CompTIA Linux+ Certification XK0-006 Flashcards - Section 3: Shell Operations

Linux shell

A tool that lets users run programs, check files, and customize how the system works.

Environmental variables

Special variables used by the system, shell, or apps to store process-specific or session data.

$USER variable

The username of the person currently logged in.

View the $USER variable

echo $USER

$HOME variable

The user's home directory path.

View the $HOME variable

echo $HOME

$SHELL variable

The path to the current shell interpreter (e.g., /bin/bash).

PS1 variable

The appearance of the command prompt.

Escape sequences in PS1

\u: username, \h: hostname, \w: working directory, \$: prompt symbol ($ or #).

PATH variable

A list of directories that Linux searches to find executable programs.

Add a directory to PATH temporarily

export PATH=$PATH:/your/custom/dir

Make PATH changes permanent

Add them to ~/.bashrc or ~/.zshrc.

DISPLAY variable

Specifies where GUI programs send their output.

DISPLAY=:0.0

Local desktop session using display 0 and screen 0.

DISPLAY=localhost:10.0

Remote X forwarding session over SSH.

Absolute path in Linux

A complete address to a file/folder starting from the root (/).

Relative path in Linux

A file/folder address based on your current working directory.

~ in Linux paths

The current user's home directory.

Environment configuration files

Files like .bash_profile, .bashrc, .profile that define shell behavior.

.bash_profile

Loads user-specific variables when starting a Bash login session.

.bashrc

Runs for every interactive, non-login shell like opening a terminal.

.profile

General login session settings, especially for POSIX-compliant shells.

Input redirection (

Redirects a command's input from a file instead of the keyboard.

Here-document (<

Allows embedding a block of text directly in a script/command.

Here-string (<<

Sends a single string as input to a command.

Standard input (stdin)

Default method for commands to receive input (usually the keyboard).

Output redirection (> and >>)

> overwrites a file with output; >> appends output to a file.

stderr in Linux

A separate output stream used for error messages.

2> and 2>> in output redirection

Redirects stderr to a file (overwrite or append).

&>

Redirects both stdout and stderr to the same file.

History command

Shows previously run commands in current and past sessions.

!!

Repeats the last command entered.

!43 or !cmd

Re-runs a specific command by number or prefix from history.

alias in Linux

A shortcut for a longer command.

make an alias permanent

Add it to ~/.bashrc or ~/.bash_profile.

regex

A series of character patterns used to search or match text.

regex symbol .

Matches any single character.

^ and $ in regex

^ matches start of line, $ matches end of line.

[ ] in regex

Define character ranges or sets (e.g., [a-z], [0-9]).

* in regex

Matches zero or more of the preceding character.

+ in regex

Matches one or more of the preceding character.

? in regex

Matches zero or one of the preceding character.

{n} in regex

Matches exactly n occurrences of the previous character.

grep

Searches files for lines that match a regex pattern.

sed

Stream editor used for modifying text in files.

awk

Text processing tool that handles structured data and prints specific fields.

cut

Extracts sections or columns from files.

tr

Translates or deletes characters in text.

wc

Counts lines, words, and characters in text.

sort

Arranges data by specified fields or values.

uniq

Filters out or counts duplicate lines in text.

xargs

Builds command lines from input (often used with find).

head

Displays the first few lines of a file.

tail

Displays the last few lines of a file; can follow live changes.

more

Displays file content page-by-page (scroll forward only).

less

Displays file content with forward/backward scrolling and searching.

echo

Prints text or variables to the screen or a file.

cat

Reads, concatenates, and displays file contents.

tee

Writes output to both a file and the screen at the same time.

printf

Prints formatted text with control over variables and layout.

bc

A command-line calculator for arithmetic operations.

uname

Displays system information (OS, kernel version, architecture).

source

Executes a script in the current shell, applying changes immediately.

nano

A simple, user-friendly terminal text editor.

nano's save and exit shortcuts

Ctrl + O to write, Ctrl + X to exit.

vi/vim

Advanced modal text editors with Insert, Command, and Execute modes.

command in vim saves and exits

:wq

quit vim without saving

:q!

replace text in vim

:s/old/new/g

highlight search matches in vim

:set hlsearch