Every file has nine access modes associated  with  it.   The
     modes  can  be switched on and off by using the "chmod" mode
     changing command.  The nine access modes are divisible  into
     three  sets  of  three switches.  Each set of three switches
     corresponds to a class of people:

          user  - the person who created the file
          group - people in a selected group
          other - everyone else on the system

     For each class of people there are three classes of  permis-
     sions:

          read    - ability to see the contents of the file
          write   - ability to change the contents of the file
          execute - ability to execute the contents of the file

     The protection fields of a UNIX file are displayed by  using
     the command 'ls' followed by the '-l' option.

          % ls -l
          total 161
          -rw-r--r--  1 parish    9487 May 19 12:34 index
          -rw-r--r--  1 parish    7235 May 19 13:57 basic
          -rw-------  1 parish    1564 May 19 23:45 tools
          %

PERMITTING ACCESS TO FILES AND DIRECTORIES

     Here is a sample directory listing, showing  the  permission
     fields and the people associated with each permission:

          :<------------special flag to indicate type of file
          :                  [e.g. d : directory, - : file]
          : u<--------------three permissions for USER
          :|||
          :||| g<--------------three permissions for GROUP
          :|||:::
          :|||::: o<---------------three permissions for OTHER
          :|||:::|||
          drwxrwxrwx  2 parish         1024 May 19 12:34 files
          -rw-------  2 parish         1024 May 19 12:34 morefils
          -rwxr-xr-x  2 parish         1024 May 19 12:34 lastfile

     The hyphen indicates that the  permission  is  disabled.  An
     enabled  permission  is shown by the appropriate letter, 'r'
     'w' or 'x.'

     The permission fields for directories are interpreted a lit-
     tle  differently  than  those  for  a file. The three fields
     (user, group, other) remain the same as those for a file but
     the three permissions mean:

          r(ead):   can look for a file name in this directory
          w(rite):  can create or delete files in this directory
          x(ecute): can search into this directory

     In other words, directory permissions protect  files  rather
     than the contents of files. For example, if someone only has
     execute permission on a directory they can  list  or  run  a
     file  in  that directory but they can't get a listing of all
     the files in that directory. For that  they  would  have  to
     know  the  exact  names  in advance. It is necessary to have
     execute permission on a directory to change (chdir) to it.

CONTROLLING ACCESS WITH CHMOD

     In order to control the access users may have to  your  file
     or directory, use the 'change mode' program, chmod.

     The chmod command allows changing of permissions  by  names,
     in  a format similar to the way that they are printed on the
     screen after issuing the 'ls -l' command.  For  example,  to
     turn off other's write permission you can issue the command:

          chmod o-w filename

     (you might translate "o-w" as "for others, take  away  write
     permission".)

     To turn write permission back on you would say:

          chmod o+w filename

     (similarly, "for others, add write permission".)

     You can group changes together with commas. For example,  in
     order  to make a file readable by the public but writable by
     your group, you might use the command:

          chmod g+rw,o+r filename

     To remove write permission from your  group  later  on,  you
     could issue the command:

          chmod g-w filename

     Another way to achieve the same result would be to  use  the
     command

          chmod g=r filename

     The = operator assigns the permission explicitly (all  other
     bits  for  that  category  (owner, group, or others) will be
     reset, i.e., g=r would remove all permission from the  group
     except  read,  and  explicitly  set read if not set already.
     (NOTE:  It is unlikely that you would ever want to give  the
     public write permission to your files or directories.)

     If you wish to  grant  access  to  a  directory  to  others,
     without  risking  changes to the directory's files, give 'r'
     AND 'x' permissions  (the  execute  flag  is  important  for
     access  along  with the read flag).  If you wish to keep the
     directory private, then remove permissions from the  'other'
     fields.   NOTE:  it is possible to delete a file in a direc-
     tory, even without having read or write access to that file,
     merely by having write access to the file's directory.

USING OCTAL VALUES TO CHANGE ACCESS

     Protection fields can also be interpreted as  octal  values.
     The following table shows the most commonly used modes.

                             Private         Public
          Directory       700             755
          Text file         600             644

     To change the mode of the protection fields, use the command

          % chmod 700 directory
          % ls -ld directory
          drwx------  2 parish         1024 May 19 12:34 directory
          % chmod 755 directory
          % ls -ld directory
          drwxr-xr-x  2 parish         1024 May 19 12:34 directory

          % chmod 600 filename
          % ls -l filename
          drw-------  2 parish         3234 May 19 12:34 filename
          % chmod 644 filename
          % ls -l filename
          drwxr-xr-x  2 parish         3234 May 19 12:34 filename

     You can also chmod a file so that you can't read, write,
     or  execute  it  even  though you own it.  There may be cir-
     cumstances in which you will want to turn off write  permis-
     sion  to yourself, so that you don't accidentally change the
     file.  You will be able to use  chmod  again  at  any  time,
     changing  the  permissions  to whatever settings you prefer.
     You can never get yourself into very much trouble with  this
     feature,  but  some people are upset when the system refuses
     to let them read or write their own files.

SUMMARY

                             chmod 755 *

---

|| Home || || The Basics || Resources || Policy || Volunteer Coordinator / Webmaster ||

---

© 2000 All Rights Reserved

---