The file structure and related utilities

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The file structure and related utilities

• CS240 Computer Science II

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The hierarchical or tree file structure
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Typical UNIX hierarchical or tree file structure
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Typical contents of various directories

• / (root) the top of the file hierarchy
• /export/home usually contains all users
  subdirectories.
• /usr traditionally contains subdirectories that
  keep system information.
• /usr/bin usually contains the standard Unix
  utilities.
• /usr/sbin contains utilities for system
  administration.
• /etc keeps configuration and other
  administration files, e.g., /etc/passwd contains
  a list of all users.
• /var contains files that vary as the system
  runs, e.g., users mailbox files, system log
  files, etc.
• /dev contains information about peripheral
  devices.
• /tmp used by programs to hold temporary files.

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Directory or file name

• Directory or file folder may hold files and
  sub-directories.
• A file or directory name is case-sensitive and
  may be up from 14 to 255 characters (depending on
  the system). Allowable characters include A-Z,
  a-z, 0-9, _, ., and ,.
• ls a utility displays all files, including
  hidden ones (those files whose name begin with a
  .), in a directory.
• ls F utility displays directories with / as the
  last character.

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Directories

• Working or current directory pwd utility
  displays your current (present) working
  directory.
• Home directory is usually your logging directory.
  It contains an important file named .profile
  (Bourne or Korn shell) or .login (C-shell) known
  as the start-up file. the start-up file contains
  shell commands and variables that customize a
  users work environment.

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The mkdir utility

• mkdir create a directory.
• It automatically puts two entries in every
  directory that you create the . and ..,
  representing the directory itself and its parent
  directory.
• the . is synonymous with the pathname of the
  working directory.
• the .. is synonymous with the pathname of the
  parent of the working directory.

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The cd utility

• cd without argument makes home directory your
  working directory.
• cd .. moves you from you present working
  directory to its parent directory.
• cd pathname moves you to the pathname that you
  specify.

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The rmdir utility

• rmdir removes or deletes a directory that is
  empty (no files or sub-directories in it.).
• If the directory is not empty, you must use rm
  utility to first remove all files.

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Relative pathname

• pwd
• /faculty/ewh
• mkdir CS240 relative to present
  working directory
• pwd
• /faculty/ewh/CS240
• mkdir /faculty/ewh/CS240 absolute
  pathname is specified
• Note that you can use either relative or absolute
  pathname in virtually anywhere that requires a
  filename or pathname for other utilities such as
  cd, ls, vi, mkdir, rm, etc.

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Examples of using pathnames

• Assuming /faculty/ewh is the present working
  directory.
• cp abstract CS480/Seminars/abstract.2002
• vi CS480/Seminars/abstract.2002
• cd CS480/Seminars
• pwd
• /faculty/ewh/CS480/Seminars
• vi abstract.2002
• cd ..

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The mv utility

• the mv can be used to rename a file.
• The mv can also be used to move files from one
  directory to another.
• mv existing-file-list directory
• The following command moves the prog1 and prog2
  files from their present working directory to its
  child CS240 sub-directory.
• mv prog1 prog2 CS240

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Access permissions

• Three types of users
• the owner of a file
• a member of the owners group
• anyone else
• Three types of access
• read
• write
• execute
• So, there can be nine different combinations.

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Access permissions continued

• ls l prog1.cpp
• - r w - r - - r - - 1 ewh faculty 524
  Nov 8 1752 prog1.cpp
• the type of the file (first character)
• the files access permission (next 9 characters)
• r (read), w (write), x (execute), and - (no
  permission)
• In the 9-character group
• first three owner
• next three group
• last three anyone else
• the number of links to the file
• the name of the owner of the file
• the name of the group that has group access
• the size of the file in bytes
• the date and time the file was created or last
  modified
• the name of the file

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The chmod utility

• adds and - removes a permission
• a all users u owner o other, g
  group. A superuser (usually system administrator)
  can have full access to all files.
• chmod aaw prog1.cpp adds read/write to all
• ls l prog1.cpp
• - r w - r w - r w - 1 ewh faculty 524
  Nov 8 1752 prog1.cpp

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Directory access permission

• Since a directory is not a file, it cannot be
  executed. Execution right of a directory has been
  defined as the right to search through the
  directory.
• The command below grants all users with
  permission to look through, read, write and
  remove all files from Dr. Hus CS240 directory.
• chmod arwx /faculty/ewh/CS240
• To view the access permission of a subdirectory,
  one can use ls with -d option.
• ls -ld /faculty/ewh/CS240

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Link a pointer to a file

• Everytime a file is created, a pointer which is
  associated with the filename and points to the
  file is stored in a directory.
• The same file can be shared by creating
  additional links with the ln utility.

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The ln utility

• Issuing a the following command in Dr. Hus home
  directory to share the prog1.cpp file in Dr. Hus
  home directory (/faculty/ewh/prog1.cpp) a new
  link will appear in Dr. Najarians directory
  /faculty/najarian
• ln prog1.cpp /faculty/najarian/newname.cpp
• Note
• the filename may differ, but both refer to the
  same physical file
• file status information such as access
  permission. owner, and the time the file was last
  modified is the same for all links!
• the rm utility removes a link. When the last link
  is removed, the files is deleted the operating
  system releases the storage space used by the
  file.
• links or pointers to a file may be placed in
  different directories of the same user.
• It is not possible to create links between
  different file systems.

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Symbolic link

• The link just described that directly points to a
  file is known as the hard link. Only the
  superuser can create a hard link to a directory.
• A symbolic link in an indirect pointer it is a
  driectory entry that contains the pathname of the
  pointed-to file. Anyone can create a symbolic
  link to a directory.
• A symbolic link can link to any file regardless
  of where the file is located in the file
  structure.
• To create a symbolic link, use the -s option of
  the ln utility.
• to create s symbolic link to a directory, use the
  -s option the the ln utility.
• Note that the pwd utility does not recognize the
  symbolic link it only recognizes the linked-to
  directory.