How to Successfully Execute a Shell Script on a Remote Server Using Expect

Struggling to run a shell script on a remote server using Expect? Discover how to resolve the 'No such file or directory' error and execute your scripts effectively.
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This video is based on the question https://stackoverflow.com/q/74559209/ asked by the user 'david' ( https://stackoverflow.com/u/6187009/ ) and on the answer https://stackoverflow.com/a/74559653/ provided by the user 'Barmar' ( https://stackoverflow.com/u/1491895/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting Remote Script Execution with Expect

When working with remote servers, executing shell scripts can sometimes lead to unexpected challenges. One common issue that users encounter is the notorious error message: bash: pwd.sh: No such file or directory. This typically occurs when using Expect to automate SSH commands. In this post, we'll explore the problem and provide a clear solution.

The Problem Explained

In your scenario, you're attempting to run a shell script named pwd.sh on a remote server using an Expect script (called ssh_cmd.sh). The script is supposed to SSH into the server and execute the pwd.sh file, but instead, it throws an error indicating that the script cannot be found.

You're able to run the command directly in the terminal without issue, which leads to confusion. This discrepancy arises due to how commands are processed in Expect, specifically how the spawn command handles arguments.

Example Code

Here’s a simplified view of how your ssh_cmd.sh is structured:

[[See Video to Reveal this Text or Code Snippet]]

Running the following command results in the error:

[[See Video to Reveal this Text or Code Snippet]]

Why Does This Happen?

The root of the problem lies in how Expect handles command arguments within the spawn command. Unlike other shell environments where command execution can interpret redirection (e.g., < pwd.sh), using spawn sends the command as literal arguments to the remote shell. Consequently, the script pwd.sh doesn’t exist on the remote machine, leading to the "No such file or directory" error.

The Solution

To solve this issue, you can change the way you spawn the command. Instead of passing the command with input redirection directly to ssh, you can execute a local bash and pass it the command. Here’s the revised line of code:

[[See Video to Reveal this Text or Code Snippet]]

Updated Expect Script

Here’s how the modified ssh_cmd.sh should look:

[[See Video to Reveal this Text or Code Snippet]]

Key Benefits of This Approach

Correct Execution: By invoking a local bash, the input redirection works as expected, finding pwd.sh on your local system and sending it correctly to the remote server.

Simplicity: This method maintains the simplicity and readability of your Expect script.

Final Thoughts

Executing scripts on remote servers can seem daunting at first, but understanding how commands are processed makes all the difference. By implementing the above changes, you'll resolve the "No such file or directory" issue and successfully execute your scripts using Expect. If you're still facing challenges or have further questions, don’t hesitate to ask for help!

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