1. Chapter 4: The Linux Filesystem
Where stuff is

2. In this chapter … What is a hierarchical filesystem
Directories and files
Pathnames: absolute vs. relative
Permissions
Links

3. What is a filesystem? A data structure Stores data Organizes data
Allows for data retrieval and manipulation

4. What is a hierarchical filesystem?
Essentially, a filesystem that allows nesting of folders under a central point
Like a pyramid or upside-down tree
Tree analogy most common – ie the directory tree
Programmers – definition of a tree applies here

5. Hierarchical Filesystem
Directories can contain other directories and/or ordinary files
Concept different from reality – in implementation everything is a file
Directories, devices, named pipes, ordinary files – all really just files

6. Lingo
Root directory
Subdirectories
Parents, children

7. Filenames
Each file within a directory must have a wholly unique filename
Can be up to 255 characters – make them longer to avoid confusion
Special characters must either be escaped out (using backslash) or in quotes
Only illegal characters are / and carriage return

8. Filenames con’t Spaces and other special characters a bad idea
Instead of spaces use underscores or periods
Ex: my_file or my.file

9. Extensions Not always essential but helps simplify and avoid confusion
Some programs like gcc depend on proper extensions
This includes case!

10. Hidden Files To make a file hidden, start it with a period
Ex .plan
A normal ls will not show hidden files
Use ls –a to reveal ALL files
Startup files, containing configuration settings for your account, hidden

11. mkdir – create directory
Syntax: mkdir directory
Directory can be a relative or absolute pathname (we’ll get to that in a minute)
You can use ls –F to show directories with a forward slash at the end of the name
If using a color terminal, directories will be a different color than ordinary files

12. Working Directory The directory you are currently working in
pwd will tell you what your working directory is
Helpful to know when using relative pathnames (again, coming up)

13. Home Directory Not to be confused with working directory
The directory you start in when you first logon
Most users it is /home/username
For root, it is /root
Can be changed by system administrator

14. cd – change working directory
Syntax: cd [directory]
Again, directory can be absolute or relative
If no argument given, changes working directory back to your home directory

15. Absolute Pathnames (finally)
Absolute pathname for a file gives the file’s location relative to the root of the filesystem
Sometimes long
Ex: /home/jhowell/Assignment1/animals
Shortcut: ~ represents your home directory
So the above could also be ~/Assignment1/animals

16. Relative Pathanmes
A pathname relative to the current working directory
Make sure you know what your working directory is!
Shorter
Ex, in my home directory: Assignment1/animals

17. . and .. Directories . is an alias for the working directory
.. is an alias for the parent of the current working directory
These pointers are placed in every directory when created by mkdir
Can be used in relative pathnames

18. Standard Filesystem Directories
Most distributions try to adhere to the Filesystem Hierarchy Standards, but it’s not uncommon to find things in odd places
Even less standardized going from Linux to BSD to UNIX
In other words – no guarantees

19. Common Directories / (root) – root of the filesystem
/bin – essential system binaries (commands)
/boot – files for the bootloader
/dev – device files
/etc – system configuration files
/home – user home directories
/lib – standard libraries and modules

20. Common con’t
/mnt – mount point for temporary filesystems (floppies, CD-ROMs, non-native partitions)
/opt – optional add-on software
/proc – kernel and process information
/root – root’s home directory
/sbin – essential system binaries
/tmp – temporary space (not swap)

21. Common con’t
/usr – common area for data / program users use frequently
/var – frequently changing data like system logs, caches, spools and mailboxes
Lots more important subdirectories – see the textbook

22. rmdir – remove directory
Syntax rmdir directory
Only deletes empty directories
Not empty? Delete the files with rm and try rmdir again
Lazy? rm –r directory will recursively delete a directory and its contents (files and directories)
Use with caution!

23. touch – create a file Syntax: touch filename
Creates an empty file (size 0)
Useful to create placeholders or while learning the interface

24. mv revisited Already used mv to rename files
If last argument is a directory, mv moves files into a different directory
If given a directory as the first argument, mv moves the directory to the new name supplied (which can either be a rename or move!)

25. Date and time created / accessed
Permissions
Use a ls –l (for long view) and you might see something like this:
drwxr-xr-x 2 jhowell jhowell 4096 Aug 18 15:46 Desktop
-rw-rw-r-- 1 jhowell jhowell 0 Sep 4 18:08 myfile
drwxrwxr-x 2 jhowell jhowell 4096 Aug 22 15:32 public_html
filename
# of links
group
Date and time created / accessed
File Permissions
user
size
Type of file

26. File types - ordinary file b block device c character device
d directory
p named pipe
l symbolic link

27. File permissions Three types of permissions – read, write and execute
Three sets of permissions – owner (user), group, and other (everyone else)
rwx rwx rwx
user
(owner)
other
group

28. chmod – CHange MODe Changes permissions Syntax: chmod [ugoa][+-][rwx]
Ex: grant everyone (all) read and write chmod a+rw myfile
Ex: remove execute permission for other chmod o-x myfile

29. chmod – alternate syntax
Instead of [ugoa][+-][rwx], use binary equivalent
rwx rwx rwx
For each section, sum up the permissions, and assemble a three digit number
So full access to everyone would be 777

30. Alternate syntax example
Grant user full access, group read and execute, and deny access to other
rwx r-x ---
So we get chmod 750 myfile
= 0
= 7
= 5

31. Permissions Caveats
To execute a shell script you must allow both read and execute permissions
To get into a directory, you must have execute permission
root can still read and write to files without read and write permissions

32. One more exception setuid and setguid
Allows a file to be executed with the permissions of the file’s owner or group
A way to let users perform privileged tasks without granting them general permissions
Should be used sparingly with files owned by root

33. Links Pointers to files Points to an exact location on disk
When a file is created, it is the first link to a particular spot on the disk
To make a file appear in multiple directories, make additional links

34. Working with Links Syntax: ln [-s] existing-file new-link
Without –s a hard link is created
With –s a soft or symbolic link is created
To delete a link use rm
Delete all the hard links and the file is ‘deleted’

35. Hard Links Points to a precise inode on the disk
Now file appears in two locations
Only one copy of the data is stored
When you create a file, you allocate disk space and create a hard link
Hard links can only be used on a single FS
Can’t link to directories

36. Soft Links Also called symbolic links or symlinks
Instead of pointing to inode, points to the pathname of a hard link
Move the original file, symlink breaks
Symlinks don’t touch the data directly – safer
When using ln existing-file should be an absolute pathname