转载：http://blog.csdn.net/vicant/article/details/2317721
有时候需要给文本加上行号，用UNIX的cat命令可以进行这项工作。
cat命令的用法如下：
NAME
cat - concatenate, copy, and print files
SYNOPSIS
cat [-benrstuv] file ...
Options
cat recognizes the following options:
-b Omit line numbers from blank lines when -n option is
specified. If this option is specified, the -n option is
automatically selected.
-e Print a $ character at the end of each line (prior to the
new-line). If this option is specified, the -v option is
automatically selected.
-n Display output lines preceded by line numbers, numbered
sequentially from 1.
-r Replace multiple consecutive empty lines with one empty
line, so that there is never more than one empty line
between lines containing characters.
-s Silent option. cat suppresses error messages about non-
existent files, identical input and output, and write
errors. Normally, input and output files cannot have
identical names unless the file is a special file.
-t Print each tab character as ^I and form feed character as
^L. If this option is specified, the -v option is
automatically selected.
-u Do not buffer output (handle character-by-character).
Normally, output is buffered.
cat -n 可以加上行号，cat -b 也可以加上行号，但会忽略空行。
如下命令可以为file1加上空行后，输入为file2（空行也加上行号）：
cat -n file1 > file2
如下命令可以为file1加上空行后，输入为file2（空行不加行号）：
cat -b file1 > file2