Bash & Linux Cheat Sheet

Linux: The Operating System for Developers & Builders

Free, open-source, runs everything from your laptop to cloud servers. Master it once, use it everywhere.

---

What It Is (Simple)

Think of it as the universal building block of software infrastructure.

• Operating System: Kernel + tools + packages + freedom
• Created: 1991 by Linus Torvalds (still maintained by global community)
• Philosophy: Free, open-source, user controls everything
• Runs: Laptops, servers, IoT devices, phones (Android), cloud infrastructure, embedded systems

Why it matters for builders:

• ✅ Free (no licensing costs)
• ✅ Open-source (see and modify anything)
• ✅ Runs your code the same way locally as in production
• ✅ Industry standard (AWS, Google Cloud, Azure all run Linux)
• ✅ Powerful command-line tools for automation
• ✅ Scales from Raspberry Pi to 10,000-server clusters

Core Linux Concepts

1. The Kernel

The core. Manages:

• CPU scheduling (which program gets to run)
• Memory management (RAM allocation)
• Process management (running programs)
• File systems (storage)
• Networking (TCP/IP stack)
• Device drivers (hardware access)

You don't interact with the kernel directly. It just works. But when things break, understanding the kernel helps.

2. File System Hierarchy

/                    ← Root (top-level)
├── /home/           ← User files (your code, documents)
├── /var/            ← Logs, databases, runtime data
├── /etc/            ← Configuration files
├── /bin/            ← Essential commands
├── /usr/bin/        ← Installed programs
├── /opt/            ← Third-party software
├── /tmp/            ← Temporary files (deleted on reboot)
└── /root/           ← Root user's home (dangerous!)

Important:

• Everything is a file (even devices are files in /dev)
• Permissions matter (rwx = read/write/execute)
• /etc is where config files live (understand this for production debugging)

Navigation & File System

Command	Explanation
pwd	Print current directory
ls	List directory contents
ls -la	List all files with details
ls -lh	Human readable sizes
cd dir	Change directory
cd ..	Move one directory up
cd ~	Go to home directory
cd -	Go to previous directory
tree	Show directory structure

---

File & Directory Operations

Command	Explanation
touch file	Create empty file
mkdir dir	Create directory
mkdir -p a/b/c	Create nested directories
cp file1 file2	Copy file
cp -r dir1 dir2	Copy directory
mv old new	Move or rename file
rm file	Delete file
rm -r dir	Delete directory
rm -rf dir	Force delete directory

---

Viewing Files

Command	Explanation
cat file	Print file contents
less file	Scrollable viewer
head file	First 10 lines
head -n 50 file	First 50 lines
tail file	Last 10 lines
tail -f log	Follow log updates
wc file	Count lines, words, bytes

---

Searching Files & Content

grep (search inside files)

Command	Explanation
grep "text" file	Search text
grep -i "text" file	Case insensitive search
grep -n "text" file	Show line numbers
grep -c "text" file	Count matches
grep -r "text" .	Recursive search

Example:

grep -i "error" app.log

---

find (search files)

Command	Explanation
find . -name "*.log"	Find log files
find /path -type f	Find files
find /path -type d	Find directories
find /logs -mtime -1	Modified in last 24 hours
find /logs -size +100M	Files larger than 100MB
find /logs -name "*.log" -exec grep -l "ERROR" {} \;	Find logs containing ERROR

These log-analysis commands are heavily used in DevOps troubleshooting.

---

Text Processing (Core Automation Tools)

Command	Explanation
sort file	Sort lines
uniq	Remove duplicates
cut -d',' -f1	Extract column
awk '{print $1}'	Print column
sed 's/a/b/g'	Replace text

Example pipeline:

cat logs.txt | grep ERROR | sort | uniq

---

File Permissions (IMPORTANT)

Linux permissions follow:

rwx rwx rwx
│   │   │
│   │   └── others
│   └────── group
└────────── owner

Each permission has numeric values:

Permission	Value
Read	4
Write	2
Execute	1

Example combinations:

Command	Meaning
chmod 755 file	Owner full, others read+execute
chmod 644 file	Owner read/write, others read
chmod 700 file	Owner full access only
chmod +x script.sh	Make script executable

Example:

chmod +x deploy.sh

Ownership:

Command	Explanation
chown user file	Change owner
chown user:group file	Change owner and group
chgrp group file	Change group

The permission structure rwx rwx rwx is shown in the Linux cheatsheet document as the standard format.

---

Compression & Archiving

tar (most common)

Command	Explanation
tar -cvf archive.tar dir	Create archive
tar -xvf archive.tar	Extract archive
tar -czvf archive.tar.gz dir	Compress archive
tar -xzvf archive.tar.gz	Extract compressed

---

zip

Command	Explanation
zip archive.zip file	Create zip
unzip archive.zip	Extract zip

---

rar (optional)

Command	Explanation
rar a archive.rar file	Create rar
rar x archive.rar	Extract rar

---

Process Management

Command	Explanation
ps	Show processes
ps aux	Detailed process list
top	Real-time process monitor
htop	Advanced monitor
kill PID	Terminate process
kill -9 PID	Force kill

---

Disk & System Monitoring

Command	Explanation
df -h	Disk space
du -sh dir	Directory size
free -h	Memory usage
uptime	System uptime
lsblk	Show disks

---

Networking

Command	Explanation
ping host	Test connectivity
curl url	Send HTTP request
wget url	Download file
netstat -tuln	Show open ports
ss -tuln	Modern port viewer

---

Piping & Redirection (Power Feature)

Symbol	Meaning
`	`	Pipe output
>	Redirect output
>>	Append output
2>	Redirect error
&>	Redirect stdout + stderr

Example:

grep ERROR app.log | sort | uniq > errors.txt

---

Command Chaining

Operator	Meaning
cmd1 && cmd2	Run cmd2 if cmd1 succeeds
`cmd1		cmd2`	Run cmd2 if cmd1 fails
cmd1 ; cmd2	Run both

---

History & Productivity

Command	Explanation
history	Show command history
!!	Repeat last command
!n	Run command number
Ctrl + R	Search history
clear	Clear terminal
man command	Show manual

---

Introduction to Bash Scripting

Bash scripting is a powerful way to automate tasks on Linux and Unix-like systems using the Bash shell. It involves writing sequences of commands in a plain text file (a script) that can be executed by the Bash shell.

Why Use Bash Scripts?

• Automation of repetitive tasks
• System administration and maintenance
• Complex workflow management
• Direct OS interaction - commands interact directly with the operating system
• Versatile and powerful - combine multiple system tools seamlessly

Key Concepts

• Variables - Store and manipulate data
• Conditionals - Make decisions in your scripts
• Loops - Repeat operations efficiently
• Functions - Create reusable code blocks
• Script arguments - Accept input parameters
• File format - Scripts are saved with .sh extension
• Execution - Make executable with chmod +x and run with ./script.sh

Creating & Running Bash Scripts

Create script

touch script.sh

Shebang (define interpreter)

#!/bin/bash

Make executable

chmod +x script.sh

Run script

./script.sh

Run without executable

bash script.sh

---

Basic Script Template

#!/bin/bash

set -e
set -u
set -o pipefail

echo "Starting script..."

DATE=$(date)

echo "Current date: $DATE"

Recommended safety options:

Option	Meaning
set -e	Exit on error
set -u	Fail if variable undefined
set -o pipefail	Catch pipe failures

These make scripts much safer in production.

---

Variables

Declare variable

NAME="Mahendra"

Use variable

echo "$NAME"

Command output into variable

DATE=$(date)
FILES=$(ls)

Best practices

UPPERCASE for constants
Quote variables "$VAR"
Define variables at top

---

Script Arguments

Access script parameters

./script.sh file1.txt

Inside script:

$0 → script name
$1 → first argument
$2 → second argument
$# → number of arguments
$@ → all arguments

Example:

FILE=$1
echo "Processing $FILE"

---

Arrays

Create array

ERRORS=("ERROR" "CRITICAL" "FATAL")

Access elements

${ERRORS[0]}
${ERRORS[@]}

Loop array

for e in "${ERRORS[@]}"
do
 echo "$e"
done

---

Conditionals

Basic syntax

if [ condition ]; then
  command
fi

Example

if [ -f file.txt ]; then
  echo "File exists"
fi

Common comparisons

Operator	Meaning
-eq	equal
-ne	not equal
-gt	greater
-lt	less
-ge	greater or equal
-le	less or equal

String comparison

if [ "$VAR" = "value" ]

---

Loops

For Loop

for file in *.log
do
  echo "$file"
done

While Loop

while read line
do
 echo "$line"
done < file.txt

---

Functions

Define function

check_logs() {
  echo "Checking logs..."
}

Call function

check_logs

---

File Tests

Test	Meaning
-f	file exists
-d	directory exists
-s	file not empty
-x	executable
-r	readable
-w	writable

Example

if [ -d /var/log ]; then
  echo "Directory exists"
fi

---

Logging Output

Print message

echo "Script started"

Write to file

echo "log message" >> log.txt

Redirect errors

command 2> error.log

Redirect all output

command > output.log 2>&1

---

Debugging Scripts

Enable debug mode

set -x

Disable debug

set +x

Run debug

bash -x script.sh

---

Scheduling Scripts (Automation)

Cron job format

* * * * * command
│ │ │ │ │
│ │ │ │ └ day of week
│ │ │ └ month
│ │ └ day
│ └ hour
└ minute

Example

0 2 * * * /scripts/backup.sh

Run script every day at 2 AM.

---

Useful Bash Built-ins

Command	Explanation
read	read user input
echo	print output
exit	exit script
source file	run script in same shell
export VAR	environment variable

---

Error Handling Pattern

Production scripts should handle errors.

Example:

if ! command; then
  echo "Command failed"
  exit 1
fi

---

Real World Script Example

Log Monitoring Script

#!/bin/bash

LOG_DIR="/var/log"
REPORT="error_report.txt"
ERROR_PATTERNS=("ERROR" "CRITICAL" "FATAL")

echo "Starting log analysis..." > "$REPORT"

LOG_FILES=$(find "$LOG_DIR" -name "*.log")

for FILE in $LOG_FILES
do
  echo "Checking $FILE"

  for PATTERN in "${ERROR_PATTERNS[@]}"
  do
    COUNT=$(grep -c "$PATTERN" "$FILE")

    if [ "$COUNT" -gt 0 ]; then
      echo "$FILE contains $COUNT $PATTERN entries" >> "$REPORT"
    fi
  done
done

echo "Analysis complete. Report saved to $REPORT"

---

Mental Model of Bash Automation

Most scripts follow this flow:

INPUT
   ↓
FIND FILES
   ↓
FILTER (grep)
   ↓
PROCESS (loops)
   ↓
CONDITIONS
   ↓
REPORT / ACTION