🐧 The Linux Terminal

"The command line is your superpower - master it, and you master the machine."

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Cheat Sheet

📑 Table of Contents

• 📖 Getting Help
  • man — Manual Pages
  • Other Help Commands
  • type — Command Type Checker
  • wc — Word Count
  • Searching All Man Pages
• ⌨️ Keyboard Shortcuts
  • Navigation
  • Editing
  • Control
  • History
• 📜 Command History
  • History Expansion (Bang Commands)
  • Managing History
  • History Environment Variables
• 🔐 Root vs Non-Root Users
  • Ways to Gain Root Privileges
  • Managing Sudo Sessions
• 🔀 Piping and Command Redirection
  • Understanding Standard Streams
  • Piping
  • Output Redirection
  • Input Redirection
  • The tee Command
• 🔍 Finding Files and Directories
  • which — Find Executables
  • locate / plocate — Database Search
  • find — Real-time Search
• 🔎 grep — Search File Contents
  • Regular Expression Basics
  • strings — Extract Readable Text
• 🎓 Additional Essential Commands
• 📚 Quick Reference Card

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📖 Getting Help

One of the most important skills in Linux is knowing how to find help. The terminal provides several built-in documentation systems.

man — Manual Pages

man <command>

Opens the manual pages for any command. This is your primary reference.

Navigation within man pages:

Key	Action
h	Display help
q	Quit
f or Space	Forward one page
b	Back one page
/pattern	Search forward
?pattern	Search backward
n	Next search result
N	Previous search result
g	Go to beginning
G	Go to end

Man page sections:

Section	Content
1	User commands
2	System calls
3	Library functions
4	Special files
5	File formats
6	Games
7	Miscellaneous
8	System administration

💡 Tip: Use man 5 passwd to view section 5 of passwd (file format) vs man passwd (the command).

Other Help Commands

Command	Description
whatis <cmd>	One-line description of a command
info <cmd>	GNU-style manual (often more detailed)
<cmd> --help	Built-in quick syntax reference
help <builtin>	Help for shell built-in commands

type — Command Type Checker

type <command>

Reveals whether a command is:

• A shell builtin (use help for documentation)
• An alias
• A function
• An external executable (use man for documentation)

$ type cd
cd is a shell builtin

$ type ls
ls is aliased to 'ls --color=auto'

$ type grep
grep is /usr/bin/grep

wc — Word Count

wc <file>

Displays line count, word count, and byte count.

Flag	Output
-l	Line count only
-w	Word count only
-c	Byte count only
-m	Character count

$ wc /etc/passwd
  45   89  2678 /etc/passwd
#lines words bytes

Searching All Man Pages

man -k <keyword>
# or
apropos <keyword>

Search through all manual page descriptions.

$ man -k "copy files"
cp (1)     - copy files and directories
install (1) - copy files and set attributes

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⌨️ Keyboard Shortcuts

Master these shortcuts to dramatically increase your terminal efficiency.

Navigation

Shortcut	Action
Ctrl + A	Move cursor to beginning of line
Ctrl + E	Move cursor to end of line
Ctrl + B	Move cursor back one character
Ctrl + F	Move cursor forward one character
Alt + B	Move cursor back one word
Alt + F	Move cursor forward one word

Editing

Shortcut	Action
Ctrl + U	Delete everything before cursor
Ctrl + K	Delete everything after cursor
Ctrl + W	Delete word before cursor
Alt + D	Delete word after cursor
Ctrl + Y	Paste (yank) previously deleted text
Ctrl + _	Undo last edit

Control

Shortcut	Action
Ctrl + L	Clear screen (same as clear)
Ctrl + C	Interrupt/kill current process
Ctrl + Z	Suspend current process (use fg to resume)
Ctrl + D	Exit shell / EOF signal
Ctrl + S	Pause terminal output
Ctrl + Q	Resume terminal output

History

Shortcut	Action
Ctrl + R	Reverse search through history
Ctrl + G	Exit history search mode
Ctrl + P or ↑	Previous command
Ctrl + N or ↓	Next command
Alt + .	Insert last argument of previous command

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📜 Command History

Your shell maintains a persistent history of commands, making it easy to recall and reuse previous work.

History Basics

history              # Display command history
history 20           # Show last 20 commands

History file location: ~/.bash_history (or ~/.zsh_history for Zsh)

History Expansion (Bang Commands)

Syntax	Description
!!	Repeat the last command
!n	Execute command number n from history
!-n	Execute command n lines back
!string	Execute most recent command starting with string
!?string	Execute most recent command containing string
!$	Last argument of previous command
!*	All arguments of previous command
!:n	The nth argument of the last command
^old^new	Repeat last command, replacing old with new

$ echo hello world
hello world

$ !!              # Runs: echo hello world
$ !echo           # Runs most recent 'echo' command
$ sudo !!         # Run last command with sudo
$ !echo:p         # Print (don't execute) the command

Managing History

history -d <n>    # Delete line n from history
history -c        # Clear entire history
history -w        # Write current history to file
history -r        # Read history file into current session

History Environment Variables

Variable	Description
$HISTSIZE	Number of commands kept in memory
$HISTFILESIZE	Number of commands saved to file
$HISTFILE	Location of history file
$HISTCONTROL	Controls what gets saved
$HISTIGNORE	Patterns to ignore
$HISTTIMEFORMAT	Timestamp format

$HISTCONTROL Values

Value	Behavior
ignorespace	Ignore commands starting with a space
ignoredups	Ignore duplicate consecutive commands
ignoreboth	Combines ignorespace and ignoredups
erasedups	Remove all previous duplicates

# Add to ~/.bashrc
export HISTCONTROL=ignoreboth:erasedups

$HISTTIMEFORMAT — Add Timestamps

export HISTTIMEFORMAT="%F %T  "
# Output: 2024-01-15 14:30:45  command

Format	Output
%F	Full date (YYYY-MM-DD)
%T	Time (HH:MM:SS)
%d	Day
%m	Month
%y	Year (2 digits)

💡 Pro tip: Add a trailing space in the format string for readability.

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🔐 Root vs Non-Root Users

Understanding privilege levels is fundamental to Linux security and administration.

The Root User

• UID 0 — The superuser with unrestricted access
• Can read, modify, or delete any file
• Can execute any command
• Use with caution! There's no "undo" for many root actions

Ways to Gain Root Privileges

Command	Description
sudo <command>	Execute single command as root
sudo -i	Interactive root shell (root's environment) ✅
sudo su -	Switch to root with root's environment ✅
sudo su	Switch to root, keep current environment
su	Switch to root (requires root password)
su - <user>	Switch to another user

⚠️ Best Practice: Prefer sudo -i or sudo su - to get a clean root environment.

Managing Sudo Sessions

sudo -k          # Invalidate cached credentials immediately
sudo -v          # Extend sudo timeout without running a command
sudo -l          # List your sudo privileges

The default sudo timeout is typically 5 minutes (configurable in /etc/sudoers).

The sudoers File

sudo visudo      # Safe way to edit sudoers

Never edit /etc/sudoers directly! Use visudo for syntax checking.

Checking Your Privileges

whoami           # Current username
id               # User ID, group ID, and groups
groups           # Groups you belong to

🛡️ Security tip: Only use root when necessary. Follow the principle of least privilege.

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🔀 Piping and Command Redirection

One of Linux's most powerful features is the ability to chain commands together and redirect their input/output.

Understanding Standard Streams

Every process has three standard streams:

Stream	File Descriptor	Description
stdin	0	Standard input
stdout	1	Standard output
stderr	2	Standard error

Piping (|)

Pipes send the output of one command as input to another.

command1 | command2 | command3

Examples:

ls -la | less                    # Browse long output
cat file.txt | sort | uniq       # Sort and remove duplicates
ps aux | grep nginx              # Find nginx processes
history | tail -20               # Last 20 commands
ls -lSh /etc | head              # 10 largest items in /etc

Output Redirection

Operator	Description
>	Redirect stdout (overwrite)
>>	Redirect stdout (append)
2>	Redirect stderr (overwrite)
2>>	Redirect stderr (append)
&>	Redirect both stdout and stderr
&>>	Append both stdout and stderr

Examples:

ls -l > listing.txt              # Save output to file
echo "log entry" >> app.log      # Append to file
find / -name "*.conf" 2> /dev/null   # Discard errors
command &> output.log            # Capture everything

Input Redirection

Operator	Description
<	Redirect stdin from file
<<	Here document (heredoc)
<<<	Here string

sort < unsorted.txt              # Sort file contents
wc -l < data.txt                 # Count lines

# Heredoc - multi-line input
cat << EOF
Line 1
Line 2
EOF

# Here string
grep "pattern" <<< "search in this string"

Combining Streams

# Redirect stderr to stdout
command 2>&1

# Redirect stdout to stderr
command 1>&2

# Send both to a file
command > output.log 2>&1

# Modern syntax (bash 4+)
command &> output.log

The tee Command

Write to both stdout AND a file simultaneously.

command | tee file.txt           # Output to screen and file
command | tee -a file.txt        # Append to file
command | tee file1.txt file2.txt   # Write to multiple files

$ ls -la | tee directory.txt
# Shows output AND saves to directory.txt

Process Substitution

Treat command output as a file.

diff <(command1) <(command2)     # Compare outputs
cat <(ls dir1) <(ls dir2)        # Concatenate outputs

💡 Pro tip: Use /dev/null as a "black hole" to discard unwanted output.

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🔍 Finding Files and Directories

Linux provides several tools for locating files, each with different use cases.

Quick Reference

Command	Search Method	Speed	Use Case
which	PATH only	Fast	Find executables
whereis	Standard locations	Fast	Find binaries, man pages, source
locate/plocate	Database	Very fast	Quick filename search
find	Real-time filesystem	Slower	Complex searches with actions

which — Find Executables

which python              # First match in PATH
which -a python           # All matches in PATH

whereis — Find Related Files

whereis bash
# bash: /usr/bin/bash /usr/share/man/man1/bash.1.gz

locate / plocate — Database Search

Searches a pre-built database (very fast but may be outdated).

locate filename           # Search for filename
locate -i filename        # Case-insensitive
locate -c filename        # Count matches
sudo updatedb             # Update the database

⚠️ Note: Database is typically updated daily via cron. Run updatedb for fresh results.

find — Real-time Search

The most powerful and flexible file search tool.

find <path> <options> <expression>

Basic Options:

Option	Description
-name "pattern"	Match filename (case-sensitive)
-iname "pattern"	Match filename (case-insensitive)
-type f	Files only
-type d	Directories only
-type l	Symbolic links only
-maxdepth n	Limit search depth
-mindepth n	Start at depth n

By Size:

Option	Description
-size +100M	Larger than 100MB
-size -1k	Smaller than 1KB
-empty	Empty files or directories

By Time:

Option	Description
-mtime -7	Modified in last 7 days
-mtime +30	Modified more than 30 days ago
-mmin -60	Modified in last 60 minutes
-newer file	Modified after reference file

By Permissions/Ownership:

Option	Description
-user username	Owned by user
-group groupname	Owned by group
-perm 644	Exact permissions
-perm -644	At least these permissions

Actions:

Option	Description
-print	Print pathname (default)
-ls	List in ls -l format
-delete	Delete matching files
-exec cmd {} \;	Execute command on each file
-exec cmd {} +	Execute command on all files at once

Examples:

# Find all .log files
find /var/log -name "*.log"

# Find files larger than 100MB
find / -type f -size +100M 2>/dev/null

# Find and delete empty directories
find . -type d -empty -delete

# Find files modified in last 24 hours
find . -mtime -1

# Find and change permissions
find . -type f -name "*.sh" -exec chmod +x {} \;

# Find with multiple conditions (AND)
find . -name "*.txt" -size +1M

# Find with OR condition
find . \( -name "*.jpg" -o -name "*.png" \)

# Find files NOT matching pattern
find . -type f ! -name "*.txt"

💡 Pro tip: Use 2>/dev/null with find to suppress "Permission denied" errors.

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🔎 grep — Search File Contents

grep (Global Regular Expression Print) searches for patterns within files.

Basic Syntax

grep [options] "pattern" <file(s)>
command | grep [options] "pattern"

Common Options

Option	Description
-i	Case-insensitive search
-v	Invert match (show non-matching lines)
-n	Show line numbers
-c	Count matching lines
-l	List only filenames with matches
-L	List filenames without matches
-w	Match whole words only
-x	Match whole lines only
-r / -R	Recursive search in directories
-A n	Show n lines after match
-B n	Show n lines before match
-C n	Show n lines before and after match
-E	Extended regex (same as egrep)
-P	Perl-compatible regex
-o	Show only matching part
--color	Highlight matches

💡 Note: grep -P is not supported everywhere (e.g., BusyBox and many BSD/macOS installs).

Examples

# Basic search
grep "error" logfile.txt

# Case-insensitive, with line numbers
grep -in "warning" /var/log/syslog

# Search entire word only
grep -w "fail" results.txt

# Recursive search in directory
grep -r "TODO" ./src/

# Search multiple patterns
grep -E "error|warning|critical" log.txt

# Exclude certain files
grep -r "pattern" --exclude="*.log" .

# Include only certain files
grep -r "function" --include="*.js" .

# Show context around matches
grep -C 3 "Exception" app.log

# Count occurrences
grep -c "GET" access.log

# Pipe with other commands
ps aux | grep nginx
cat /etc/passwd | grep -v "nologin"

Regular Expression Basics

Pattern	Meaning
.	Any single character
*	Zero or more of previous
+	One or more of previous (use -E)
?	Zero or one of previous (use -E)
^	Start of line
$	End of line
[abc]	Any character in brackets
[^abc]	Any character NOT in brackets
[a-z]	Range of characters
\b	Word boundary
\d	Digit (use -P)
\s	Whitespace (use -P)

# Lines starting with "Error"
grep "^Error" log.txt

# Lines ending with a number
grep "[0-9]$" data.txt

# Lines containing IP addresses (basic)
grep -E "[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+" access.log

strings — Extract Readable Text

Extracts printable ASCII strings from binary files.

strings /usr/bin/ls           # See strings in binary
strings -n 10 binary          # Minimum 10 characters
strings binary | grep "pass"  # Search for patterns

💡 Pro tip: Combine grep with find using xargs:

find . -name "*.py" | xargs grep "import"

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🎓 Additional Essential Commands

xargs — Build Commands from Input

Convert input into arguments for other commands.

# Find and remove files
find . -name "*.tmp" | xargs rm

# Parallel execution
cat urls.txt | xargs -P 4 -I {} curl {}

# Handle filenames with spaces
find . -name "*.txt" -print0 | xargs -0 cat

cut — Extract Columns

cut -d':' -f1 /etc/passwd     # First field, colon delimiter
cut -c1-10 file.txt           # First 10 characters

sort — Sort Lines

sort file.txt                 # Alphabetical
sort -n file.txt              # Numerical
sort -r file.txt              # Reverse
sort -u file.txt              # Unique (remove duplicates)
sort -k2 file.txt             # Sort by 2nd column

uniq — Filter Duplicates

sort file.txt | uniq          # Remove duplicates (requires sorted input)
sort file.txt | uniq -c       # Count occurrences
sort file.txt | uniq -d       # Show only duplicates

head & tail — View File Parts

head -n 20 file.txt           # First 20 lines
tail -n 20 file.txt           # Last 20 lines
tail -f /var/log/syslog       # Follow file (live updates)
tail -F log.txt               # Follow with retry (handles log rotation)

less — Pager for Large Files

less large_file.txt

Key	Action
Space	Next page
b	Previous page
/pattern	Search forward
?pattern	Search backward
n	Next search result
N	Previous search result
q	Quit
g	Go to beginning
G	Go to end

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📚 Quick Reference Card

Essential Navigation

pwd                 # Print working directory
cd <dir>            # Change directory
cd ~                # Go to home directory
cd -                # Go to previous directory
ls -la              # List all files with details

File Operations

cp src dest         # Copy file
mv src dest         # Move/rename file
rm file             # Remove file
rm -r dir           # Remove directory recursively
mkdir -p dir/sub    # Create nested directories
touch file          # Create empty file / update timestamp

Viewing Files

cat file            # Display entire file
less file           # Page through file
head -n 10 file     # First 10 lines
tail -n 10 file     # Last 10 lines
tail -f file        # Follow file changes

Getting Help

man command         # Manual page
command --help      # Quick help
whatis command      # One-line description
type command        # What type of command

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📝 Note: This guide covers Bash, the default shell on most Linux distributions. Some features may differ in other shells like Zsh or Fish.