A shell variable is a variable that is available only to the current shell. In contrast, an environment variable is available system wide and can be used by other applications on the system.A shell is the operating system's command interpreter. It processes the commands entered on the command line or read from a shell script file.
A relational database management system (RDBMS) is a database engine/system based on the relational model specified by Edgar F. Codd--the father of modern relational database design--in 1970.
Most modern commercial and open-source database applications are relational in nature. The most important relational database features include an ability to use tables for data storage while maintaining and enforcing certain data relationships.
In 1970, Edgar F. Codd, a British computer scientist with IBM, published “A Relational Model of Data for Large Shared Data Banks.” At the time, the renowned paper attracted little interest, and few understood how Codd’s groundbreaking work would define the basic rules for relational data storage, which can be simplified as:
Codd later published another paper that outlined the 12 rules that all databases must follow to qualify as relational. Many modern database systems do not follow all 12 rules, but these systems are considered relational because they conform to at least two of the 12 rules.
Most modern commercial and open-source database systems are relational in nature and include well-known applications, e.g., Oracle DB (Oracle Corporation); SQL Server (Microsoft) and MySQL and Postgres (open source).
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