[root@stu227 he]# touch he.txt

[root@stu227 he]# set -o noclobber

[root@stu227 he]# echo "123" > he.txt

bash: he.txt: cannot overwrite existing file

如果要取消限制，就把set -o 改为set +o

[root@stu227 he]# set +o noclobber

[root@stu227 he]# echo "123" > he.txt

[root@stu227 he]# cat he.txt

123