Create and Rename Files with Fish

I am an avid user of the fish shell. Here is a small demo of a common task I have. How to efficiently rename a long list of files in a directory?

# lets create 10 empty text files:
floswald@PTL11077 ~/test> for i in (seq 1 10)
                              touch testfile$i.txt
                          end

floswald@PTL11077 ~/test> ls
testfile1.txt   testfile2.txt   testfile4.txt   testfile6.txt   testfile8.txt
testfile10.txt  testfile3.txt   testfile5.txt   testfile7.txt   testfile9.txt

no suppose I realize that testfile is actually a poor name for this. I want to call them trainfile instead. Here is how to change those filenames in batch instead of manually one by one. The concept is called command substition, as in

echo (pwd)  

which evalutes first the command in the bracket before giving it to the echo command.

in our case, we use a sed call to substitute parts of each filename. sed is extremeley powerful, look here for a concise manual. In our case, we want to substitute test with train. The s command in sed does that. The syntax is s/old/new/. So we want to substitute test with train. We can do that like this:

floswald@PTL11077 ~/test [64]> for i in *.txt
                                   mv $i ( echo $i | sed 's/test/train/g' )
                               end

floswald@PTL11077 ~/test> ls
trainfile1.txt   trainfile2.txt   trainfile4.txt   trainfile6.txt   trainfile8.txt
trainfile10.txt  trainfile3.txt   trainfile5.txt   trainfile7.txt   trainfile9.txt