[root@newgromit ~]# echo blah > testfile.txt
[root@newgromit ~]# cat testfile.txt 
blah
[root@newgromit ~]# sed 's/blah/woot/g' testfile.txt 
woot
[root@newgromit ~]# cat testfile.txt 
blah
[root@newgromit ~]# sed 's/blah/woot/g' testfile.txt > newtestfile.txt
[root@newgromit ~]# cat testfile.txt 
blah
[root@newgromit ~]# cat newtestfile.txt 
woot