CompTIA Linux+ key terms

The default bootloader on most Linux distributions; it presents a menu, loads the kernel and initramfs into memory, and passes boot parameters. Its configuration lives in /boot/grub/grub.cfg, usually generated by helper scripts.

A small compressed filesystem loaded together with the kernel at boot. It contains the drivers and tools needed to locate and mount the real root filesystem before handing control over to it.

A configuration file listing filesystems and where they are mounted at boot, including mount options and the dump and fsck order. Entries are typically identified by UUID or device path.

The directory in an existing filesystem tree where another filesystem is attached, making the new filesystem's contents appear at that path until it is unmounted.

A virtual block device carved from a volume group by the Logical Volume Manager, letting administrators resize, snapshot and move storage without repartitioning physical disks.

Disk area used as overflow for memory when pressure rises, or to hold inactive pages. It can be a dedicated partition or a swap file activated with swapon.

An INI-style file describing a resource systemd manages, such as a service, socket, mount, timer or target. It declares dependencies, the command to run and restart behavior.

The primary command for controlling systemd, used to start, stop, restart, enable or disable units and to inspect their status and dependencies.

A grouping unit that pulls in a set of other units to represent a system state, such as multi-user.target or graphical.target, replacing the older SysV runlevels.

A world-readable text file holding one line per local account with username, UID, GID, home directory and login shell. Password hashes are not stored here on modern systems.

A root-only file containing password hashes plus aging fields for each account, such as last change day, minimum and maximum age, warning period and expiry date.

A low-level command that creates an account entry, assigns a UID and primary group, and can optionally create the home directory and copy in skeleton files.

A command that displays or edits password aging information in /etc/shadow, including expiry dates, minimum and maximum days between changes, and warning days.

Pluggable Authentication Modules, a framework of shared libraries that lets programs such as login and sshd use configurable stacked authentication, account, session and password checks.

The configuration file, edited with visudo, that defines which users or groups may run which commands as which target users, and whether a password is required.

A public and private cryptographic key used for passwordless, encrypted authentication. The public half is placed in authorized_keys while the private half stays secret on the client.

A mandatory access control system built into the kernel that labels processes and files and enforces policy rules, running in enforcing, permissive or disabled mode.

A kernel security module that confines programs using per-program profiles of allowed file paths and capabilities, shipped mainly with Debian and Ubuntu systems.

A dynamic firewall manager that uses zones and services to define allowed traffic and applies rules through nftables or iptables backends without dropping existing connections.

A value that masks permission bits when new files and directories are created, subtracted from the defaults of 666 for files and 777 for directories.

The setuid, setgid and sticky bits shown as s or t in permission strings. They run a program as its owner, inherit a directory's group, or restrict deletion in shared directories.

A tool for public-key encryption and digital signing, used to encrypt files and email and to verify the signatures on packages and software repositories.

The userspace daemon that receives kernel audit events, writes them to log files and can raise alerts. It is used to record security-relevant activity such as privilege changes.

The first line of a script beginning with #!, which names the interpreter that should execute the file, for example #!/bin/bash.

A number from 0 to 255 returned when a command finishes, where 0 means success. Scripts inspect it through $? or in conditionals to control flow.

A per-user schedule table for cron made of five time fields followed by a command; edited with crontab -e and stored under /var/spool/cron.

A unit that triggers another unit on a calendar or monotonic schedule, offering logging, dependency handling and accuracy options beyond those of traditional cron.

A YAML file describing an ordered set of tasks and a desired state to apply to managed hosts, run over SSH without an agent installed on the targets.

A stream editor that reads text line by line and applies scripted substitutions, deletions and other edits, commonly used inside shell pipelines.

A pattern-action text processing language that splits each input line into fields, making it suited to column extraction, reformatting and report generation.

Operators such as >, >>, < and 2>&1 that connect a command's standard input, output or error stream to files or to other file descriptors.

A command that prints the kernel ring buffer, showing hardware detection, driver messages and errors recorded during boot and by later events.

The command for querying the systemd journal, able to filter entries by unit, boot, priority or time range and to follow new messages live.

A diagnostic tool that traces the system calls a process makes and the signals it receives, useful for finding why a program fails or hangs.

A tool that dumps socket statistics, showing listening ports, established TCP connections, addresses, and the processes that own each socket.

A command that reports used and available space for each mounted filesystem, with -i showing inode usage instead of block usage.

A command that lists open files and the processes holding them, including network sockets, which helps identify what blocks an unmount or occupies a port.

A tool that maps the path packets take to a destination by sending probes with increasing TTL values and reporting each responding hop along the way.