Linux Glossary

The vocabulary behind the commands, in plain language. 39 concepts — what each one means, why it matters, and which commands put it to work. Start anywhere; every entry links onward.

Absolute vs relative path: Full address from / versus directions from where you stand.
Bash: The default shell on most Linux distributions.
Cron: The scheduler that runs commands at fixed times.
Daemon: A background process providing a service, usually started at boot.
Distribution (distro): The kernel plus everything else, packaged as a usable system.
DNS: The system that translates names into IP addresses.
Environment variable: A named value processes inherit when they start.
Exit code: The number a command returns: 0 means success.
File permissions: Who may read, write and execute — nine bits per file.
Filesystem hierarchy: The standard layout of directories under /.
Glob (wildcard): Patterns like *.txt, expanded by the shell before the command runs.
Hard link: A second, equally real name for the same file.
Inode: The structure that IS a file — metadata plus pointers to its data.
Kernel: The core of the operating system, managing hardware and processes.
Mount: Attaching a filesystem to a directory in the tree.
Package manager: The tool that installs, updates and removes software.
PATH: The ordered list of directories the shell searches for commands.
PID (Process ID): The number that identifies a running process.
Pipe (|): Connects one program's output to the next one's input.
Port: A number identifying a specific service on a machine.
Process: A running program, with its own ID, memory and environment.
Redirection (> and >>): Sending a program's streams to files instead of the screen.
Regular expression: A pattern language for matching text, used by grep, sed and awk.
Root: The administrator account with unrestricted privileges.
Shebang (#!): The first line telling the system which interpreter runs a script.
Shell: The program that reads your commands and runs them.
Signal: An asynchronous message sent to a process, like SIGTERM or SIGKILL.
SSH key: A key pair that replaces passwords for remote login.
stderr (standard error): Stream 2 — the separate channel for errors and diagnostics.
stdin (standard input): Stream 0 — where a program reads its input from.
stdout (standard output): Stream 1 — where a program writes its normal results.
sudo: Running a single command with administrator privileges.
Symbolic link (symlink): A small file containing a path, resolved on every access.
System call: How a program asks the kernel to do something.
systemd: The init system that starts and supervises services on modern Linux.
Terminal: The window that displays a shell session.
TTY: The terminal device a process is attached to — or is not.
umask: The permission bits automatically removed from new files.
Working directory: The folder your shell is currently "standing" in.