What does `nl -b a` do?

The nl -b a command is used to number all lines in a file, including blank lines. Here's a breakdown of the options:

• nl: This is the command to number lines in a file.
• -b: This option specifies the numbering behavior.
• a: This argument for -b means to number all lines, regardless of whether they are blank or not.

So, when you run nl -b a filename, it will number every line in the specified file, including any blank lines.