tail [-f] [-c number|-n number] [file]

tail [-f] [-c number|-n number] [file]
tail -r [-n number] [file]

-n number

This option shall be equivalent to -c number, except the starting location in the file shall be measured in lines instead of bytes. The origin for counting shall be 1; that is, -n +1 represents the first line of the file, -n -1 the last.

If neither -c nor -n is specified, -n 10 shall be assumed.

-n number

If -r is not specified, this option shall be equivalent to -c number, except the starting location in the file shall be measured in lines instead of bytes. The origin for counting shall be 1; that is, -n +1 represents the first line of the file, -n -1 the last.

If -r is specified, number shall specify the number of lines to read (in reverse) from the end of the input file. The application shall ensure that number does not have a sign.

-r

Copy the lines in reverse order (last line first). If -n is specified, that many lines of the file, starting with the last line, shall be copied. If -n is not specified, every line of the input file shall be copied.

If none of the -c, -n or -r options is specified, -n 10 shall be assumed.

The functionality made available by tail -r has been historically provided on some systems by a separate utility (tac), although tac traditionally lacked support for -n to limit the output. While both <tt>tail -n$n | tac</tt> and <tt>tac | head -n$n</tt> can be used to output a fixed length of reversed line output, the standard developers decided that it was preferable to have a single utility <tt>tail -r -n$n</tt> for the same purpose. Furthermore, in deciding whether to standardize tac rather than tail -r, it was determined that more implementations that have achieved POSIX certification had already implemented tail -r as an extension.