A shell script is a concise program designed to automate tasks on a computer. These scripts are plain text files, editable with any text editor, and executed by an interpreter without prior compilation.

Your Initial Script

Create a file named greeting.sh with the following content:

#!/usr/bin/env bash
echo "Greetings, Universe!"

The shebang (#!) specifies the interpreter. Grant execute permission and run it:

chmod u+x greeting.sh
./greeting.sh

Working with Variables

Defining Variables

Assign values using an equals sign without spaces.

username="dev_user"

Rules:

• No spaces around =.
• Name can contain letters (case-sensitive), numbers (not at start), and underscores.
• Avoid reserved keywords.
• Conventionally, uppercase denotes constants.

Accessing Variables

Prefix the variable name with a dollar sign.

username="dev_user"
echo "User: $username"

Command Substitution vs. Variable Expansion

• $() executes a command and substitutes its output. Backticks ` ` do the same.
• ${} ensures correct variable expansion, especially when adjacent to other text. Example:

current_date=$(date +%Y-%m-%d)
echo "Today is $current_date"
# Equivalent with backticks
timestamp=`date +%s`
echo "Timestamp: $timestamp"

Immutable Variables

Mark a variable as read-only with readonly.

config_path="/etc/app/config"
readonly config_path

Removing Variables

Use unset to delete a variable. This fails for read-only variables.

unset temp_var

Predefined Variables

Special variables provide runtime information.

• $?: Exit status of the last command.
• $1, $2, ...: Positional parameters. For the tenth and beyond, use ${10}.

Other useful built-in variables:

#!/usr/bin/env bash
read -p "Enter input: "
echo "Parent PID: $PPID"
echo "Current directory: $PWD"
echo "Line number: $LINENO"
echo "Random number: $RANDOM"
echo "Last input (non-option): $REPLY"
echo "Option arg: $OPTARG, Option index: $OPTIND"
echo "Script duration (seconds): $SECONDS"

Environment Variables

View all with env. They are typically uppercase.

env

Set a new variable for the current session:

export APP_MODE="production"
export PATH="$HOME/.local/bin:$PATH"

To make permanent, add to ~/.bashrc (user-specific) or /etc/profile (system-wide).

Data Types and Structures

Declare different types of variables:

app_name="Data Processor"  # String
declare -i item_count=100   # Integer
data_set=(10 20 30 40 50)   # Indexed array
declare -A metadata         # Associative array
metadata["version"]="1.2"
metadata["author"]="admin"

String Manipulation

Strings can use single or double quotes. Double quotes allow variable expansion and escape sequences. Single quotes treat everything literal.

app_desc="Processes data efficiently."
len=${#app_desc}
sub_str=${app_desc:0:9}
char_pos=$(expr index "$app_desc" "aeiou") # Find first vowel

Advanced extraction using patterns:

url="https://example.com/docs/api-guide.html"
echo "Full URL: $url"
echo "After first slash: ${url#*/}"          # Removes up to first '/'
echo "After last slash: ${url##*/}"         # Removes up to last '/'
echo "Before last slash: ${url%/*}"         # Keeps part before last '/'
echo "Before first slash: ${url%%/*}"        # Keeps part before first '/'

Arrays

Access elements by index, starting from 0.

data_set=(10 20 30 40 50)
third_element=${data_set[2]}
all_elements=("${data_set[@]}")
element_count=${#data_set[@]}
length_of_third=${#data_set[2]}

Associative Arrays

Store key-value pairs. Declare with -A.

declare -A server_info
server_info=(["ip"]="192.168.1.10" ["role"]="web")
# Or assign individually
server_info["status"]="active"
echo "IP Address: ${server_info[\"ip\"]}"