[fix] Fixed cmd line too long

[daniel-santos]

Checklist

• [no] I have read the OpenWISP Contributing Guidelines.
• [yes] I have manually tested the changes proposed in this pull request.
• [n/a] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [n/a] I have updated the documentation.

Reference to Existing Issue

Closes #159

Description of Changes

Uses --data's special treatment of @ to load data from file instead of in-lining it on the command line.

[nemesifier]

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Review triggered.

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📝 Walkthrough

Walkthrough

The change modifies the data transmission mechanism in the monitoring agent script. Instead of reading file contents into an in-memory variable using cat and passing it via the -d parameter, the script now uses curl's --data @filename`` syntax to send the file directly from disk. The script additionally checks for file existence and gracefully skips transmission if the file is missing, rather than failing on a missing file.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: fixing a command-line length issue by avoiding inline data passing.
Description check	✅ Passed	The description covers the key aspects including manual testing confirmation and a clear explanation of the technical change made, though it lacks detail in some checklist items.
Linked Issues check	✅ Passed	The code changes directly address issue #159 by replacing inline JSON command-line data with file-based curl transmission using --data @$filename.
Out of Scope Changes check	✅ Passed	All changes are focused on the specific objective of addressing the shell command-line length limit issue; no extraneous modifications detected.

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📜 Recent review details

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between d631074 and f6efa39.

📒 Files selected for processing (1)

• openwisp-monitoring/files/monitoring.agent

🔇 Additional comments (2)

openwisp-monitoring/files/monitoring.agent (2)

154-158: Good defensive check for file existence after decompression.

This correctly handles the edge case where the decompressed file might not exist (e.g., if gzip -d failed silently or the file was removed between operations). The continue gracefully skips to the next file.

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177-177: Core fix correctly addresses the command-line length limitation.

Using -d "@$filename" makes curl read the payload directly from the file, bypassing shell expansion and command-line length limits. This is the idiomatic solution for large payloads.

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