The File System

• The file system in Unix/Linux is structured as a tree.
  Each node is either a directory (called folder in other OS's), or a file. A directory contains other directories and files.
  The top node is denoted by a forward-slash, /.

• Each user has a home-directory, where his/her files are contained.
  This can be refered to by either its full path (e.g. /home/users/adam for the user with login-name adam), or one of the following two "shorthand" notations:
  • a tilde followed by the login-name (e.g, the home directory of the user adam can be refered to by ~adam)
  • only the tilde, ~, if a user wants to refer to his/her own home-directory.
  Here are some commands to navigate in the file system.

• There are two special entries in each directory:
  • . (a dot) denotes the directory itself
  • .. (two dots) denotes the parent directory
  Therefore:
  • the command   cd ..   changes directory to the directory directly above the one where the command is issued.
  • the command   cd .   does not do anything, except maybe printing the current position in the file system.

• Each user's files and directories can be protected from other users (and therefore other users cannot read or change them).
  Some files are protected by default, other are left open to be read (but most likely never to written) by the other users.
  To (un)protect files, see the commands related to file permissions and ownership, in particular chmod.

• To see the permissions for a file, do ls -l FILE_NAME.
  For a directory do ls -ld DIRECTORY_NAME or ls -ld if you are in the directory you want to check. Alternatively, use ls -la and see the information for ., the directory itself.
  Other information provided by ls -l are the owner, group, size, and date/time of last change.

  To change the permissions, use the command chmod.

  Here are some example, and their meaning.

  
  phoenix> ls -l papers/Fermat-conjecture.tex
  -rw-r-----   1 adam    users  3495 Aug 18 15:23 papers/Fermat-conjecture.tex
  
  phoenix> ls -ld images
  drwxr-x--x   3 bob     www    4096 Apr  1  2003 images/
  
  phoenix> cd images
  phoenix> ls -ld
  drwxr-x--x   3 bob     www    4096 Apr  1  2003 ./
  
  

  The permissions are given for three categories: user, group, other (i.e., the rest of the world).
  Each category can be allowed to read, write and/or execute the file/directory.
  These permissions are listed in the last nine letters of the ten-letter string produced by ls -l.
  • The file Fermat-conjecture.tex is owned by user adam, and belongs to the group users.
    The string describing the permissions is -rw-r-----:
    • the first character, -, shows that this is a "plain" file
    • the next three characters, rw-, mean: the user (adam) can read and write, but not execute the file
    • the next three characters, r--, mean: the group (users) can also read, but not write or execute the file
    • the next three characters, ---, mean: others can not read, write or execute the file
  • The directory images is owned by user bob, and belongs to the group www.
    The string describing the permissions is drwxr-x--x:
    • the first character, d, shows that this is a directory
    • the next three characters, rwx, mean: the user (bob) can read (in this case, see what files are in the directory), write (in this case: add/remove files) and execute (in this case, access files in) the directory
    • the next three characters, r-x, mean: the group (www) can read and execute, but not write
    • the next three characters, --x, mean: others can only execute, but not read or write