Introduction

This guide covers concepts focusing on arrays, loops, file operations, arithmetic operations, and string manipulation.

1. Arrays in Bash

Arrays in Bash are zero-indexed lists of values. There are several ways to declare and initialize arrays.

Declaring and Initializing Arrays

Using declare:

declare -a my_array

Implicit declaration with initialization:

my_array=(apple banana cherry)

Empty array declaration:

my_array=()

Declaring with specific indices:

my_array=([0]="apple" [1]="banana" [5]="cherry")

Declaring empty and adding elements:

my_array=()
my_array+=("apple")
my_array+=("banana")

Declaring a read-only array:

declare -ar my_readonly_array=(apple banana cherry)

Accessing and Manipulating Arrays

# Access elements
echo ${my_array[0]}  # Output: apple

# Get all elements
echo ${my_array[@]}  # Output: apple banana cherry

# Get array length
echo ${#my_array[@]}  # Output: 3

# Slicing an array
echo ${my_array[@]:1:2}  # Output: banana cherry (from index 1, take 2 elements)

# Finding the indices of an array
echo ${!my_array[@]}  # Output: 0 1 2 (or 0 1 5 for the array with specific indices)

2. Loops in Bash

Bash provides several types of loops for repetitive tasks.

For Loops

Traditional for loop:

for i in 1 2 3 4 5
do
    echo $i
done

C-style for loop:

for ((i=0; i<5; i++))
do
    echo $i
done

For loop with array:

fruits=(apple banana cherry)
for fruit in "${fruits[@]}"
do
    echo $fruit
done

For loop with command substitution:

for file in $(ls)
do
    echo $file
done

While Loops

Basic while loop syntax:

count=1
while [ $count -le 5 ]
do
    echo "Count is: $count"
    ((count++))
done

3. File Operations

Reading from and writing to files is a common task in Bash scripting.

Reading a File Line by Line

while IFS= read -r line
do
    echo "Line: $line"
done < "input.txt"

Writing to a File

echo "Hello, World!" > output.txt
echo "Appending this line" >> output.txt

4. Arithmetic Operations in Bash

Bash offers various methods for arithmetic operations:

# Using let
let result=5+3
echo $result  # Output: 8

# Using (( ))
((result = 5 + 3))
echo $result  # Output: 8

# Using $[ ]
result=$[5 + 3]
echo $result  # Output: 8

# Using expr (note the spaces)
result=$(expr 5 + 3)
echo $result  # Output: 8

5. String Manipulation

String Case Checking

This example checks if given strings start with uppercase or lowercase letters:

#!/bin/bash

declare -a upper
declare -a lower

words=("Hello" "world" "BASH" "script" "ARRAY" "example")

for word in "${words[@]}"
do
    if [[ $word == [[:upper:]]* ]]
    then
        upper+=("$word")
    elif [[ $word == [[:lower:]]* ]]
    then
        lower+=("$word")
    fi
done

echo "Uppercase words: ${upper[@]}"
echo "Lowercase words: ${lower[@]}"

Advanced Pattern Matching

This script demonstrates number detection and pattern matching:

#!/bin/bash

declare -a num_var
declare -a userid_var
declare -a other_var

strings=("123" "abc" "456" "1234abcd56" "789xyz" "2023user01" "hello123")

for str in "${strings[@]}"
do
    if [[ $str =~ ^[0-9]+$ ]]
    then
        # String is a number
        num_var+=("$str")
    elif [[ $str =~ ^[0-9]{4}[a-z]{4}[0-9]{2}$ ]]
    then
        # String matches pattern \d{4}[a-z]{4}\d{2}
        userid_var+=("$str")
    else
        # String doesn't match any specific category
        other_var+=("$str")
    fi
done

echo "Numbers: ${num_var[@]}"
echo "User IDs: ${userid_var[@]}"
echo "Other strings: ${other_var[@]}"

6. Comprehensive Example: File Processing and Data Categorization

This example combines file reading, loops, and string manipulation to categorize data from a file:

#!/bin/bash

# Initialize arrays for different types of data
declare -a names
declare -a numbers
declare -a emails
declare -a mixed

# Read the file line by line
while IFS= read -r line
do
    # Check if line is a name (contains only letters and spaces)
    if [[ $line =~ ^[[:alpha:][:space:]]+$ ]]
    then
        names+=("$line")
    # Check if line is a number
    elif [[ $line =~ ^[0-9]+$ ]]
    then
        numbers+=("$line")
    # Check if line is an email
    elif [[ $line =~ ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}$ ]]
    then
        emails+=("$line")
    # If it doesn't match any of the above, it's mixed content
    else
        mixed+=("$line")
    fi
done < "data.txt"

# Print the categorized data
echo "Names: ${names[@]}"
echo "Numbers: ${numbers[@]}"
echo "Emails: ${emails[@]}"
echo "Mixed Content: ${mixed[@]}"

This script reads a file named data.txt, categorizes each line based on its content, and stores it in the appropriate array.

Conclusion

This tutorial has covered essential Bash scripting concepts including:

Array declaration and manipulation
For and while loops
File reading and writing operations
Arithmetic operations
String manipulation and pattern matching
A comprehensive example combining multiple concepts

array, aictionary, for and while loops to score 25 marks in System Commands OPPE