Answer by cas for sed - Confused what each field does

The sed script prints the first line of input and then prints a line of * characters of exactly the same width as the first line.

1{p;s/./*/g}

• 1 is an address matching the first line of input only. i.e. the sed commands in the {} block apply only to the first line.
• p - print the input without change
• ; is a command separator, in this case separating the p command and the s/./*/g command.
• s/./*/g - change every character to a *, and print the result.

Any remaining input (lines 2+) are printed unmodified if they exist as per sed's default behaviour (if used without the -n option)

The whole command:

column -t -s : <(echo Name:Age:Gender:Height:Weight) test.txt | sed "1{p;s/./*/g}"

uses column -t -s : to tabulate input consisting of the echoed string "Name:Age:Gender:Height:Weight" and the contents of file.txt. column's output is piped to sed to produce the line of ******s separating the header from the rest of the output.