Chapter 4: The Linux Filesystem

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 –rdirectory 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. 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 writechmod a+rw myfile • Ex: remove execute permission for otherchmod o-x myfile

29. chmod – alternate syntax • Instead of [ugoa][+-][rwx], use binary equivalent rwx rwx rwx 421 421 421 • 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 + 0 + 0 = 0 4 + 2 + 1 = 7 4 + 0 + 1 = 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 lnexisting-file should be an absolute pathname