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The xp_cmdshell is a powerful extended procedure installed by default in SQL Server. It enables the database system to issue operating system commands like those entered at the DOS prompt or the command prompt. This article will delve into the use of xp_cmdshell to control file system access with SQL Server 2016, SQL Server 2017, MySQL 5.7, DB2 11.1, and Azure SQL.

The xp_cmdshell comes deactivated by default in SQL Server due to security reasons. To enable it, you can use the sp_configure stored procedure. Here is an example of enabling xp_cmdshell in SQL Server:

EXEC sp_configure 'show advanced options', 1;
RECONFIGURE;
EXEC sp_configure 'xp_cmdshell', 1;
RECONFIGURE;

Once enabled, xp_cmdshell can be used to perform a wide array of tasks. For instance, to list all files in a specific directory, you could execute a command like the example below:

EXEC xp_cmdshell 'dir C:\Users\Public';

With xp_cmdshell, you can also manipulate files. For example, you can copy a file from one location to another as demonstrated in the code snippet below:

EXEC xp_cmdshell 'copy C:\Users\Public\Document1.docx D:\Backup\';

For security reasons, xp_cmdshell should be strictly controlled and its use minimized. Permissions to execute this command should be granted sparingly. In Azure SQL, the use of xp_cmdshell is not supported due to its potential security implications.

For MySQL 5.7, there is no direct counterpart to xp_cmdshell. However, similar functionality can be achieved using sys_exec and sys_eval UDFs from the lib_mysqludf_sys library. DB2 11.1 also lacks a direct xp_cmdshell counterpart but you can use ADMIN_CMD procedure to run some specific sets of commands.

In conclusion, xp_cmdshell is a powerful tool that allows SQL Server to interact with the operating system. However, due to its potential security risks, it should be used with caution and controlled meticulously. Alternatives exist for other database systems like MySQL and DB2, but they also need to be used wisely to prevent security vulnerabilities.

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