Long Running Tasks in SQLcl

SQLcl 24.1 introduced a way to run tasks in parallel. Any command that makes sense to run in the background — a long-running query, a Liquibase changeset generation, a data export — can now be kicked off without blocking your session.

Three commands work together to make this happen: background, jobs, and wait4.

The commands

background (or bg) takes any SQLcl or SQL*Plus command and runs it as a background task. You can name the task and optionally wait for another task to complete before it starts.

SQL> help background syntax
 background|bg [-wait4|-w4 <wait4>] [-taskname|-tn <taskname>] <commandspec>

jobs lists and manages background tasks — check status, view logs, or clean up finished work.

wait4 pauses execution until a specific task (or list of tasks) completes, or waits for a given number of milliseconds. Useful for scripting dependencies between tasks.

Running a command in the background

Take a Liquibase changeset generation as an example. Normally you’d run it in the foreground:

SQL> liquibase generate-db-object -object-name employees -object-type table -sql
--Starting Liquibase at 2024-03-19T23:10:47 (version 4.26.0)
Changelog created and written out to file employees_table.xml
Operation completed successfully.

That blocks your session until it finishes. With background, you can fire it off and keep working:

SQL> background -taskname lb-employees liquibase generate-db-object -object-name employees -object-type table -sql
Started task with id: 2

SQL> jobs
 2: [ Running ] lb-employees (/Users/bamcgill/.sqlcl/jobslogs/lbemployees.log)

The task runs in the background. Check on it with jobs, and pull the output with jobs logs:

SQL> jobs logs -id 2
--Starting Liquibase at 2024-03-19T23:21:09 (version 4.26.0)
Changelog created and written out to file employees_table.xml
Operation completed successfully.

SQL> jobs
 2: [ Finished ] lb-employees (/Users/bamcgill/.sqlcl/jobslogs/lbemployees.log)

The output is identical to running the command in the foreground — it’s just not blocking you while it works.

Where this matters

The obvious use case is kicking off several independent tasks at once — generate changesets for multiple schemas, run exports in parallel, or execute long-running reports while you continue working in the same session.

Combined with wait4, you can script task dependencies: start task A and task B in parallel, then wait4 both before starting task C. That’s a simple but effective orchestration pattern without leaving the command line.

Download the latest SQLcl from oracle.com and give it a try.