Tcl after Command

last modified October 20, 2026

The Tcl after command schedules script execution after a time delay. It's essential for creating timers and delayed actions in Tcl. The delay is given in milliseconds, and the command can either pause the program or schedule a script callback.

Basic Definition

The after command delays script execution or returns timer IDs. A scheduled script runs once; a repeating event is made by having the script schedule itself again. Scheduled scripts run from Tcl's event loop, at the global level, so they only run while the program is processing events (for example inside vwait, or in a Tk application).

Syntax: after ms ?script script ...?. With one argument, it pauses execution. With a script, it schedules script execution after ms milliseconds and returns an ID. Related forms are after cancel id, after idle script and after info.

Simple Delay

This example shows the basic delay functionality of the after command.

basic_delay.tcl

puts "Start"
after 2000
puts "End"

This script prints "Start", waits 2000 milliseconds (2 seconds), then prints "End". The after command with one argument pauses execution; it blocks the whole program, and no events are processed while it sleeps.

Scheduled Script Execution

This demonstrates scheduling a script to run after a delay while the main script continues.

scheduled_script.tcl

puts "Main script starts"
after 1000 {puts "Delayed message"}
puts "Main script continues"
vwait forever

The main script starts, schedules a message to appear after 1 second, then continues. The vwait keeps the event loop running to process the delayed script. The variable forever is never set, so the script keeps waiting after the message appears and has to be stopped with Ctrl+C (like all the examples below that end with vwait forever).

Canceling a Scheduled Event

The after cancel command cancels a previously scheduled event using its ID.

cancel_event.tcl

set id [after 3000 {puts "This won't appear"}]
after 1000 {after cancel $id; puts "Event canceled"}
vwait forever

This schedules a message for 3 seconds but cancels it after 1 second. The after cancel command uses the ID returned by the first after; the ID can also be replaced by the script text itself.

Repeating Timer

This example creates a repeating timer by having the callback reschedule itself.

repeating_timer.tcl

proc repeat {} {
    puts "Tick [clock format [clock seconds] -format %T]"
    after 1000 repeat
}
repeat
vwait forever

The repeat procedure prints the current time every second. Each invocation schedules itself to run again after 1000 milliseconds. The delay is counted from the end of the callback, so the ticks drift slightly later over time.

Multiple Timers

This shows how to manage multiple independent timers with different intervals.

multiple_timers.tcl

proc timer1 {} {
    puts "Timer1: [clock seconds]"
    after 1500 timer1
}

proc timer2 {} {
    puts "Timer2: [clock seconds]"
    after 2500 timer2
}

timer1
timer2
vwait forever

Two independent timers run with different intervals (1.5s and 2.5s). Each timer maintains its own scheduling by calling after in its callback.

After with Arguments

This demonstrates passing arguments to a callback procedure using after.

after_with_args.tcl

proc callback {msg count} {
    puts "$msg $count"
    incr count
    after 1000 [list callback $msg $count]
}

callback "Count:" 1
vwait forever

The callback procedure receives and increments a counter. The list command ensures proper argument passing when scheduling the next execution.

Best Practices

Units: Remember time is in milliseconds (1000ms = 1s).
Event Loop: Use vwait to keep the event loop running.
Memory: Cancel timers when no longer needed.
Arguments: Use list for callback arguments.
Blocking: after ms without a script freezes the program, including a GUI; use the callback form in event-driven code.

This tutorial covered the Tcl after command with practical examples showing timer creation, delayed execution, and event scheduling.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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