Linux LPIC-1 107.2: Scheduling Jobs with at Command

Scheduling Jobs with at

• Valid time specifications for at include: at 08:30 AM next week, at midday, at now +4 days, at tomorrow 08:30 AM
• Invalid time specifications for at include: at 01-01-2020 07:30 PM, at 21:50 01.01.20, at 10:15 PM 31/03/2021

Reviewing and Deleting at Jobs

• To review at job commands, use at -c
• To review pending at jobs, use at -l or atq
• To delete at jobs, use at -d or atrm

Using systemd-run as an Alternative to at

• systemd-run can be used to schedule one-time jobs, similar to at
• Example: # systemd-run --on-calendar='2019-10-06 11:30' date to run the date command at 11:30 AM on 2019/10/06
• Example: # systemd-run --on-active="2m" ./foo.sh to run the foo.sh script after 2 minutes

Reviewing systemd-run Logs

• systemd-run logs are stored in the systemd journal
• To review logs, use the journalctl command

Viewing Routing Tables

• netstat -r, ip route, and route commands are used to view the routing table.
• The output of these commands displays the destination, gateway, genmask, flags, and interface.
• The ip route command does not show IPv6 routes by default; use ip -6 route to view IPv6 routes.
• The route -6 and netstat -6r commands are used to view IPv6 routing tables.

Flags Column

• The U flag indicates that a route is up.
• The ! flag means reject route, i.e., a route with a ! won't be used.
• The n flag means the route hasn't been cached.
• The G flag indicates a gateway.

Metric or Met Column

• The Metric or Met column refers to the administrative distance to the target.
• This administrative distance is used by routing protocols to determine dynamic routes.

Adding and Deleting Routes

• The route -6 add and ip route add commands are used to add routes.
• The route -6 del and ip route del commands are used to delete routes.
• Adding a route can make a network reachable, while deleting a route can make a network unreachable.

Guided Exercises

• The ip link and ip addr commands can be used to list network interfaces.
• An interface can be temporarily disabled using the ip link set dev down command and re-enabled using the ip link set dev up command.
• A reasonable subnet mask for IPv4 is 255.255.255.0.
• The netstat -r, ip route, and route commands can be used to verify the default route.
• A second IP address can be added to an interface using the ip addr add command.

Explorational Exercises

• The ip link add command can be used to configure VLAN tagging.
• A default route can be configured using the ip route add command.
• Detailed information about the ip neighbour command can be obtained using the ip neighbour help command.