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Bash Scripting

1. Introduction

  • Bash (Born Again Shell) is a command-line utility used to interact with Unix and Linux systems.

  • We use CLI commands that are executed by the Bash.

  • This typing of commands could be a tedious task if many commands are to be executed with various parameter options. We need a way to automate the long processes such as managing files, installing software, or configuring systems.

  • A Bash script is a text file that contains a series of Bash commands, allowing users to automate tasks they would normally type manually in the terminal.

  • Bash is not just a command-line utility; it's also a simple programming language. It includes constructs like variables, arrays, loops, and conditional statements, which allow users to run commands in a structured and logical way.

  • In this module, we describe various Bash scripting techniques, each explained with practical examplesd.

2. echo and comments

  • echo is one of the simplest and most useful commands in Bash.
  • It displays text or variable values on the screen — or sends them to a file.

echo "Hello, world!"
"Output is"
Hello, world!

name="Mujeeb"
echo "Hello, $name!"
Output:
Hello, Mujeeb!

In Bash, comments are simple and start with the # symbol. Everything after # on the same line is ignored by the shell.

# This is a single-line comment
echo "Hello, Bash!"  # This is an inline comment

2.1 Special Characters and Escapes

  • By default, echo doesn’t interpret escape sequences like \n (newline)
  • The -e option in echo enables interpretation of escape sequences 

echo "Line1\nLine2"
ouput:
Line1\nLine2

echo -e "Line1\nLine2"
ouput:
Line1
Line2


echo -e "Bash scripting basics:\n1. Variables\n2. Loops\n3. Conditions"
Output: 
Bash scripting basics:
1. Variables
2. Loops
3. Conditions

2.2 List of escape Characters

Escape Sequence Description Example Command Example Output
\n New line echo -e "Line1\nLine2" Line1
Line2
\t Horizontal tab echo -e "Col1\tCol2" Col1 Col2
\v Vertical tab echo -e "A\vB" A

B
\r Carriage return (moves cursor to line start) echo -e "Hello\rWorld" World
\b Backspace (deletes previous character) echo -e "abc\b" ab
\a Alert (system bell sound) echo -e "Beep\a" (beep sound)
\\ Prints a single backslash (\) echo -e "\\n" \n
\" Prints double quotes echo -e "\"Hello\"" "Hello"
\0NNN Character with octal value NNN echo -e "\041" !

2.3 Basic file creation with echo

echo "Hello, Bash world!" > hello.txt

This creates (or overwrites) a file named hello.txt with the text: 

Hello, Bash world!

2.4 printf vs echo

Feature echo printf
Purpose Prints text or variables to the terminal Prints formatted text with control over layout
Automatic newline Adds newline automatically Does not add newline unless specified (\n)
Escape sequences Needs -e option to interpret (\n, \t) Always interprets escape sequences
Formatting Limited Powerful (supports %s, %d, %f, etc.)
Portability Behavior can vary between shells More consistent across systems
Error handling Always returns 0 Returns nonzero on output error
Typical use Simple messages or logs Structured, aligned, or formatted output

Example:

echo "Hello, World!"
printf "Hello, %s!\n" "World"
Output: 
Hello, World!
Hello, World!

3. Creating variables

Topic Description Example
Creating a Variable In Bash, variables are created by assigning a value without spaces around =. name="Mujeeb"
Printing a Variable Use $ before the variable name or ${} for clarity. echo $name → Mujeeb
echo ${name} → Mujeeb
Reading Input from Keyboard Use the read command to take input from the user. read username
echo "Hello, $username!"
Variable Naming Rules - Must start with a letter or underscore _
- Can contain letters, numbers, underscores
- Case-sensitive		 Valid: _var1, name
Invalid: 1name, my-var
Types of Variables Bash has only strings by default. Numbers are treated as strings unless used in arithmetic.
Special types: arrays, associative arrays, environment variables		 num=10
declare -a arr=(1 2 3)
declare -A assoc=([key1]=val1 [key2]=val2)
Unsetting Variables Remove a variable using unset. unset name
Default Values Use ${var:-default} to provide a default if variable is empty/unset echo ${username:-Guest} → prints Guest if username is empty

4. Arithmetics

4.1 Variables are Strings by default

Bash has only strings by default. Numbers are treated as strings.

Example 1: Assign a number to a variable 

num="10"

Example 2: Add two values 

result="$num + 5"
echo "Result without arithmetic: $result"
Output: 
Result without arithmetic: 10 + 5

4.2 Perform arithmetic

Using $(( )), Bash interprets the variable as a number and performs the calculation.

# Arithmetic evaluation
num=10
sum=$((num + 5))
echo "Result with arithmetic: $sum"
Output: 
Result with arithmetic: 15

4.3 Caution

Expression Description Output / Behavior
result=$num + 5 No quotes, no arithmetic result becomes 10; Bash tries to run + 5 as a command → Error: bash: +: command not found
result="$num + 5" Quotes around expression result becomes the string 10 + 5 (no calculation)
result=$((num + 5)) Arithmetic context result becomes 15 (calculation performed)
echo $result Display the value Prints the value of result depending on above assignment

4.4 List of arithmetic operation

Operation Symbol Example Output
Addition + a=10; b=5; sum=$((a + b)); echo $sum 15
Subtraction - a=10; b=5; diff=$((a - b)); echo $diff 5
Multiplication * a=10; b=5; prod=$((a * b)); echo $prod 50
Division / a=10; b=5; div=$((a / b)); echo $div 2
Modulus (remainder) % a=10; b=3; mod=$((a % b)); echo $mod 1
Exponentiation ** a=2; b=3; pow=$((a ** b)); echo $pow 8
Increment ++ a=5; ((a++)); echo $a 6
Decrement -- a=5; ((a--)); echo $a 4

(Under construction)