Reference
Glossary
Every term, command, and concept in the 30-day course, defined plainly.
Absolute path
A file path that starts from the root directory (/), such as /home/user/file.txt. It describes the complete location regardless of where you currently are.
Argument
Extra information passed to a command after the command name, such as filenames, options, or values.
Bash
The Bourne Again SHell — the default command-line interpreter on Ubuntu. It reads what you type and translates it into instructions for the computer.
Builtin
A command implemented inside the shell itself (like cd or echo), rather than being a separate program file on disk.
Environment variable
A named value stored by the shell that programs can read. Examples include HOME, PATH, and USER.
File descriptor
A number the operating system uses to identify an open file. 0 is stdin, 1 is stdout, 2 is stderr.
Glob
A pattern using wildcards (*, ?, []) that the shell expands into matching filenames before a command runs.
Hard link
An additional directory entry that points to the same inode (data) as another file. Deleting one name does not delete the data until all names are removed.
Inode
A data structure on disk that stores a file's metadata (size, permissions, timestamps) and pointers to its data blocks.
Option / Flag
A modifier starting with - or -- that changes how a command behaves, such as ls -la.
PATH
An environment variable containing a colon-separated list of directories where the shell looks for executable programs.
PID
Process ID — a unique number the operating system assigns to every running program.
Pipe
The | symbol. It connects the output of one command to the input of another, letting you chain commands together.
Redirection
Using >, >>, <, or 2> to change where a command reads input from or writes output to.
Relative path
A file path that starts from your current working directory, such as ../documents/file.txt.
Root directory
The top-level directory of the Linux filesystem, written as /.
Shebang
The #! line at the start of a script that tells the system which interpreter to use, such as #!/bin/bash.
Signal
A message sent to a process to request an action, such as SIGTERM (polite stop) or SIGKILL (force stop).
Stderr
Standard error — the output stream (file descriptor 2) where commands write error messages.
Stdin
Standard input — the input stream (file descriptor 0) that commands read from, usually your keyboard.
Stdout
Standard output — the output stream (file descriptor 1) where commands write normal results.
Sudo
Superuser do — runs a command with administrator (root) privileges. Use sparingly and only when you understand the consequences.
Symbolic link (symlink)
A special file that stores a path pointing to another file or directory. Unlike a hard link, it can span filesystems and point to directories.
Terminal
A text-based interface for interacting with the operating system by typing commands.
Wildcard
A special character used in glob patterns. * matches any sequence of characters, ? matches exactly one character.
Working directory
The folder you are currently inside in the terminal. Use pwd to see it.