How can I repeat a command many times?

Shell variables are also used in loops, which repeat commands many times. If we run this command:

for filetype in gif jpg png; do echo $filetype; done

it produces:

gif
jpg
png

Notice these things about the loop:

1. The structure is for …variable… in …list… ; do …body… ; done
2. The list of things the loop is to process (in our case, the words gif, jpg, and png).
3. The variable that keeps track of which thing the loop is currently processing (in our case, filetype).
4. The body of the loop that does the processing (in our case, echo $filetype).

Notice that the body uses $filetype to get the variable's value instead of just filetype, just like it does with any other shell variable. Also notice where the semi-colons go: the first one comes between the list and the keyword do, and the second comes between the body and the keyword done.