In system administration, there are often situations where you need to execute scripts on remote machines. A common approach involves copying the script to the remote machine, then executing it. However, there’s a more streamlined way to do this without having to copy the script over first. This is where the power of ssh and command redirection come into play.

This guide will explain how the following command works to run a script on a remote machine without copying the script:

ssh user@remote_host 'bash -s' < /my/desired/directory/myscript.sh

• Prerequisites
  • Steps for SSH Key Authentication
• Understanding the Command
  • Command Breakdown
  • What Happens Internally?
• Why Use This Method?
• Use Cases
• Example: Running a Disk Check Remotely
• Conclusion

Prerequisites

For this method to work, passwordless SSH authentication should already be set up between your local machine and the remote host. If SSH key-based authentication isn’t configured, the command will still prompt for a password each time you run it, which could hinder automation.

Steps for SSH Key Authentication

Generate SSH Keys: If you haven’t set up SSH keys, you can generate a key pair using the following command:

ssh-keygen -t ecdsa -b 384

Copy the Public Key to the Remote Host: You can then copy the public key to the remote host:

ssh-copy-id user@remote_host

Test the Connection: Ensure that you can log in without a password by running:

ssh user@remote_host

If you authenticate without being prompted for a password, you’re all set.

Understanding the Command

Let’s break down the key components of the command and explain how they work:

Command Breakdown

ssh user@remote_host 'bash -s' < /my/desired/directory/myscript.sh

ssh user@remote_host:

• The ssh command is used to establish a secure connection to a remote host.
• Replace user with your remote username and remote_host with the hostname or IP address of the machine you’re targeting.

‘bash -s’:

• This part tells the remote machine to run a bash shell in a non-interactive mode (-s flag).
• The -s flag allows bash to read from standard input, meaning it will receive the script content piped from the local machine.

< /my/desired/directory/myscript.sh:

• The < operator redirects the content of the script located on the local machine (in this case, /my/desired/directory/myscript.sh) as input to the ssh command.
• Essentially, it sends the script content over the SSH connection for execution on the remote machine.

What Happens Internally?

• The script myscript.sh from your local machine is not copied to the remote machine.
• Instead, it’s streamed over the SSH connection, where it’s immediately executed by the remote host’s bash shell.

Think of it like this: the local script’s content is passed as if it were typed directly into the terminal session on the remote machine.

Why Use This Method?

No Need to Transfer Files: There’s no need to use tools like scp to copy the script to the remote machine first. This saves time and avoids cluttering the remote system with temporary script files.

Quick and Efficient: Ideal for running quick tasks or maintenance scripts on multiple remote machines without leaving any trace of the script behind.

Automation-Friendly: Since the script is executed directly from your local machine, you can easily integrate this method into larger automation workflows where you need to run scripts across multiple servers.

Use Cases

This command is particularly useful in the following scenarios:

• Routine Maintenance: Running simple scripts like updates or service restarts across many servers.
• Ad-hoc Tasks: Executing a one-off command on a remote system without the hassle of uploading the script.
• Multi-server Management: Ideal when managing a fleet of servers, allowing you to quickly execute commands on several systems in succession by changing the target host in your SSH command.

Example: Running a Disk Check Remotely

Let’s say you have a script called check_disk.sh that checks for disk space usage, and you want to run it on a remote server.

Your local script might look like this:

#!/bin/bash
df -h

To run this on the remote server 192.168.1.50, use the following command:

ssh user@192.168.1.50 'bash -s' < /path/to/check_disk.sh

The check_disk.sh script is executed directly on the remote machine, and you’ll see the output of the df -h command displayed in your local terminal.

NOTE:

You may also run the below command to get the same results of df -h, however, this is just an example. This script can be much more in depth, and it will still execute successfully on the remote machine.

ssh user@192.168.1.50 'df -h'

Conclusion

The ssh user@remote_host 'bash -s' < /my/desired/directory/myscript.sh command is a simple but powerful tool for executing local scripts on remote systems without copying files. Whether you’re managing a handful of machines or a large infrastructure, this method can greatly simplify your workflow, saving you time and avoiding unnecessary file transfers.

Remember, this approach relies on having SSH key-based authentication set up between the machines. Once that’s in place, you’re ready to efficiently manage remote systems with just a few commands!

📝 For more information about this bash command, review this bash man page!