linux crash course chapter 04 2

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 forwar

Chapter 4:

The Linux Filesystem

Where stuff is

In this chapter …

• What is a hierarchical filesystem

• Directories and files

• Pathnames: absolute vs relative

• Permissions

• Links

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

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

• Root directory

• Subdirectories

• Parents, children

• 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

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

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

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)

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

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

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

and Directories

• is an alias for the working directory

• is an alias for the parent of the current

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

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

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)

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

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!

touch – create a file

• Syntax: touch filename

• Creates an empty file (size 0)

• Useful to create placeholders or while learning the interface

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!)

• 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

Type

of file

File Permissions

# of links

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

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

chmod – alternate syntax

• Instead of [ugoa][+-][rwx], use binary

Alternate syntax example

• Grant user full access, group read and execute, and deny access to other

• root can still read and write to files without

read and write permissions

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

• 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

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’

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

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