Everything about Batch processing

An attachment is a row in wp_posts; its files sit somewhere else, and deleting one does not delete the other. What to measure before you start, why "unused" is so hard to prove, and the order of operations that keeps the site up.

A watermark can go on the original, on the sub-sizes WordPress generates, or on top at display time. Only two of those put pixels in the file, and only one of them is reversible.

WordPress renames every file you upload, using rules almost nobody has read. Here is what sanitize_file_name actually changes, why a name with an umlaut resolves on one host and 404s on another, and why core answers a collision by adding a number instead of overwriting. Clean your names in the browser first, because after the upload the name is a URL.

Square, story, banner, avatar, email header. Six shapes and one idea, which is two hours of dragging things around if you do it the obvious way.

Core converts on upload through a single filter and ignores everything already in the library. Here is what that filter really covers, what regeneration adds, and why the uploads folder grows before it shrinks.

Uploading the same file twice gives you a numbered copy, not a match, because core only tests the file name against one directory. Here is how the duplicates get made, three detection passes ranked by what they actually prove, and a deletion order that will not break a live page.

Somebody needs two hundred name badges by Thursday. The spreadsheet with all those names already exists, and typing them a second time is pure waste.

You build the layout once, tell a few layers where to get their content, and from then on every post, product or page renders its own version.

Regenerating thumbnails and resizing the original are different jobs, and only one of them is irreversible. What the scaled pair actually costs you, how to measure it with read only commands, what a safe bulk resize has to do step by step, and why most sites should fix the upload threshold instead.

Regenerating thumbnails re-encodes every registered size from the original file. Here is when it is genuinely needed, what each WP-CLI flag really does, why --only-missing quietly keeps the old files, and the order that stops your uploads folder growing.