BEGIN/END (The GNU Awk User’s Guide)

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Gawk/docs/latest/BEGIN 002fEND

7.1.4 The BEGIN and END Special Patterns

All the patterns described so far are for matching input records. The BEGIN and END special patterns are different. They supply startup and cleanup actions for awk programs. BEGIN and END rules must have actions; there is no default action for these rules because there is no current record when they run. BEGIN and END rules are often referred to as “BEGIN and END blocks” by longtime awk programmers.

Using BEGIN/END    How and why to use BEGIN/END rules.
I/O And BEGIN/END    I/O issues in BEGIN/END rules.