Bash OOP

The library adds an object abstraction, that has many features modern OOP-oriented have out-of-the-box.

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Features

All objects can be of type class or type instance. An instance of a class gets all methods and attributes of the class. They are independent of the class and can be changed, not affecting the parent class.

The library implements Multilevel Inheritance. Subclasses inherit from other subclasses. All subclasses inherit from base class Object.

Encapsulation is implemented strictly (as in Golang), in contrast to Python or Ruby. It is not possible to directly access a method or an attribute marked as private.

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Requirements

The library requires bash>=4.3.

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How to use

Clone the repo or copy the lib directory contents and import lib/main.sh to install the library.

# Clone the repo:
git clone https://github.com/mora9715/bash_oop

# Import the main library entrypoint in your scripts:
source bash_oop/lib/main.sh

Once the library is set up, make sure to check the Library API Reference.

For an even deeper understanding of what you can and cannot do with the library, take a look at the test cases.

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API Reference

The section documents public library API exposed to end-users.

Class declaration

A function class can be used to declare a new class. Class Name is passed as the first argument. Optional parent class can be passed as the second argument.

# Declare a class with the name Employee
class Employee

# Declare a new subclass SalaryEmployee of a class Employee
class SalaryEmployee Employee

Attribute access/declaration

A keyword : is here for attributes-related operations.

# Declare an attribute base_salary of the class SalaryEmployee
SalaryEmployee : base_salary = 5000
SalaryEmployee : active? = true

# Access the attribute
SalaryEmployee : base_salary

Method access/declaration

Methods-related operations are called using a dot keyword .. A new method can be declared by sending the method code to the STDIN of the method handler.

# Declare a method calculate_salary of the class SalaryEmployee
SalaryEmployee . calculate_salary << 'EOF'
  local extra_coefficient
  extra_coefficient="${1}"; shift
  
  echo "$((`${this} : base_salary` * ${extra_coefficient}))"
EOF

SalaryEmployee . suspend! << 'EOF'
  ${this} : active? = false
EOF

# Call a method calculate_salary of the class SalaryEmployee passing `extra_coefficient` argument
SalaryEmployee . calculate_salary 1.25

Instance creation

Call a class with an instance name as the first argument to create a new instance of the class.

A class can implement special init method that gets called after an instance is created.

SalaryEmployee . __init__ << 'EOF'
  ${this} : name = ${1}; shift
EOF

SalaryEmployee employee Joe

employee : name

Instance deletion

It is possible to destroy an instance of a class, removing all its declared methods and attributes. It is done using the destroy function.

A class can implement a special destroy method that gets called before an instance is destroyed.

SalaryEmployee . __destroy__ << 'EOF'
  run_some_cleanup
EOF

SalaryEmployee employee Joe

destroy employee

Protected attributes/methods

It is possible to declare a protected attribute/method. Such methods cannot be accessed from outside of the object context.

Add an underscore _ before attribute/method name to make it protected.

SalaryEmployee . set_position << 'EOF'
  ${this} . validate_position "${1}"
  ${this} : _position = "${1}"
EOF

SalaryEmployee . get_position << 'EOF'
  ${this} : _position
EOF

Exceptions

A special Exception class is available for your use. You can throw exceptions with custom messages / exit codes together with a raise function. The class accepts message and exit code as optional parameters:

Exception MyException

raise MyException "custom message" 255

# ==== Produces:
  ...
  main @ /path/to/your/script.sh:8

MyException: custom message

You can override or extend how exceptions are raised by customizing the magic __raise__ method.

Special attributes

Each object has a set of special attributes available to a developer.

• Object : __name__: Name of the object itself (name of a class/instance).
• Object : __parent__: Name of the object parent.
• Object : __type__: Type of the object (class/instance).

Special methods

A class can implement special methods that are invoked under certain conditions.

• Object . __init__(this, *args): Gets invoked each time a new instance of a class is created.
• Object . __destroy__(this, *args): Gets invoked before an instance of a class is destroyed.