fold:tldr:84e0f

This command is using the fold utility with the option -w30 to wrap lines in a text document to a maximum width of 30 characters. ${filename} represents the name of the file that needs to be processed.

The fold command is commonly used to format text files so that lines do not exceed a certain width. By specifying -w30, it sets the maximum width for each line to 30 characters. If a line in the file is longer than 30 characters, fold will insert a line break at the nearest whitespace character to keep the text within the specified width.

For example, let's assume we have a file called "example.txt" with the following content:

This is an example line that is longer than 30 characters and needs to be wrapped.

Executing the command fold -w30 example.txt will modify the text in "example.txt" as follows:

This is an example line that
is longer than 30 characters
and needs to be wrapped.

Now, each line is limited to a maximum of 30 characters, and if it exceeds that limit, it is wrapped to a new line. The modified content is then displayed on the terminal screen or could be redirected to another file.