IT 244 – Chapter 4
The Filesystem:
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Hierarchical
o Root at the top
o Directory tree can come off any directory node as subdirectory
Files: directory, special, and ordinary
o Special files are system level files (symbolic link, device, etc.)
o Directories
 Contain references to any other type of file (directory, special,
or ordinary).
 Are a parent directory to files that it contains, which are its
children.
 A Pathname is a series of directories separated by /.
 Absolute if the path begins with / or ~.
 Relative if the path begins with anything else (a
directory name, a “.”, or a “..”).
o Ordinary files
 Files that are neither special or directory files are ordinary files
created and used by users or programs that users run.
o Filenames:
 A-Z, a-z, 0-9, “_”, “.”, “,”
 Problems can arise from:
 Can contain other characters if they are escaped using \.
 Extensions
 If a program is built to use a particular extension
 UNIX does not care about extensions
Directories of note:
 Home directory
o Special pathname “~”.
 ~/ is the current user’s home directory.
 ~sam/ is the home directory for the user sam.
o Special Startup (config) files (beginning with a dot)
 These files are invisible to ls unless the –a or –A option is given
 Current directory = “.”
 Parent directory = “..”
 Standard system directories:
o / - Root
o /bin – Essential command binaries
o /boot – Files for the system boot loader (all files necessary for boot)
o /dev – All device files
o /etc – System configuration files
 /etc/opt – configuration files for some added software
 /etc/X11 – configuration for the X Window System
o /home
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/lib – System level shared libraries
/lib/modules – Loadable kernel modules (extend the kernel)
/mnt – Mount point for temporary filesystems
/opt – Add-on software packages (optional)
/proc – Kernel and process information
/sbin – Essential system binaries
 utilities for boot process (see /usr/sbin)
/tmp
/Users – Mac OS X default for user home directories
/usr – Contains subdirectories with system-level information
/usr/bin – Contains standard Linux and OS X utility programs
/usr/lib – Libraries needed by system and many other programs
/usr/local – Locally important files for applications added to the
system
/var – Varying files, such as log, mail, spool, and temporary files.
Commands:
 pwd – Present working directory
 mkdir [-pv] –m mode] <dirname> – Creates a directory
o –p – creates intermediate directories if necessary
o –v – verbose creation
o –m – set the permissions to <mode> which can be anything acceptable
to chmod
 rmdir – Delete an empty directory
 cd – Change directory
o “cd –“ – toggle between this and the previous directory.
 touch <filename> - Create an empty file called <filename>
 mv [-fin] <filename> <newfilename> – Rename a file
o If given a new name, this is a simple rename.
o If given a new home directory without a new name, this is a move of
that file’s inode from being owned by its current directory to the new.
o If given a new home directory on a different partition, the file is
copied to the new partition, its inode is entered as a child of the new
home directory, and then the old file is deleted.
o –i – prompt before moving
o –f – force no prompt (overrides a previously defined –i)
o –n – do not overwrite an existing file (overrides any previously
defined –i or –f)
 cp [-fin] <file> <newfile> – Copy a file
o –i – prompt before copying
o –f – force the overwrite of an existing file (overrides previously
defined –n)
o –n – do not overwrite an existing file (overrides previously defined –f
or –i)
inodes:
 Fundamental concept on UNIX systems and their descendants
 An inode is:
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o A system generated data structure on UFS or ext3 filesystems
o The identifier for files containing the actual pointer to the location on
disk, and the following associated attributes:
 Permissions (read, write, execute, suid, sgid, sticky)
 Owner
 Group
 File size
 File access/change/modification time
 File deletion time
 Number of links (soft/hard)
 Extended attributes
 Access Control List (ACL)
ls –i – shows the inode number for a file (also available with stat)
Links:
 Links are not files, but system structures containing a reference to another
file.
o They appear as files, having names and being listed by ls
o If in the same directory as the linked file, the link must have another
name.
 Useful to share a file or files
o In the situation where the shared file(s) should be able to be
exchanged for another without having to overwrite the current
file/directory.
o Indirection: Links can be quickly created and destroyed, thus
switching the path to a program’s files (back and forth) seamlessly.
 Hard links (ln <file> <linkfile>):
o Older, and becoming outdated
o Restrictions:
 Can only link to a file that exists in the same partition.
 Cannot link to a directory.
o Basically an instance of the inode of the linked file is added to the
directory where the link exists, even if the linked file is in the same
directory.
o An ls listing shows no difference. Using the –i option, you can see they
both have the same inode.
o NOTE: if you create a hard link and then remove the original file, the
file on disk still exists, accessible through the hard link.
 Soft (symbolic / symlinks) links (ln [–s | --symbolic] <file> <linkfile>):
o Newer, used all the time
o An indirect (vs. direct for hard) link to a file
 A new special file containing the path to the file.
o An ls -l listing shows the link permissions starting with an “l”, and an
ls –i listing shows two different inodes.
o Created to overcome both hard link restrictions
o NOTE: if you create a soft link and then remove the original, there is
no file to be retrieved.
o Added advantage:
The ability for a soft link to point to nothing
 Can remove and replace the file and the soft link still
works.
o Disadvantage:
 Unlike hard links, the status information (create, modify, etc.)
of the soft link does not reflect that of the linked file.
 You MUST use full path to the linked file, or there can be
problems in retrieving the file through the soft link.
 If you change directory through a symbolic link with cd, and
then do pwd, you will see a totally different path.
Removing links:
o rm removed links of both types.
o Hard links:
 When you remove the last hard link to a file (including its
original hard link from its creation) the file itself will then be
deleted.
o Soft links:
 When you remove all soft links to a file the file remains, since
the file itself is represented by at least one hard link.
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Standard Access Permissions:
 ls –l – “-rwxrwxrwx+”
o First position – file type (ordinary, directory, symlink)
o Last position – ACL flag
o Middle – Permissions - three sections of rwx (user, group, other)
 chmod [[n]nnn | [ugo][+-][rwxs]] – Change the non-ACL access permissions
of a file
o Can either use a 3-4 digit permissions setting, or symbolic arguments
for adding/removing rwxs for ugo (user,group,other)
 If s is used, this is for user and group only
o If use a 4 digit permission, a binary representation of the leftmost
number is used to set SUID, SGID, and sticky bits.
 SUID = 4 = 100
 Represented by a “S” if not executable for user
 Represented by a “s” if also executable for user
 SGID = 2 = 010
 Represented by a “S” if not executable for group
 Represented by a “s” is executable for group
 Sticky = 1 = 001
 Represented by a “T” if not executable for other
 Represented by a “t” if executable for other
 Bitwise and’ing these together will allow for adding more than
one.
 chown <user> <file> - Change the ownership of the file
 chgrp <group> <file> - Change the group associated with the file
 umask <mask> - sets the mask for permissioning in the current session
o 777 bitwise or’ed with the mask will be the permissions set by default.
ACL Access Permissions:
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Provide finer-grained control over permissioning
Disadvantages:
o NOTE: reduces performance – NEVER enable these on a filesystem
containing system files
o Most utilities do not preserver ACL settings
 Find out if the utility has an option that you can give on the
command line for preserving ACL permissions.
 mv, cp, tar are good examples.
Enabling ACL
o Install the acl software package
o Linux supports ACL on ext2, ext3, ext4 filesystems.
o Mount the disk to be ACL enabled using the acl option in /etc/fstab
 Remount a disk if the disk was already mounted when
/etc/fstab was changed.
Can set default rules for permissions of files created within a directory
setfacl – Sets a file’s ACL permissions
getfacl – Retrieves a file’s ACL permissions