More Information About Modes

There are three types of permissions for a file or directory: read, write, and execute.

File

• Read: it is possible to see the contents of the file
• Write: it is possible to change the contents of the file
• Execute: the file contains machine code, or the file contains a script and the name of a program to execute the script

Directory

• Read: it is possible to see the list of names of the files that are in the directory
• Write: it is possible to change the list of names of the files that are in the directory
• Execute: it is possible to access the files that are in the directory. It is possible to change (cd) to the directory.

Each file has three different access permissions

• User: the owner of the file
• Group: the group that the user belongs to. A group allows different users to read and write a set of files. A group allows different users to read and write a set of files.
• Other: every other account that is neither the user nor a member of the file's group.

The output of thels -al command lists all the permissions. There are ten characters

• First character - type of file: d is directory, l is link, - is file
• The remaining are r (read), w (write), x (execute) and - (no permissions)
• First set of three - user permissions
• Second set of three - group permissions
• Third set of three - other permissions

Each permission is assigned a numeric value. Each number from 0 - 7 in binary indicates a unique combination of permissions of rwx.

• none: 0 (000)
• e: 1 (001)
• w: 2 (010)
• we: 3 (011)
• r: 4 (100)
• re: 5 (101)
• rw: 6 (110)
• rwe: 7 (111)

Calculate the number from 0 to 7 for each access category: user, group, other. Put them together to get a three digit permission number.

• chmod 755 public_html
• chmod 644 index.html
• chmod 711 ~
• chmod 600 httpd.conf

To get more information on the chmod command, use the man pages.

• On ocelot: man chmod
• Here are the explanations of the other modes in the ls command
  • l - file locking. Only one process can access the file at one time.
  • t - sticky bit. If directory has write access, only owner of the directory or a file in the directory can rename or remove the file.
  • T - sticky bit but no access. Sticky bit is on, but directory does not have execute permission.
  • s - set-user or set-group ID. An executable file will run with the permissions of the user or group of the file.
  • S - set-user or set-group ID but no access. Set-ID bit is on, but directory does not have execute permission.

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Symbolic Link

Symbloic links are used in UNIX much like a shortcut is used in Windows. A directory can create a link to a file in another directory. To the operating system, this looks like the file exists in the current directory, eventhough it actually exists in a different physical location.

To create a symbolic link:

1. Change to the directory where the link is to be created.
2. Issue the ln -s command with the path to the actual file that is being linked
   ln -s /path/to/real/someFile.ext

If the link was created, then you will see the name of the linked file in the current directory, but it will be labeled as a link in a directory lising,
lrwxrwxrwx someFile.ext
This means that the file is a link and that it will have the same permissions as the actual file.

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