Variables in Bash

Variables in Bash are used to store and manipulate data. They are essential for writing flexible and reusable scripts.

1. Declaring Variables

To declare a variable, simply assign a value to a name without spaces around the = sign.

Syntax:

variable_name=value

Examples:

name="Alice"
age=25

2. Accessing Variables

To use the value of a variable, prefix its name with a $.

Examples:

name="Alice"
echo "Hello, $name!"  # Output: Hello, Alice!

Using Curly Braces

Use curly braces {} to explicitly delimit the variable name.

greeting="Hello"
echo "${greeting}, World!"  # Output: Hello, World!

3. Types of Variables

a) Local Variables

These variables are available only within the current shell session or script.

b) Environment Variables

These variables are available globally and can be accessed by any subprocess.

Examples:

export PATH="/usr/local/bin:$PATH"
echo $PATH

c) Special Variables

Bash provides predefined variables for specific purposes, such as:

$0: The name of the script.
$1, $2, ...: Positional parameters for command-line arguments.
$#: Number of arguments passed.
$?: Exit status of the last executed command.
$$: Process ID of the current shell.
$@ and $*: All arguments passed to the script.

4. Rules for Variable Names

Variable names can contain letters, numbers, and underscores (_).
They must start with a letter or underscore.
Avoid using special characters or spaces in names.
Variable names are case-sensitive.

Examples:

my_var=10       # Valid
MyVar=20        # Valid (different from my_var)
1var=30         # Invalid (cannot start with a number)
var-name=40     # Invalid (special characters not allowed)

5. Default Variable Values

Use ${variable:-default} to provide a default value if the variable is unset or empty.

Examples:

name=${name:-"Guest"}
echo "Hello, $name!"  # Output: Hello, Guest! (if name is unset)

6. Modifying Variables

a) Arithmetic Operations

Use $((expression)) for arithmetic.

Examples:

num1=5
num2=10
sum=$((num1 + num2))
echo $sum  # Output: 15

b) String Concatenation

first="Hello"
second="World"
greeting="$first, $second!"
echo $greeting  # Output: Hello, World!

c) Increment and Decrement

counter=5
((counter++))  # Increment
echo $counter  # Output: 6

7. Reading User Input into Variables

Use the read command to get input from the user.

Examples:

echo "Enter your name:"
read name
echo "Hello, $name!"

8. Exporting Variables

To make a variable accessible to child processes, use export.

Examples:

export MY_VAR="exported_value"
bash -c 'echo $MY_VAR'  # Output: exported_value

9. Unsetting Variables

To remove a variable, use the unset command.

Examples:

name="Alice"
unset name
echo $name  # Output: (empty)

10. Best Practices for Using Variables

Always Quote Variables: Quoting prevents issues with spaces and special characters.
```
file_name="My File.txt"
echo "$file_name"
```
Use Meaningful Names: Choose descriptive names for better readability.
```
total_users=100
```
Initialize Variables Before Use: Avoid referencing uninitialized variables.
Use Local Variables in Functions: Prevent conflicts by declaring variables with local inside functions.
```
my_function() {
    local var="local_value"
    echo $var
}
```

11. Debugging Variables

Use the declare or set command to list all variables in the current session.

Examples:

declare -p
set

Conclusion

Variables in Bash are the backbone of any script, enabling data manipulation, configuration, and communication between processes. By following best practices and mastering variable operations, you can create efficient and flexible scripts.

Let me know if you’d like more advanced examples or a breakdown of environment variables!

Souy Soeng

Variables in Bash