I recently hacked together an ESP-01 and a cheap solar garden lantern so that I can have warm flickering candlelight whenever the mood strikes. For example, by using at(1), I can make teatime¹ just a little more magical:

$ echo "curl 'http://ember.local/on'" | at teatime

at(1) is a Unix utility for scheduling one-off commands. In the example above, a request to turn on the lantern will be sent at 4pm.

Whereas cron(8) is used for scheduling recurring jobs, at(1) is used for scheduling “do-this-thing-ONCE-at-some-point-in-the-future” kind of jobs, for example:

Scheduling a server restart during off-hours:

ssh admin@server 'echo "sudo systemctl reboot" | at 02:00'

Grabbing fresh data before a sync with the team:

echo "curl api.example.com/stats | jq . | ~/pipe-to-slack.sh" | at 9am monday

Stopping that debug container you’re likely going to forget:

echo "docker stop my-debug" | at now + 2 hours

You can even implement cron-like recurring jobs by executing at at the end of a job. (Using -f to read from a file makes this even easier to manage):

$ cat ~/daily.sh
#!/bin/bash
# Do some work here...
at -f ~/daily.sh 5pm tomorrow

$ at -f ~/daily.sh 5pm

On macOS, the atrun(8) daemon is disabled by default. You have to enable it using launchctl:

sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.atrun.plist

at(1) captures the entire shell environment from where you schedule the job. This means that when atrun(8) executes your job, it runs with your working directory, env vars, umask, etc.

When a job produces output (or you use the -m flag), it will be sent to you via local sendmail(8). On macOS, mail lands in /var/mail/$USER by default, and you can read it with the mail command.

Other useful bits:

• atq lists pending jobs
• at -c <id> shows what a job will run
• atrm <id> cancels a job
• at -f script.sh 3pm reads from a file instead of stdin
• batch is a sibling that runs jobs when system load is low rather than at a specific time