Скачать или смотреть Understanding Why ps/pgrep Cannot Find Scripts Without # !/bin/bash

An in-depth explanation of how the shebang affects the execution and searching of bash scripts on Linux systems.
---
This video is based on the question https://stackoverflow.com/q/62214223/ asked by the user 'camino' ( https://stackoverflow.com/u/440403/ ) and on the answer https://stackoverflow.com/a/62222953/ provided by the user 'alani' ( https://stackoverflow.com/u/13596037/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: ps/pgrep cannot find running script missing # !/bin/bash

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Why ps/pgrep Cannot Find Scripts Without # !/bin/bash

If you've ever run into the puzzling situation where your script seems to be running but cannot be detected by ps or pgrep, you're not alone. This can happen if your script lacks the crucial # !/bin/bash shebang at the beginning. In this guide, we'll explore why this is the case and how adding this line can resolve the issue.

The Importance of the Shebang

What is a Shebang?

The shebang (# !) is the first line of a script that indicates which interpreter to use to execute the script. For example, adding # !/bin/bash at the beginning of a script tells the operating system to run the script using the Bash shell. Without it, the script may not be executed as intended by the system.

How the Shebang Affects Script Execution

When you run a script that begins with # !/bin/bash:

The kernel recognizes the shebang: It understands to invoke the specified interpreter, in this case, Bash.

Execution appears appropriately in process listings: If you check with ps ux, you'll see it executing with the full command line like /bin/bash ./sample.sh.

Conversely, if you run a script without a shebang:

The script remains unrecognized by the kernel: If it's not a compiled executable or a file showing the # ! line, the kernel cannot execute it.

Error Messages: Attempting to run such a script from other programs (like Python or C) will often result in an “Exec format error”.

What Happens When You Run Without # !/bin/bash

Consider the example of running a script called sample.sh without the shebang line:

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

When executed, the following occurs:

Bash provides fault tolerance: While you intended to run the script in a subshell, an exec command fails because it can't find the interpreter it needs to run. Instead of terminating, Bash then reads and executes the commands directly.

Process List Indication: When checking active processes, you only see the original process, shown as bash, without any reference to sample.sh in its command line.

Example Output from Process Check

When checking processes using ps, you may only see:

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

Where pgrep might yield nothing, as the script isn't recognized as a distinct running process.

Fixing the Issue

The solution is both simple and effective: Add the shebang line to your script. Here's how you can adjust it:

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

Re-running the Script

Once you’ve added the shebang, the script should now run and be identifiable in the process list. Execute it again, and perform the following checks:

Start the Script:

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

Use ps or pgrep:

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

Now, you should see the script being executed as expected.

Conclusion

In summary, the # !/bin/bash shebang is not just a convention; it plays a crucial role in how scripts are executed on Linux systems. Without it, the script may run under different circumstances that make it difficult or impossible to find using tools like ps or pgrep. Always ensure your shell scripts start with this essential line to avoid confusion and ensure proper execution tracking.