Concepts

The command reference tells you what each flag does. These pages tell you why gapline is built the way it is, and what guarantees you can count on when writing pipelines. Read them once; skim them when something surprises you.

• GTFS feeds — what a feed actually is, which of the 17 files gapline touches, and where it is strict versus forgiving.
• Severities — the line between ERROR, WARNING, and INFO, and how --min-severity promotes warnings to gate failures.
• Query language — the small expression language behind every --where clause.
• Referential integrity — reverse indexes, cascade previews, and why delete refuses to orphan stop_times.
• .gl session model — how batch scripts keep the feed in memory until an explicit save.
• Exit codes — the five codes gapline emits and how to branch on them in CI.