Lecture 25

Files and Folders (OS Level Imperatives)

File system operations

• We’ll be using some handy-dandy functions in the file-operations package

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(require "file-operations.rkt")

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Path names�and tree-structured file systems

• Modern operating systems allow files to be placed in directories (folders)
• And directories to be placed in other directories
• And so on and so on
• Creating a branching structure – in other words… a tree!
• The start of the tree is called the root directory

Tree-structured File Systems

File paths

root

a

b

d

e

f

g

c

• Naming a file involves telling the system how to get to it
• Starting with the root
• List all the directories you need to take
• To get to the file
• This is called a path for the file

root -> b -> f

Specifying files using pathnames

• A pathname is a string describing exactly where a file or directory is in a computer’s file system
• It lists all the directories in the path, separated by /’s
• On Windows
• Elements are separated with \’s
• And also has a drive letter

/Users/connor/Desktop

The pathname of my Desktop on my Mac

C:\Users\connor\Desktop

The pathname the same folder on my PC

C:\Windows\sys32\virus.exe

A file called “virus.exe” inside the sys32 folder inside the Windows folder.

D:\foo\bar\baz.txt

A text file on the D drive of my PC

Files, directories, and their paths

Delimiters in pathnames

• The first hierarchical file system was MULTICS, which used the > character to separate directory names, as in:

>foo>bar>baz.txt

• AT&T did a simplified version of MULTICS called Unix. It used slashes instead of >:

/foo/bar/baz.txt

• DOS used Unix-style pathnames (with drive letters for backward compatibility with an earlier operating system called CP/M):

C:/foo/bar/baz.txt

Windows pathname delimiters

• Then AT&T sued the (then) small company Microsoft for infringing their patent on using / as a directory separator
• Microsoft changed the / to a \
• C:\foo\bar\baz.txt
• Unfortunately, \ means something special in almost every programming language, in a string and so you have to type it as two \’s:
• “C:\\foo\\bar\\baz.txt”
• This is a pain, so windows also lets you use /

Mac pathname delimiters

• When Apple introduced hierarchical file systems, they used colons because they didn’t want to get sued like Microsoft had
• Mac HD:My stuff:Diary:November
• But when they converted to OSX (what macOS used to be called), they converted to slashes, since ATT’s patent had run out
• /Applications/Racket/DrRacket

Linux pathname delimiters

• Linux always was Unix, so it always used slashes
• /usr/local/bin/csh

Path objects in Racket

• When typing pathnames in code, you type them as strings
• But Racket represents them internally as Path objects
• Paths print as #<path:whatever>

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/Users/connor/Desktop

#<path:/Users/ian/Desktop>

C:\Users\connor\Desktop

#<path:C:\Users\connor\Desktop>

C:\Windows\sys32\virus.exe

#<path:C:\Windows\sys32\virus.exe>

D:\foo\bar\baz.txt

#<path:D:\foo\bar\baz>

Creating a Path in DrRacket

; build-path : string ... -> Path

; builds a path from a series of strings

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> (build-path "test" "test_2" "bar.txt")

/test/test_2/bar.txt

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> (build-path "test" "test_3")

/test/test_3/

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• To build a path, we can use the build-path function!

Getting JUST the filename from a Path

(path-filename path)

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Returns just the filename (no directories etc.) from a path

Converting Paths to/from Strings

(define p (build-path "test" "test_3"))

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> (path->string p)

"test/test_3"

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> (string->path "test/test_2/bar.txt")

#<path:test/test_2/bar.txt>

• Sometimes it’s useful to treat a Path as a String
• (or convert a string to a Path).
• path->string takes in a Path and returns the corresponding string
• string->path takes in a string and returns the corresponding path

Walking the file system

Finding all the files in a subtree

• directory-files gives us all the files inside a given Path to a directory
• It will return a (listof Path)
• (directory-files (build-path “.”))
• directory-subdirectories gives us all the subdirectories inside a given Path to a directory
• It will also return a (listof Path)
• (directory-subdirectories (build-path “.”))
• How do we find all the files in the directory and its subdirectories?
• Use recursion!

Finding all files in a subtree

(define (all-files directory)

(append (directory-files directory)

(apply append

(map all-files

(directory-subdirectories

directory)))))

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