Linux File Management Commands

Linux Filesystem Management

• Linux filesystem management is covered in chapter 4

Objectives

• Find files and directories on the filesystem
• Understand and create linked files
• Explain the function of the Filesystem Hierarchy Standard
• Use standard Linux commands to manage files and directories
• Modify file and directory ownership
• Define and change Linux file and directory permissions
• Identify the default permissions created on files and directories
• Apply special file and directory permissions
• Modify the default access control list (ACL)
• View and set filesystem attributes

The Filesystem Hierarchy Standard

• This is a standard set of directories for Linux and UNIX systems
• It contains standard file and subdirectory contents
• Simplifies finding specific files
• Gives Linux software developers the ability to locate files on any Linux system
• Allows to Create non-distribution-specific software

Managing Files and Directories

• The mkdir (make directory) command creates new directories
• Arguments specify directory's absolute or relative pathname
• The mv (move) command moves files
• Requires a minimum of two arguments
  • Source file/directory
  • Target file/directory
• Pathnames can be absolute or relative
• Wildcards can be used in pathname for multiple files
• Can also be used to rename files
• The cp (copy) command copies files
• Uses the same arguments as the mv command
• Also used to make copies of files
• To copy a directory full of files requires the cp command and the recursive option -r
• This copies files and subdirectories
• The target must be a file that exists, so both the mv and cp commands warn the user that the target file will be overwritten and will ask whether to continue
• This is a feature of the default configuration in Fedora Linux because the BASH shell contains aliases to the cp and mv commands
• To see the aliases present in the current shell, type alias at the prompt
• Interactive mode prompts user before overwriting files, but the -f option (force) will override interactive mode
• The rm (remove) command: removes files
• Arguments are a list of files
• Wildcards can be used
• It is in Interactive mode by default, but the -f option will override this
• The rmdir (remove directory) command removes directories, and only if they are empty

Common Linux file management commands

• mkdir creates directories
• rmdir removes empty directories
• mv Moves/renames files and directories
• cp copies files and directories full of files (with the -r or -R option)
• alias Displays BASH shell aliases
• rm Removes files and directories full of files (with the -r or -R option)
• unlink removes files

Finding Files

• The locate command searches for files in the Linux directory tree
• It looks in a premade indexed database of all files on system
• This database can be updated with the updatedb command
• The information returned by the command may not fit on screen, use more or less commands
• The find command recursively searches for files starting from a specified directory
• It is slower than the locate command, but more versatile
• Format: find -criteria
• Ensure that wildcard metacharacters are interpreted by the find command by placing them in quotation marks
• To reduce search time, specify subdirectory to be searched
• The PATH variable lists directories on system where executable files are located
  • Allows executable files to be run without specifying absolute or relative path
• The which command searches for an executable file, searching the PATH variable
• If the file is not found, it lists the directories that were searched
• Alternatives that could also be used are the type command and whereis command

Linking Files

• Files can be linked to one another
• Symbolic link (symlink) means one file is a pointer or shortcut to another
• Hard link means two files share the same data
• To better understand how files are linked, you must understand how files are stored on a filesystem, with the:
  • Superblock
  • Inode table
  • Data blocks
• The Superblock contains information about the filesystem
  • It includes number of inodes and data blocks, and the size of each data block.
• The inode table consists of several inodes
  • Each Describes a file or directory and contains a unique inode number for identification
  • Stores file size, data block locations, last date modified, permissions, and ownership
• Data blocks: data making up contents of a file
  • 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 In (link) command and specify two arguments
• The existing file to hard-link and the target file that will be created as a hard link to the existing file
• To remove hard linked files, delete one of the linked files, which reduces the link count for the file

Hard vs Symbolic Links

• Symbolic linked files do not share the same inode and data blocks with their target file
• A symbolic linked file is a pointer to the target file
• Data blocks in the linked file only contain a pathname to the target file
• Editing a symbolic linked file actually edits the target file
• If the target file is deleted, the symbolic link serves no function
• To create a symbolic link, use the -s option with the In command
• Arguments can be relative or absolute pathnames, as with hard links
• You can use the Is -I command to view both hard link and symbolic link files
• Symbolic links need not reside on the same filesystem as their target

File and Directory Permissions

• All users must login with a username and password
• Users are identified by username and group memberships
• Access to resources depends on username and group membership
• Must have required permissions

File and Directory Ownership

• During file creation, that user's name and primary group becomes the owner and group owner of the file, which is the same for directory creation
• The whoami command views current user name
• The groups command views group memberships and primary group
• The touch command creates an empty file
• The chown (change owner) command is used to change ownership of a file or directory, with two arguments:
  • New owner
  • File or directory to change
• You can use the -R option to change permissions recursively throughout the directory tree
• The chgrp (change group) command changes the group owner of a file or directory
• Same arguments and options as for chown command

Managing File and Directory Permissions

• The mode is the inode section that stores permissions and uses:
  • User permissions: owner
  • Group permissions: group owner
  • Other permissions: everyone on system
• There are Three regular permissions that may be assigned to each user:
  • Read
  • Write
  • Execute
• User or owner: refers to users with read, write, and execute permission
• Other: refers to all users on system
• Permissions are not additive, the system assigns the first set of permissions that are matched in the mode order: user, group, other
• Linux permission should not be assigned to other only

Access Control

• Read allows a user to open and read the contents of a file or to list the contents of the directory if the user has also been given execute permission
• Write allows a user to open, read, and edit the contents of a file or to add or remove files to and from the directory if the user has also been given execute permission
• Execute allows a user to execute the file in memory if it is a program file or script, and allows a user to enter the directory and work with directory contents

Changing Permissions

• The chmod (change mode) command changes mode (permissions) with a minimum of at least two arguments
  • Criteria is used to change permissions
  • Filenames to change
• If the permissions to be changed are identical for the user, group, and other categories, you can use the "a" character to refer to all categories
• u (user)
• g (group)
• o (other) -a (all categories)

Operations in chmod table

Operation | Permission

• ------------ | ------------- +(adds a permission) | r (read)
• (removes a permission) | w (write) = (makes a permission equal to)| x (execute)

Default Permissions

• New files are given rw-rw-rw- permissions by default
• The umask variable is a special variable that takes away permissions on new files and directories.
• The umask command displays the umask
• You can change the umask by setting a new umask as an argument to the umask command

Special Permissions

• There are three special permissions for files and directories
• SUID (Set User ID)
• SGID (Set Group ID)
• Sticky bit
• If SUID is set on a file, the user who executes the file becomes the owner of the file during execution
  • No special functionality when set on a directory
  • Only applicable to binary compiled programs
  • It cannot be used on shell scripts
• SGID also applicable to files and directories
  • If set on a file, the user who executes the file becomes a member of the file's group during execution
  • If a user creates a file in a directory with SGID set, the file's group owner is set to be the directory's group owner and not the user's primary group
• Previously the sticky bit locked files in memory, currently it is only applicable to directories
• The sticky bit ensures that a user can only delete his/her own files when given write permissions in a directory
• Special permissions require execute
• They mask the execute permission when displayed by the Is -l command
• May be set even if file or directory does not have execute permission
  • Indicating letter in the mode will be capitalized
• Add special permissions via chmod command

Access Control List

• Access control list (ACL) is a list of users or groups that you can assign permissions
• The setfacl (set file ACL) command modifies ACL entries for a particular Linux file or directory
• Use the -m option to modify the ACL or the -b option to remove all extra ACL assignments on a particular file or directory
• The getfacl (get file ACL) command is used to list all additional entries in the ACL

Managing Filesystem Attributes

• Linux has file attributes that can be set which work outside Linux permissions, and are filesystem-specific
• The lsattr (list attributes) command lists filesystem attributes
• The chattr (change attributes) command adds or removes filesystem attributes
• Immutable attribute (i) prevents the file from being modified in any way

Summary

• the Linux directory tree obeys the Filesystem Hierarchy Standard, which allows system files to be located in standard directories
• Many file management commands exist to create, change the location of, or remove files
• Files can be found using different commands such as: locate, which and find
• Files can be linked two different ways:
  • Symbolic link: a file serves as a pointer to another
  • Hard links: one file is a linked duplicate of another
• Each file and directory has an owner and a group owner
  • the owner can change permissions and grant ownership
• Permissions can be set on the owner of a file, members of the group of the file, and everyone on the system (other)
• There are three regular file and directory permissions (read, write, execute) and three special file and directory permissions (SUID, SGID, sticky bit)
• Permissions can be changed using the chmod command
• New files and directories receive default permissions
• The root user has all permissions to all files and directories on the Linux filesystem
  • The Root user can change the ownership of any file or directory
• The default ACL on a file or directory can be modified to include additional users or groups
• Filesystem attributes can be set on Linux files to provide low-level functionality such as immutability