The wait command is the synchronization primitive in Bash that allows parent scripts to coordinate with child processes. When you launch processes in the background using &, they run independently and asynchronously. If your script finishes and exits while background jobs are still running, those jobs might be terminated (depending on shell configuration) or become orphaned. The wait command allows the parent script to pause execution until specific child processes or all background jobs have completed.

You can use wait in several ways. If you provide a specific Process ID (e.g., wait $pid), execution pauses only until that particular process terminates. The exit status of the wait command itself becomes the exit status of that process, allowing you to check whether the background job succeeded or failed. This is crucial for error handling in scripts that depend on the results of parallel tasks.

If you run wait with no arguments, it blocks until all currently active background jobs spawned by the current shell have finished. This implements the "barrier" synchronization pattern: "Start tasks A, B, and C in parallel, but don't proceed to step D until everyone is finished." This is fundamental for workflows where subsequent operations depend on the completion and results of multiple parallel tasks. Some shells also support wait -n to return as soon as any single background job completes, enabling more sophisticated parallelism patterns.