How can I change the permission of my homedirectory
For all user (Windows, SGI, NeXT, SUN, Linux, MAC)
Open a telnet (or ssh) connection to the fileserver on which your homedir
is stored.
Use the chmod - command to change the permissions:
Every directory and file on the system has an owner, and also an associated group. It also has a set of permission flags which specify separate read, write and execute permissions for the user (owner), group, and other (everyone else with an account on the PP&B cluster)
The 'ls' command shows the permissions and group associated with files when used with the -l option. An example of the output produced by 'ls -l' is shown below
drwxr-xr-x 8 weil pc 1024 May 18 2002 Mail/ -rw-r--r-- 1 weil pc 289596 May 21 2002 USER.DAT drwxr-xr-x 15 weil pc 512 Aug 1 2003 profile/ drwx------ 8 weil pc 13824 Aug 9 2001 weil/
Understanding how to read this output is useful to all users, but especially people using group access permissions.
| Field 1: | a set of ten permission flags |
| Field 2: | link count (don't worry about this) |
| Field 3: | owner of the file |
| Field 4: | associated group for the file |
| Field 5: | size in bytes |
| Field 6-8: | date of last modification (format varies, but always 3 fields) |
| Field 9: | name of file (possibly with path, depending on how ls was called) |
The permission flags are read as follows (left to right)
| position | meaning |
| 1 | directory flag, 'd' if a directory, '-' if a normal file |
| something else occasionally may appear here for special devices | |
| 2,3,4 | read, write, execute permission for User (Owner) of file |
| 5,6,7 | read, write, execute permission for Group |
| 8,9,10 | read, write, execute permission for Other |
| value | meaning |
| - | in any position means that flag is not set |
| r | file is readable by owner, group or other |
| w | file is writeable. On a directory, write access means you can add or delete files |
| x | file is executable (only for programs and shell scripts - not useful for data files). Execute permission on a directory means you can list the files in that directory |
The default file permissions (umask, for UNIX users):
Each user has a default set of permissions which apply to all files created by that user, unless the software explicitly sets something else. This is often called the 'umask', after the command used to change it. It is either inherited from the login process, or set in the .cshrc or .login file which configures an individual account, or it can be run manually.
Typically the default configuration is equivalent to typing umask 022 which produces permissions of:
-rw-r--r-- for regular files, or drwxr-xr-x for directories.
In other words, user has full access, everyone else (group and other) has read access to files, lookup access to directories. <p> When working with group-access files and directories, it is common to use 'umask 2' which produces permissions of:
-rw-rw-r-- for regular files, or drwxrwxr-x for directories.
For private work, use umask 077 which produces permissions:
-rw------- for regular files, or drwx------ for directories.
The logic behind the number given to umask is not intuitive. The command to change the permission flags is "chmod". Only the owner of a file can change its permissions.
The command to change the group of a file is chgrp. Only the owner of a file can change its group, and can only change it to a group of which he is a member.
See the online manual pages for details of these commands on any particular system (e.g. man chmod).
Examples of typical usage are given below:
chmod g+w myfile give group write permission to "myfile", leaving all other permission flags alone chmod g-rw myfile remove read and write access to "myfile", leaving all other permission flags alone chmod g+rwxs mydir give full group read/write access to directory "mydir", also setting the set-groupID flag so that directories created inside it inherit the group chmod u=rw,go= privatefile explicitly give user read/write access, and revoke all group and other access, to file "privatefile" chmod -R g+rw give group read write access to this directory, and 'everything' inside of it (-R = recursive) chgrp -R th change the ownership of this directory to group 'th' and 'everything' inside of it (-R = recursive). The person issuing this command must own all the files or it will fail.
WARNINGS:
Putting umask 002 into a startup file (.login or .cshrc) will make
these settings apply to everything you do unless manually changed.
This can lead to giving group access to files such as saved email in
your home directory, which is generally not desireable.
Making a file group read/write without checking what its group is can lead to accidentally giving access to almost everyone on the system. Normally all users are members of some default group such asusers, as well as being members of specific project-oriented groups. Don't give group access to users when you intended some other group.
Remember that to read a file, you need execute access to the directory it is in AND read access to the file itself. To write a file, you need execute access to the directory AND write access to the file. To create new files or delete files, you need write access to the directory. You also need execute access to all parent directories back to the root. Group access will break if a parent directory is made completely private.