05. Directories


Topics:
  • Files in Linux
  • Directories
  • Directory Tree Diagram
  • Root Directory
  • Home Directory
  • Directory Path
  • Commands
    • pwd and cd
    • ls and directories
    • mkdir and rmdir
    • cp and Directories
    • mv and Directories
    • which and whereis
Files in Linux
  • The Linux philosophy is “everything is a file” 
  • The term “file” not only means a file of data (the typical meaning), it can also refer to an input device (such as a scanner), an output device (such as the monitor), a hardware component (such as the hard drive), or a process (such as the shell that you work with)
  • Linux divides files into 7 different types:
1. regular file: a file of data. Data can be text (human readable) or binary (machine readable)
2. directory: a file that can “contain” other files (equivalent to folders in Windows)
3. character special file: IO device that processes one character at a time, such as a printer
4. block special file: IO device that processes a block of characters at a time, such as a hard disk
5. symbolic link: a file that holds the address of another file
6. FIFO: a file used for inter-process communication
7. socket: a file used for network communication

Directories
  • Directories help you organize files by providing ways to group similar files together
  • In Linux, files are grouped into directories (which themselves are files), and directories are grouped under other directories, in “tree” form called a directory tree or directory hierarchy
  • Each file in the hierarchy is called a node
  • The top node is called root
  • Except for root, each node can have 
    • 1 node above it, called its parent node
    • 0 or more nodes below it, called its child nodes
  • A parent directory can have directory child nodes, called subdirectories

Directory Tree Diagram
  • In this sample directory tree:
    • There are 9 files
    •  Each file is a node
    •  One root node
    •  DirectoryC is the child node of root, and is the parent node of a regular file 
    •  DirectoryD is the subdirectory of DirectoryB


Root Directory
  • There is only one root node for every directory hierarchy
  • The actual name you type in for root is  /
  • Some common subdirectories under root are
    • bin:  (for binary) contains general Linux utilities, in binary format
    • sbin: (for system binary) contains utilities for system administration
    • etc: contains configuration files for the system
    • usr: (for user) contains applications for the users
    • lib: (for library) shared libraries
    • var: (for variable) contains data files that change when the system is running
    • tmp: (for temporary) contains temporary files
    • dev: (for device) contains files for hardware devices
    • mnt: (for mount) contains mount points for storage devices
    • home: contains home directories of users

Home Directory
  • Every user is assigned a unique directory to store his / her files. This directory is called the home directory
  • It has the same name as your log in name
  • You cannot change your home directory name or its location in the system
  • When you first log in the system, you are at the home directory 
  • Often you will not be at the home directory. Instead you will move to a different directory, depending on the task you are doing. The directory where you are working is called the current directory or the working directory

Directory Path
  • Since the Linux directory tree is a huge hierarchy of files, it is important to know the path to a file in order to access it
  • The path of a file is the location of the file in the directory tree
  • A path is formed by 
    • listing node names that are connected to each other in the tree structure
    • separate node names with  /   (not the same as root)
  • 2 types of paths: absolute path and relative path
  • Absolute path
    • Shows the location of a file, starting from root  /
    • The absolute path of a file will not change unless the file is moved to another place in the hierarchy
  • Relative path
    • Shows the location of a file, starting from your current directory
    • Since the relative path is relative to your current directory, the relative path of a file will change if you change your current directory

  • Assume your home directory is ImaStudent:
    •  The absolute path of your home directory is /home/student/ImaStudent
    •  From the root directory, you can use a relative path to the distribution directory by traversing down to home, then down to distribution. The relative path from root is:    home/distribution






  • All commands that accept a filename as an argument will accept a filename with a path
  • When a filename with no path is the argument, the file needs to be in the current directory or the shell will not be able to find it
  • When a filename with a path is the argument, the file needs wherever the path indicates
  • Examples:
    • ls  fileA       will list fileA in the current directory
    • ls  /labs/fileA will list fileA that is in the labs directory which is under root
    • cp  fileA  fileB   will make a copy of fileA in the current directory and store it as fileB in the current directory
    • cp fileA  /files/fileB   will make a copy of fileA in the current directory and store it as fileB in the files directory, which is under root directory

pwd and cd
  • pwd: (for print working directory) shows the absolute path of where you are in the directory tree
  • cd: (for change directory) moves you to another directory
  • Common format:   cd  path   
    • where path can be:
      • Nothing: moves you to your home directory
      • Absolute path: make sure it starts with /  (root)
      • Relative path: make sure the first node in the path is connected to your current directory
        • To go to a subdirectory:  type the name of the subdirectory
        • To go to a parent directory:  type  .. 
        • To go to the current directory:   type  .
      • Path with special symbols: 
        • ~   your home directory
        • ~userID    home directory of user  userID

ls and Directories
  • Recall: 
    • ls will list filenames in the current directory
    • ls  filename will echo filename if file exists and is a regular file
  • If filename is the name of a directory, then ls will list filenames under that directory
  • To see the file type of a file:
    • Use the long listing:   ls –l
      • The first character in the mode column will tell you the file type:             d for directory, l for link, - for regular file
    • Use:   ls –F 
      • The filenames will be listed with an additional symbol at the end:            / for directory, @ for link, * for executable regular file, nothing for text file (which is also a regular file)

mkdir and rmdir
  • mkdir: (for make directory) creates a new directory in the current directory, or under a different directory if a path is given
    • Common format:    mkdir   directory_name
    • When first created, the directory is an empty directory (no files in it, except 2 hidden files . and ..)
  • rmdir: (for remove directory) deletes an empty directory
    • If the directory is not empty, you must delete all files in it first
    • Common format:    rmdir   directory_name
    • Alternatively, to remove a non-empty directory, use             rm –r   directory_name         where -r is for recursive
    • Removes the directory and recursively go down all its subdirectories and remove all the files under them
    • Caution: this can remove a large number of files, make sure you don’t run this command ‘by accident’

cp and Directories
  • Recall that cp will copy the source file to the destination file. Now we discuss all the combinations of cp with regular files and with directories
    • File below means regular file, Dir below means directory
  • Copying a source regular file
    • cp existingFile  nonExistingFile       new nonExistingFile is created
    • cp existingFile1 existingFile2           existingFile2 is overwritten
    • cp existingFile  nonExistingDir      new regular file created with the      name of nonExistingDir
    • cp existingFile  existingDir               new file called existingFile created      under existingDir
    • Copying a source directory (need to use –r option, for recursively copy, all files and subdirectories will also get copied)
    • cp –r  existingDir  nonExistingDir    new nonExistingDir is created
    • cp –r  existingDir1 existingDir2        existingDir1 is copied and put      under existingDir2
    • cp –r existingDir nonExistingFile     new directory called nonExistingFile      is created
    • cp –r existingDir existingFile            not possible

mv and Directories
  • Recall that mv will move the source file to the destination file, and the source file will no longer exist. Now we discuss all the combinations of mv with regular files and with directories
    • File below means regular file, Dir below means directory
  • Move a source regular file
    • mv existingFile  nonExistingFile      new nonExistingFile is created
    • mv existingFile1 existingFile2          existingFile2 is overwritten
    • mv existingFile  nonExistingDir      new regular file created with the      name of nonExistingDir
    • mv existingFile  existingDir              new file called existingFile created                     under existingDirectory
    • Moving a source directory (don’t need option, all files and subdirectories will move)
    • mv  existingDir  nonExistingDir        new nonExistingDir is created
    • mv  existingDir1 existingDir2           existingDir1 moves under existingDir2
    • mv existingDir nonExistingFile         new directory called nonExistingFile      is created
    • mv existingDir existingFile               not possible

which and whereis
  • These commands give the path (location) of a particular utility. They only accept a utility name as an argument
  • which  utility_name
    • shows the actual utility that runs when utlity_name is typed on the command line
    • Example:  [testuser@abc ~]$ which vi
        • alias vi='vim'
        • /usr/bin/vim
  • whereis utility_name
    • shows the location of the utility in the system
      • Example:  [testuser@abc ~]$ whereis vi
        • vi: /bin/vi /usr/share/man/man1/vi.1.gz                       /usr/share/man/man1p/vi.1p.gz
    • If you know the location of a utility and you want to run it, type the absolute path of the utility on the command line
      • Example: To run vi instead of vim: [testuser@abc ~]$ /bin/vi filename

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