清空文件内容的方法：
1.echo：
[cti@minicc ~]$ cat test.txt
test—test
[cti@minicc ~]$ echo “” >test.txt //将空字符写入test.txt，覆盖文件中已有的内容
[cti@minicc ~]$ cat test.txt
[cti@minicc ~]$ du test.txt
4 test.txt
注：文件中有一个空字符，文件大小为4k
2.重定向：
[cti@minicc ~]$ echo “acd” >test.txt
[cti@minicc ~]$ cat test.txt
acd
[cti@minicc ~]$ >test.txt //重定向
[cti@minicc ~]$ cat test.txt
[cti@minicc ~]$ du test.txt
0 test.txt
5.cp
[cti@minicc ~]$ echo “test” >test.txt
[cti@minicc ~]$ cp /dev/null test.txt
[cti@minicc ~]$ cat test.txt
[cti@minicc ~]$ du test.txt
0 test.txt
6.sed
[cti@minicc ~]$ echo “test”>test.txt
[cti@minicc ~]$ cat test.txt
test
[cti@minicc ~]$ sed -i ‘1,$’d test.txt
[cti@minicc ~]$ cat test.txt
[cti@minicc ~]$ du test.txt
0 test.txt