Linux File System Concepts

Linux Filesystem Management

• Covers finding files and directories
• Includes linking files, understanding the Filesystem Hierarchy Standard
• Explores managing files and directories with standard Linux commands
• Explains modifying file and directory ownership
• Explains defining and change Linux file and directory permissions
• Shows identifying the default permissions created on files and directories
• Describes applying special file and directory permissions
• Covers modifying the default access control list (ACL)
• Shows how to View and set filesystem attributes

The Filesystem Hierarchy Standard

• Includes standard set of directories for Linux and UNIX systems
• Covers standard file and subdirectory contents
• Simplifies finding specific files
• Gives Linux software developers the ability to locate files on any Linux system
• Supports creating non-distribution-specific software

Managing Files and Directories

• The mkdir command creates new directories, using absolute or relative pathnames
• The mv (move) command moves files and requires a minimum of two arguments: a source file/directory and a target file/directory
• Pathnames in mv can be absolute or relative, and wildcards can be used for multiple files; also used to rename files.
• The cp (copy) command copies files
• To copy a directory recursively (with all files and subdirectories), use the -r option with cp command.
• Both the mv and cp commands warn if the target is an existing file and prompt for confirmation in Fedora Linux that has BASH aliases for these commands.
• To see shell aliases, type alias at the prompt.
• Interactive mode prompts the user before overwriting files
• The -f option with rm forces overwriting in interactive mode.
• The rm (remove) command removes files, and takes a list of files as arguments and can use wildcards.
• It operates in interactive mode by default, but -f overrides this.
• The rmdir (remove directory) command removes directories, but only if they are empty

Finding Files

• The locate command searches for files in the Linux directory tree using a pre-made indexed database
• Use updatedb command to update the database used by locate
• The output from locate may require using more or less commands to view
• The find command recursively searches for files starting from a specified directory
• Has format: find -criteria .
• find is slower but more versatile than locate
• Wildcard metacharacters must be quoted to be interpreted by find.
• Specifying a subdirectory reduces search time
• The PATH variable lists directories where executable files are located, allowing them to be run without specifying the full path.
• The which command searches for an executable file by searching the PATH variable; if not found shows directories searched
• The type and whereis commands are alternatives to which

Linking Files

• Files can be linked to each other using symbolic links (symlinks).
• A symbolic link is a pointer or shortcut to another file
• Files can be linked to each other using hard links, where two files share the same data
• Improved understanding of file storage on a filesystem requires defining: superblock, inode tables, and data blocks.
• The superblock stores filesystem information like the number of inodes and data blocks, as well as the size of each data block
• The inode table contains inodes, each describing a file or directory with a unique inode number
• The inode stores file size, data block locations, last date modified, permissions, and ownership
• Data blocks contain the data making up the contents of a file and are referenced by the inode.
• Hard linked files share the same inode and inode number and must reside on the same filesystem
• To create a hard link, use the ln (link) command with two arguments: the existing file and the target file to be created as a hard link
• Removing a hard linked file reduces the link count for the file
• Symbolic linked files do not share the same inode and data blocks with their target file
• A symbolic link is a pointer to its target, storing the pathname to the target file in its data blocks so editing the symbolic link edits the target file
• If the target file is deleted, the symbolic link become useless.
• To create a symbolic link, use the -s option with the ln command.
• Symbolic links can use relative or absolute pathnames and are viewed with ls -l
• Symbolic links need not reside on the same filesystem as their target

File and Directory Permissions

• Linux requires users to log in with a username and password.
• Usernames and group memberships determine access to resources based on required permissions.

File and Directory Ownership

• During file creation, the user's name and primary group become the owner and group owner
• The same applies for directory creation
• whoami command shows the current user name
• groups command shows group memberships and primary group
• touch command creates an empty file.
• The chown (change owner) command changes the ownership of a file or directory specified by two arguments, New owner, and file or directory to change
• The -R option with chown changes permissions recursively throughout the directory tree.
• chgrp changes the group owner of a file or directory, using the same arguments and options as chown.

Managing File and Directory Permissions

• The mode is the inode section that stores permissions for the user (owner), group owner, and others (everyone)
• Read, write, and execute are the three regular permissions that may be assigned to each user
• The mode order is user, group, other.
• The system assigns the first set of permissions that are matched
• Focus on assigning permissions to the user and group, not just to "other"

Interpreting Permissions

• Read permission allows a user to open and read the file's contents
• Write permission allows a user to open, read, and edit file contents
• Execute permission allows a user to run the file in memory if it is a program or script
• Read permission on a directory lets a user list the contents when execute is granted
• Write permission on a directory lets a user add or remove files and requires execute
• Execute permission on a directory lets a user enter the directory and work with its contents

Changing Permissions

• The chmod (change mode) command changes the permissions (mode) of files or directories
• The chmod command has at least two arguments, criteria used to change permissions and the filenames to change
• When permissions are the same for all categories, use the "a" character to apply to all categories
• The values:
  • u (user) - + (adds permission)
  • g (group) - -(removes permission)
  • o (other) - = (makes permission equal to)
  • a (all categories)
• Permissions:
  • r (read)
  • w (write)
  • x (execute)
• Numeric representation of the mode
  • User, group, other
  • Values for each, 4 read, 2 write, 1 execute
  • The mode number is the sum of these values

Default Permissions

• New files have rw-rw-rw- permissions by default
• New directories have rwxrwxrwx permissions by default
• The umask variable is a special variable that takes away permissions on new files and directories.
• umask command is used to display the umask
• To change the umask, set a new umask as an argument to the umask command.

Special Permissions

• These include SUID, SGID, and the sticky bit

Defining special permissions

• If SUID is set on a file, the user executing it gains ownership during execution (e.g., passwd command).
• SUID has no function on directories and only works on binary compiled programs, not shell scripts.
• SGID is applicable to both files and directories.
• If set on a file, the executing user becomes a member of the file's group.
• With SGID set on a directory, file creations in that directory are set to the directory's group owner, not the user's primary group
• The sticky bit was used to lock files in memory, but is currently limited to directories.
• It ensures users with write permissions can only delete their own files

Setting Special Permissions

• Special permissions require execute permissions
• They mask the execute permission when displayed with Is –l command
• May be set even if file or directory does not have execute permission
• Indicating letter in the mode will be capitalized
• Special persmissions added via chmod command

Setting Custom Permissions in Access Control List (ACL)

• An ACL is a list of users or groups that you can assign permissions to
• The setfacl command is used to modify ACL entries for a Linux file or directory, using the -m option
• Use the -b option to remove all extra ACL assignments on a particular file or directory
• The getfacl command lists all additional entries in the ACL

Managing Filesystem Attributes

• Linux files have settable attributes separate from Linux permissions
• These attributes are specific to the filesystem
• The lsattr command lists filesystem attributes
• Thechattr command adds or removes filesystem attributes.
• The immutable attribute (i) prevents a files from being modified.