modify basic filestructure

It's unclear what's 'basic', so `basic/` notes have been mostly moved.
The remainder became `shell/`.

system/at.md

@@ -0,0 +1,77 @@
+---
+title: "at"
+tags: [ "basics", "time" ]
+---
+Install with:
+
+```sh
+sudo apt install at
+```
+
+Enable the daemon service with:
+
+```sh
+sudo systemctl enable --now atd
+```
+
+Then jobs can be specified with absolute time, such as:
+
+```sh
+at 16:20
+```
+
+```sh
+at noon
+```
+
+```sh
+at midnight
+```
+
+```sh
+at teatime
+```
+
+Type in your command, e.g.:
+
+```sh
+touch /tmp/$FILE.txt
+```
+
+The jobs can also be specified relative to the current time:
+
+```sh
+at now +15 minutes
+```
+
+Finally, accept the jobs with ^D.
+
+# Managing `at` Jobs
+
+Display a list of commands to run with:
+
+```sh
+atq
+```
+
+> 2	Sat Oct 20 16:00:00 2018 a roach-1
+
+This will print all pending IDs.  Remove a job by the ID with:
+
+```sh
+atrm 2
+```
+
+Check `/var/spool/atd/` to see the jobs.
+
+## Automation
+
+Automatically add a job for later, by setting the date, then using echo for the command.
+
+```sh
+t="$(date -d "2 minutes" +%R)"
+echo "fortune > ~/$FILE" | at "$t"
+watch cat $FILE
+```
+
+The `$t` here outputs the day in minutes, but you could also do `t="$(date -d "2 days" +%m/%d/%Y)"`.