Because the shell is an interpreted language, it is comparatively slow. Many operations on files are best done with external commands that implicitly loop over the lines of a file. At other times, the shell itself is more efficient. This chapter looks at how the shell works with files – both shell options that modify and extend file name expansion and shell options that read and modify the contents of files. Several external commands that work on files are explained, often accompanied by examples of when not to use them.