用UNIX的cat命令用于给文本加行号

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