Jobs at specific times

Execute job once at specific time

Note: at is not installed by default on most of modern distributions.

To execute a job once at some other time than now, in this example 5pm, you can use

echo "somecommand &" | at 5pm

If you want to catch the output, you can do that in the usual way:

echo "somecommand > out.txt 2>err.txt &" | at 5pm

at understands many time formats, so you can also say

echo "somecommand &" | at now + 2 minutes
echo "somecommand &" | at 17:00
echo "somecommand &" | at 17:00 Jul 7
echo "somecommand &" | at 4pm 12.03.17

If no year or date are given, it assumes the next time the time you specified occurs. So if you give a hour that already passed today, it will assume tomorrow, and if you give a month that already passed this year, it will assume next year.

This also works together with nohup like you would expect.

echo "nohup somecommand > out.txt 2>err.txt &" | at 5pm

There are some more commands to control timed jobs:

atq lists all timed jobs (atqueue)
atrm removes a timed job (atremove )
batch does basically the same like at, but runs jobs only when system load is lower than 0.8

All commands apply to jobs of the user logged in. If logged in as root, system wide jobs are handled of course.

Doing jobs at specified times repeatedly using systemd.timer

systemd provides a modern implementation of cron. To execute a script periodical a service and a timer file ist needed. The service and timer files should be placed in /etc/systemd/{system,user}. The service file:

[Unit]
Description=my script or programm does the very best and this is the description

[Service]
# type is important!
Type=simple
# program|script to call. Always use absolute pathes 
# and redirect STDIN and STDERR as there is no terminal while being executed 
ExecStart=/absolute/path/to/someCommand >>/path/to/output 2>/path/to/STDERRoutput
#NO install section!!!! Is handled by the timer facitlities itself.
#[Install]
#WantedBy=multi-user.target

Next the timer file:

[Unit]
Description=my very first systemd timer
[Timer]
# Syntax for date/time specifications is  Y-m-d H:M:S 
# a * means "each", and a comma separated list of items can be given too
# *-*-* *,15,30,45:00  says every year, every month, every day, each hour,
# at minute 15,30,45 and zero seconds

OnCalendar=*-*-* *:01:00  
# this one runs each hour at one minute zero second e.g. 13:01:00