Bash Guide

Introduction to Shell Scripting¶

A shell is a command-line interface that allows users to interact with the operating system by typing commands. It acts as a bridge between the user and the kernel.

Types of Shells¶

Bash (Bourne Again Shell)
C Shell (csh)
Korn Shell (ksh)
T C Shell (tcsh)
Z Shell (zsh)

This guide focuses on Bash, which is the default shell in most Linux distributions.

Why Write Shell Scripts?¶

Automate repetitive tasks
Schedule background jobs
Manage system operations
Rapidly prototype command pipelines

Writing Your First Bash Script¶

Create a script file with .sh extension: bash vim hello.sh
Add a shebang (defines the interpreter): bash #!/bin/bash echo "Hello, World"
Make it executable: bash chmod +x hello.sh
Run the script: bash ./hello.sh

Output:


Hello, World

Variables in Bash¶

Defining Variables¶

name="Chronos"
echo "Hello, $name"
````

> Note: No spaces before or after the `=` sign.

---

## Arithmetic Expressions

### Basic Operators

| Operator | Description         |
| -------- | ------------------- |
| `+`      | Addition            |
| `-`      | Subtraction         |
| `*`      | Multiplication      |
| `/`      | Division            |
| `**`     | Exponentiation      |
| `%`      | Modulus (remainder) |

### Examples

Using `expr`:

```bash
expr 11 + 1
expr 16 % 11

Using $(( )) for inline evaluation:

result=$((3 + 9))
echo $result

Reading User Input¶

Syntax¶

read variable_name
read -p "Prompt: " variable_name

Example¶

#!/bin/bash
echo "Enter a number:"
read a

read -p "Enter another number: " b

sum=$((a + b))
echo "Sum is: $sum"

Conditional Statements¶

Numeric Comparison¶

Operator	Description
`-eq`	Equal
`-ne`	Not equal
`-gt`	Greater than
`-lt`	Less than
`-ge`	Greater than or equal to
`-le`	Less than or equal to

Syntax¶

if [ condition ]; then
    # commands
fi

Example¶

read x
read y

if [ $x -gt $y ]; then
    echo "X is greater than Y"
elif [ $x -lt $y ]; then
    echo "X is less than Y"
else
    echo "X is equal to Y"
fi

Logical Operators¶

-a → AND
-o → OR

Example: Triangle Type¶

read a
read b
read c

if [ $a -eq $b -a $b -eq $c ]; then
    echo "EQUILATERAL"
elif [ $a -eq $b -o $b -eq $c -o $a -eq $c ]; then
    echo "ISOSCELES"
else
    echo "SCALENE"
fi

Looping Constructs¶

For Loop (Numbers)¶

for i in {1..5}; do
    echo $i
done

For Loop (Strings)¶

for fruit in apple orange grape; do
    echo $fruit
done

While Loop¶

i=1
while [[ $i -le 10 ]]; do
    echo $i
    ((i++))
done

Reading Files Line-by-Line¶

#!/bin/bash

LINE=1
while read -r CURRENT_LINE; do
    echo "$LINE: $CURRENT_LINE"
    ((LINE++))
done < "text-file.txt"

Command Substitution¶

Using Backticks¶

var=`df -h | grep tmpfs`

Prefer $(command) over backticks for readability:

var=$(df -h | grep tmpfs)

Passing Arguments to Scripts¶

Use $@ to refer to all positional arguments.

Example¶

#!/bin/bash

for arg in "$@"; do
    echo "Argument: $arg"
done

Execution:

$ ./script.sh 1 2 3
Argument: 1
Argument: 2
Argument: 3

Automating with Cron¶

Cron is used to schedule scripts or commands.

Crontab Syntax¶

* * * * * command
│ │ │ │ │
│ │ │ │ └─ Day of Week (0–7, Sunday=0/7)
│ │ │ └─── Month (1–12)
│ │ └───── Day of Month (1–31)
│ └─────── Hour (0–23)
└───────── Minute (0–59)

Examples¶

10 1 * 1 * → Every Jan 1st at 01:10
0 12 * * * → Every day at 12:00 PM
0 22 * * 7 → Every Sunday at 10:00 PM

Editing Cron Jobs¶

crontab -e

Example Entry¶

0 6 * * * /home/user/scripts/backup.sh

Best Practices¶

Always start with #!/bin/bash
Use set -e to stop script on first error
Quote variables: "$var" to prevent globbing and word splitting
Validate input when reading from user or CLI
Use comments generously to document code
Avoid hardcoding paths; use variables
Log output to files for long-running cron jobs