Q-R

quota

In Linux, a quota can limit users and/or groups by number of inodes or disk space. Quotas can include hard and soft limits.

quotacheck

The quotacheck command scans and creates user and group quota files.

quotaon

The quotaon command activates configured quotas.

RAID (Redundant Array of Independent Disks)

RHEL supports software RAID. You can use Anaconda to set up software RAID 0, 1, 5, and 6 arrays. You can also set up RAID arrays using the fdisk or parted command with mdadm. Also known as Redundant Array of Inexpensive Disks.

RAID 0

A RAID 0 array requires two or more partitions or hard drives. Reads and writes are done in parallel, increasing performance, filling up all partitions or hard drives equally. RAID 0 includes no redundancy; if any partition or hard drive in the array fails, all data in the array is lost.

RAID 1

A RAID 1 array requires two or more partitions or hard drives. RAID 1 is also known as mirroring, because the same information is written to both partitions. If one disk is damaged, all data will still be intact and accessible from the other disk.

RAID 5

A RAID 5 array requires three or more partitions. Parity information is striped across all partitions. If one partition fails, the data can be rebuilt. It can be automatically written to a spare disk.

RAID 6

A RAID 6 array requires four or more partitions. Parity information is striped twice across all partitions. If one or two partitions fail, the data can be rebuilt. It can be automatically written to a spare disk.

Red Hat Certified Engineer (RHCE)

Perhaps the elite certification available for Linux systems administrators. Designed to qualify Linux administrators with significant experience in configuring Linux LANs with Red Hat Enterprise Linux.

Red Hat Certified Technician (RHCT)

Another elite certification for newer Linux administrators. Designed to qualify Linux administrators with significant experience in configuring Linux workstations with RHEL. RHCEs must also meet all RHCT requirements.

Red Hat Hardware Compatibility List

The Red Hat Hardware Compatibility List (HCL) specifies all hardware that has been tested on systems running the various Red Hat operating systems. Red Hat provides installation support for any hardware that is listed as "support" on their HCL.

Red Hat Network (RHN)

The Red Hat Network (RHN) supports remote control and administration of systems with RHN subscriptions.

Red Hat Package Manager (RPM)

The Red Hat Package Manager is a system that sets up software in discrete packages. The associated rpm command allows you to add, remove, and upgrade packages.

refresh rate

The refresh rate regulates the rate at which the image you see on your screen is redrawn, in hertz (Hz).

repquota

The repquota command reports disk consumption.

resize2fs

The resize2fs command allows you to change the size of a filesystem, often used after increasing the space associated with an LVM.

reverse (inverse)zone

A DNS reverse (inverse) zone can be required by some servers, such as Apache and sendmail, to make sure an IP address points to a real computer. If the reverse zone host name does not match the IP address, the server might not respond.

rndc

The rndc command is used to manage the operation of a DNS server; it's preferred over commands such as service named start.

root

This word has multiple meanings in Linux. The root user is the default administrative user. The root directory (/) is the top-level directory in Linux. The root user's home directory, /root, is a subdirectory of the root directory (/).

router

A computer that transfers messages between LANs. Computers that are connected to multiple networks often serve as routers.

rpmbuild

The rpmbuild command allows you to build source code based on information in a .spec file.

runlevel

RHEL includes six available runlevels, as defined in /etc/inittab. Key runlevels include 1, single-user mode; 3, text login; and 5, GUI login.