When you need a diff and don't have a repo
Version-controlled files get git diff. But operations work constantly produces text that lives outside git: the nginx config on the server vs. the one in your runbook, two dumps of environment variables, yesterday's and today's output of iptables-save, a colleague's "working" compose file vs. yours. This tool does a proper longest-common-subsequence line diff on any two blobs of text, the same algorithm family as diff itself, with removals in red and additions in green.
Why line-based, and why LCS
Naive comparison ("line 5 vs. line 5") falls apart the moment one side gains a line, marking everything after it as changed. LCS finds the longest sequence of lines the two texts share, so an inserted block shows as exactly that block, and the hundred identical lines after it stay quiet. That's the difference between spotting the changed directive in a 400-line config and reading all 400 lines twice.
The whitespace checkbox
"Ignore leading/trailing whitespace" trims each line before comparing. Turn it on when comparing YAML or configs that passed through different editors, tabs-vs-spaces and trailing-space churn otherwise buries the real change. Turn it off when whitespace is the suspect: YAML indentation errors and Makefile tab problems are precisely whitespace bugs, and hiding them defeats the purpose.
Privacy, again
Config files are secrets-adjacent by nature, connection strings, internal hostnames, occasionally an API key someone shouldn't have hardcoded. Everything here is computed in your browser tab; nothing is uploaded. For files rather than pasted text, the command-line equivalents are diff -u old new or the friendlier git diff --no-index old new, which works on any two paths even outside a repository.