc.

The following explains the difference between exec and shell:

In the SSH protocol, the “exec” and “shell” commands are used to execute commands or open interactive sessions on a remote system. These two commands serve different purposes and provide different functionality when interacting with the remote server via SSH.

1. “exec” Command (Execute a Remote Command)

The “exec” command in SSH is used to execute a single command on the remote server without opening a full interactive shell session. It is non-interactive and is typically used for running specific commands remotely, such as running a script, retrieving information, or performing system tasks.

Key Characteristics of the “exec” Command:

• Single Command Execution – The “exec” command executes only one command. Once the command finishes execution, the SSH session is closed or ends.
• Non-Interactive – There is no interactive shell involved, meaning you cannot provide further input after the command starts running. The command runs and returns its output (or error) back to the client.
• Common Usage – Used in automation scripts or one-off command execution scenarios where interactivity is not required.
• Efficiency – Because it only executes one command and does not involve an interactive shell, it tends to be more efficient than launching a full shell session.

Example of Using “exec” in an SSH session:

ssh user@remote-server 'ls /var/log'

• In this example, the “ls /var/log” command is executed on the remote server and the output is returned to the client. The SSH session ends immediately after the command completes.

When to Use “exec”:

• Running commands like “ls”, “cat”, “uptime”, or scripts that require no further interaction.
• Automating tasks such as remote deployments, system administration tasks, or monitoring without manual intervention.
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2. “shell” Command (Interactive Shell Session)

The “shell” command opens a full interactive shell on the remote server. This allows the user to interact with the server just as they would in a local terminal, typing multiple commands and interacting with the system over the course of the session.

Key Characteristics of the “shell” Command:

• Interactive – A full interactive shell session is started, meaning you can type and run multiple commands, get prompts, and interact dynamically with the server. This mimics using a local terminal.
• Multiple Commands – You can execute as many commands as needed during the session. It behaves like an open-ended terminal session until you exit or log out.
• Standard Use Case for SSH – When you SSH into a remote server using the typical “ssh user@host” command, you are opening an interactive shell session by default.
• User Input – The user can provide input, see real-time output, run different commands, and manage the system interactively.

Example of Using “shell” in an SSH session:

ssh user@remote-server

• In this case, an interactive shell is opened. You can then type multiple commands interactively like “cd”, “ls”, “cat”, or even start editors like “vi” or “nano”. The session remains open until the user explicitly logs out (e.g., using “exit” or “logout”).

When to Use “shell”:

• When you need to interact with the remote system continuously, run multiple commands, or work in an interactive environment (e.g., debugging, development, or administrative tasks).
• If you need to edit files, start interactive programs, or perform real-time operations on the server.