Chapter 3 Some additional notes

1
Chapter 3 Some additional notes

• File permissions
• A file has three types of permissions (read,
  write and execute).
• Available to three categories of users (user,
  group and others).
• Can see the permission settings using ls l
• apache/public_html/CS3375 ls -l
• total 6428
• -rw-r--r-- 1 kkhan cs 3961 Jan 29
  2058 3375.html
• -rw-r--r-- 1 kkhan cs 70656 Jan 21
  0311 Syllabus_3375_sp08.doc
• -rw-r--r-- 1 kkhan cs 763392 Jan 7
  1327 Your_UNIX_2e.ppt
• -
  r-x - - - - - -
• The total line tells how many blocks (usually
  1024 bytes per block) are contained in this
  directory.
• Directory, followed by three sets of 3 bits
  indicating permissions
• First set? Owner
• Second set? Group

Owner can read and execute this file
2
Chapter 3 Some additional notes

• Copying Permissions
• As one other shortcut, its possible to tell
  chmod give users of one class the same
  permissions that some other class has.
• Lets say we have these files
• d------rwx joe acctg sales
• -rw-r--r-- joe acctg info.dat
• The other users have full permissions on the
  sales directory.
• Wed like to say the user and group should be
  assigned () the permissions belonging to
  others. That translates to
• chmod ugo
• Similarly, to make info.dat readable and writable
  to the group, we can say
• chmod gu info.dat (you can read this as the
  group is assigned () the permissions currently
  held by the user.)
• You may also use and - to add and subtract the
  permissions that currently belong to a different
  class of user.
• You cant mix the standard permissions (r, w, and
  x) with the coyping shortcuts. chmod will protest
  if you give it something like this
• chmod gwu info.dat

3
Need to be able to change the file permissions

• Why?
• Protect files from accidental deletion or
  modification (archive)
• Allow team members to read, modify, execute,
  possibly even delete a file without having to
  send them a copy
• Setting up your webpage on unix have to make
  the html files accessible or no one will be able
  to see it!
• Unix command is chmod
• Command allows us to change the permission
  settings
• Examples (can use relative or absolute
  permissions)
• chmod orwx testfile1
• -r-x---rwx 1 kkhan cs 24 Sep 7
  1041 testfile1
• // o is for other, means add permission

4
Need to be able to change the file permissions

• Examples cont.
• Change it back?
• chmod o-rwx testfile1
• Results in
• -r-x------ 1 kkhan cs 24 Sep 7
  1041 testfile1
• // - means remove permission
• Can also use absolute (binary) mode
• chmod 507 testfile1
• Results in
• -r-x---rwx 1 kkhan cs 24 Sep 7
  1041 testfile1
• 5 0 7
• 101 000 111
• r-x - - - rwx
• (Bits are set or cleared)

5
Need to be able to change the file permissions

• Access to files is not independent of the
  permissions on the directories
• The directory is the parent permissions on the
  parent cannot be over-ridden by permissions on
  its files
• Examples
• Directory is set with no write access
• Cannot add or delete a file in the directory no
  matter what its permissions are
• Question could we edit a file in this directory?
  
• Only if we have write permission for the file
• Directory is set with no read or execute access
• Question could we run an html file in this
  directory?
• No (this is a common problem setting up webpages)

6
Need to be able to change the file permissions

• Man chmod
• chmod -fR absolute-mode file...
• chmod -fR symbolic-mode-list file...
• -f Force. chmod will not complain if it fails
  to change the mode of a file.
• -R Recursively descends through directory
  arguments, setting the mode for each file as
  described above.
• Problem Im building my website and I want to
  set all my files in the directory public_html so
  everyone can read and execute my html file
• chmod R arx public_html
• Review chmod Solves Problems http//catcode.com/t
  eachmod/no_prob.html

7
File Permission Exercise

• Set the permissions as follows
• testdir1 group
  other has no access
• testfile1.html group other
  has no access
• testfile2.html group other
  has read access
• testfile3.html group other
  has write access
• testfile4.html group other
  has execute access
• testfile5.html group other
  has read and execute access
• testfile6.html group other
  has read and write access
• testfile7.html group other
  has write and execute access
• testfile8.html group other
  has read, write, and execute access

8
File Permission Exercise

• testdir2 group other
  has read access
• // 8 files
• testdir3 group other
  has write access
• // 8 files
• testdir4 group other
  has execute access
• // 8 files
• testdir5 group other
  has read and execute access
• // 8 files
• testdir6 group other
  has read and write access
• // 8 files
• testdir7 group other
  has write and execute access
• // 8 files
• testdir8 group other
  has read, write, and execute access

9
File Permission Exercise

• Now, try to
• display the contents of each directory
• e.g., ls testdir1
• display a long listing of each directory
• e.g., ls l testdir1
• why dont these two list commands behave the same
  way?
• display the contents of each file in each
  subdirectory
• e.g., more testfile1.html
• change the contents of each file
• e.g., echo trying to change the file gtgt
  testfile1.html
• delete each file
• e.g., rm testfile1.html

10
File Permission Exercise

• To test the ability to load a .html file in a
  browser, you need to set up a softlink (aka
  symbolic link) under your public_html directory
• cd
• cd public_html
• ln -s ltoriginalgt ltlink namegt
• - creates a softlink (called link name), which
  points to the original subdirectory
• e.g., ln s kkhan/public_html/testfiles
  testdir
• cd ltlink namegt takes you to the original
  subdirectory
• cd testdir
• Now, in the browser (for example running on the
  pc) you can use the open file option to test
  loading the .html files
• If the file loaded successfully, then there is a
  brief text displayed from the file
• Try loading one that definitely should work
• testdir8/testfile8.html