The `s6-format-filter` program

s6-format-filter processes lines according to its arguments, and prints the result.

Interface

     s6-format-filter format [ args... ]

format is a format string that can contain the following special sequences: %%, %s, and %0 to %9.
For every line it reads on stdin, s6-format-filter prints format on stdout, replacing %% with %, %0 with the command name (probably s6-format-filter), %1 to %9 with the first to the ninth argument in args, and %s with the input line.
s6-format-filter exits 0 when it reads EOF.