https://lh3.googleusercontent.com/-5cd-clmKnbk/AAAAAAAAAAI/AAAAAAAAADM/-SXesH19Ido/s4/etc/hosts.allow

The correct keyword for restricting the records that are returned from a SELECT query based on a supplied criteria for the values in the records is WHERE. The WHERE clause is used to filter records based on one or more conditions. The syntax of the WHERE clause is:
SELECT column1, column2, ... FROM table_name WHERE condition;
The condition can be a logical expression that evaluates to true, false, or unknown. The condition can also use comparison operators, logical operators, and wildcards to specify the criteria. For example, the following query selects all the records from the employees table where the salary is greater than 50000:
SELECT * FROM employees WHERE salary > 50000;
The other options are incorrect because they have different purposes in SQL:
* LIMIT is used to specify the maximum number of records to return from a query. For example, the following query returns only the first 10 records from the employees table:
SELECT * FROM employees LIMIT 10;
* FROM is used to specify the table or tables from which to retrieve data. For example, the following query selects all the columns from the employees table:
SELECT * FROM employees;
* IF is used to execute a block of code conditionally. For example, the following query updates the salary of an employee based on their performance:
UPDATE employees SET salary = IF(performance = 'excellent', salary * 1.1, salary) WHERE employee_id =
123; References:
https://bing.com/search?q=SQL+statements+restrict+records+based+on+criteria
https://stackoverflow.com/questions/11611931/sql-query-to-select-records-based-on-criteria

The echo command is a built-in Linux feature that prints out arguments as the standard output1. The echo command can take various options and arguments to display different types of information. One of the arguments that can be used with the echo command is $$, which represents the process ID (PID) of the current shell2. A process ID is a unique number that identifies a running process in the system. The current shell is the shell that is executing the echo command. For example, if you are using the Bash shell and run the following command:
echo $$
The output will show the PID of the Bash shell, such as:
1234
The echo
commandcanbeusefultocheckwhichshellyouareusing,ortofindoutthePIDofthecurrentshellfordebuggingormonitoringpurposes.Theecho command is different from the following commands:
echo $!: This command displays the PID of the last command executed in the background2. A background command is a command that runs without blocking the shell, allowing you to continue using the shell while the command executes. For example, if you run the following command:
sleep 10 &
This command will put the sleep command, which pauses the execution for 10 seconds, in the background. The output will show the PID of the sleep command, such as:
1 2345
If you then run the following command:
echo $!
The output will show the same PID of the sleep command, such as:
2345
echo $?: This command displays the exit status of the last command executed2. The exit status is a number that indicates whether the command was successful or not. A zero exit status means the command was successful, while a non-zero exit status means the command failed or encountered an error. For example, if you run the following command:
ls /home
This command will list the contents of the /home directory. If the command succeeds, the output will show the files and directories in the /home directory, such as:
alice bob charlie
If you then run the following command:
echo $?
The output will show the exit status of the ls command, which is zero, meaning the command was successful:
0
echo $0: This command displays the name of the current shell or script2. The name of the current shell is the name of the executable file that runs the shell, such as bash, zsh, ksh, etc. The name of the current script is the name of the file that contains the script, such as script.sh, script.py, etc. For example, if you are using the Bash shell and run the following command:
echo $0
The output will show the name of the current shell, such as:
bash
Reference:
2