[talyn256@silver ~]$ mkdir testrename
[talyn256@silver ~]$ cd testrename/
[talyn256@silver testrename]$ touch File\ One.txt
[talyn256@silver testrename]$ touch File\ Two.txt
[talyn256@silver testrename]$ touch File\ Three.txt
[talyn256@silver testrename]$ ls
File One.txt  File Three.txt  File Two.txt
[talyn256@silver testrename]$ rename \  _ *.txt
[talyn256@silver testrename]$ ls
File_One.txt  File_Three.txt  File_Two.txt
[talyn256@silver testrename]$