RHCSA key terms

A command-line utility that searches text for lines matching a pattern and prints those lines. It supports regular expressions and is commonly used to filter command output or file contents.

A stream editor that reads text line by line and applies edits such as substitution or deletion. It is often used in scripts with the -i option to modify files in place.

A command that recursively searches directory trees for files matching criteria such as name, size, owner, or modification time, and can run actions like delete or exec on matches.

A shell feature that changes where a command's standard input, output, or error stream goes, using operators like >, >>, <, and 2> to write to or read from files.

The shell operator | that connects one command's standard output to the next command's standard input, letting users build chains of commands that filter or transform data.

A command that changes the permission bits of a file or directory, using symbolic modes such as u+x or numeric octal modes such as 755 to grant or deny read, write, and execute access.

A special file that points to another path by name and is created with ln -s. Resolving it follows the stored path, and deleting the link does not remove the target file.

The high-level package manager on modern Red Hat systems. It installs, updates, removes, and queries RPM packages while resolving dependencies from configured repositories.

The low-level Red Hat package manager. It installs or queries individual .rpm files and can verify package contents, but it does not automatically resolve dependencies from repositories.

A configuration file under /etc/yum.repos.d/ with a .repo suffix that tells dnf where to download packages from, including base URLs, enabled status, and GPG key settings.

The first line of a script, beginning with #!, that names the interpreter to run it, such as #!/bin/bash. The kernel uses it when the script is executed directly.

A shell variable such as $1 or $2 that holds an argument passed to a script or function in the order given on the command line; $0 is the script name.

A number from 0 to 255 that a command or script returns to its parent, where 0 usually means success. It is stored in the $? variable and can be tested.

The command used to manage systemd services and other units, allowing users to start, stop, restart, enable at boot, disable, and query the status of units.

A configuration file that describes something systemd manages, such as a service, mount, timer, or target. Unit files are typically stored under /usr/lib/systemd/system or /etc/systemd/system.

A command that reads the systemd journal, the centralized log of system messages and service output. It can filter by unit, priority, time range, or boot.

A group of systemd units that represents a system state, similar to a runlevel. Targets such as multi-user.target or graphical.target define which services start together.

A scheduled task defined in a crontab file, with five time fields followed by a command. The cron daemon runs the command automatically at the specified minutes, hours, days, months, and weekdays.

A structure on a disk that records the start and size of partitions. MBR supports up to four primary partitions and disks up to 2 TiB, while GPT supports many partitions and larger disks.

Logical Volume Manager, a storage layer that pools physical disks into volume groups and carves flexible logical volumes from them, allowing resizing and snapshots more easily than plain partitions.

A disk or partition initialized with pvcreate for use by LVM. It stores metadata that lets the volume group track which extents belong to which logical volume.

An LVM pool made from one or more physical volumes. It provides free extents that can be allocated to logical volumes and can be extended by adding more physical volumes.

A resizable block device created from free extents in an LVM volume group, used like a normal partition for file systems or swap. It is managed with commands such as lvcreate and lvextend.

Disk space used as virtual memory when RAM is full. It can be a dedicated partition or a swap file, is enabled with swapon, and is listed in /etc/fstab for persistence.

The file that lists file systems to mount automatically at boot, including their device or UUID, mount point, type, and options. Errors in it can prevent the system from booting cleanly.

The default Red Hat file system, known for high performance on large files and volumes. It supports online growth with xfs_growfs but cannot be shrunk.

A universally unique identifier assigned to a file system, used in /etc/fstab instead of a device name so mounts still work if disks are added or reordered. It is shown by blkid or lsblk -f.

A service that mounts file systems on demand when a directory is accessed and unmounts them after inactivity. It is configured through map files under /etc/auto.master.d and related files.

The command-line client for NetworkManager. It lists connections, brings them up or down, and creates or modifies settings such as IP addresses, gateways, DNS servers, and Wi-Fi profiles.

A command from the iproute2 package that shows or changes network interfaces, addresses, routes, and neighbor entries. It replaces older tools such as ifconfig and route.

The Secure Shell client, which opens an encrypted remote login session or runs commands on another host. It uses key-based or password authentication and defaults to port 22.

A command that creates a new local user account, adding entries to /etc/passwd and /etc/shadow, creating a home directory when asked, and setting defaults from /etc/default/useradd.

A world-readable file listing local user accounts, with colon-separated fields for username, password placeholder, UID, GID, description, home directory, and login shell.

A root-only file storing password hashes and aging information for local users, with fields for last change, minimum and maximum age, warning period, and account expiration.

A command that creates a new local group, assigning it a GID and adding an entry to /etc/group. Users can then be made members to share file access.

A mandatory access control system built into Red Hat Enterprise Linux. It labels processes and files and enforces policy rules that restrict what each process may access, even for root.

The label attached to a file, process, or port, made of user, role, type, and level fields. The type field is what most policy rules check.

The default firewall management service on Red Hat systems. It organizes rules into zones and services and applies them dynamically without restarting the whole firewall.

The command-line interface to firewalld. It adds or removes services, ports, and rich rules in zones, and can make changes permanent with the --permanent option followed by --reload.

A program that lets permitted users run commands as another user, usually root, according to rules in /etc/sudoers. It prompts for the user's own password by default.