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echo:tldr:07833
This command is using the echo command to append the string "${Hello World}" to the file represented by the variable ${file-txt}.
The "${Hello World}" is enclosed in double quotes to treat it as a single string rather than separate words. It is typical to see double quotes used when the string contains spaces or special characters. However, "${Hello World}" itself does not have any special functionality and will be treated as a regular string.
The >> operator is used to append the output of the echo command to the specified file. If the file does not exist, it will be created. If it already exists, the string will be appended to the existing content rather than overwritten.
${file-txt} represents a variable that represents the file name with the extension .txt. The exact value assigned to ${file-txt} will determine the file that this command appends the string to.