File Management

UNIX File System: The UNIX File System (files and directories) are organized in a hierarchical fashion very similar to the fashion in which many corporations are organized (technically designated as a tree structure since it looks like an upside down tree). Similar files are clustered into a directory. Directories can have subdirectories etc.This is very similar to the concept of folders and sub-folders that is well understood by users of Windows95 and Macintosh systems.

At the base of this hierachy is the root directory. The root directory is designated by a single slash /. There are certain major directories which are underneath the root directory (eg. /usr, /home, /man, /tmp). These major directories contain further sub-directories (eg. /usr/bin/ is the bin sub-directory of the usr directory of root.). This structure extends further and gets complex. All subdirectories of a directory are represented by typing in the sub-directory name after the directory name separated by a slash symbol.

Example: /usr/local/bin represents the directory bin which is a subdirectory of local which is a subdirectory of usr which is a directory off root (/).

UNIX File System | File Properties | File Permissions | File Management | Directory Management

As can be seen above, the properties are divided (from left to right) Permissions, Owner, Group, Size of the File, Date of Last Modification and Name of the File. All of these properties can be modified or changed. The permissions are changed using the chmod command, the owner using the chown and the Group using the chgrp command. The file size changes when you add or remove data from the file. When you edit and save a file, its date of last modification reflects the time the change was saved. The name of the file can also be changed using the mv (move) command.

UNIX File System | File Properties | File Permissions | File Management | Directory Management

UNIX File permissions can take some getting used to since they are a bit cryptic and unlike anything that PC or Mac users would have experienced. A file or a directory can have Read, Write and Execute permissions and since UNIX is a multi-user system, a file or directory has a set of Read, Write and Execute permissions for :

• the Owner (i.e. the User who owns the file),
• the Group (i.e. other Users who are members of the Group that the file belongs to)
• and Others (i.e. everybody else other than the User or members of the Group).

These sets of file permissions are abbreviated to 3 sets of three characters R, W and X. The presence of a character implies that the permission has been turned on and the absence (dash) indicates that the permission has been turned off. The very first character in the permission's string is either a dash (meaning that it is a file) or a 'd' (meaning that it represents a directory. The next three characters rwx represent the permissions for the user, the next three characters the group and the last three others.

In the above example, the first line represents a regular file named index.html. This is a file since the first character of the permissions string is a dash (i.e. absence of any character). You will note that the next three characters are 'rw-'. This implies that the User (i.e. owner) of the file has Read and Write permissions but does not have Execute permissions. The Group and Others do not have any permissions i.e. niether can they Read, Write nor Execute the file.

The second line represents a directory named public_html. This is a directory since the first character of the permissions string is a 'd'. The Owner of the directory has Read, Write and Execute permissions. While the Group and Others have only Read and Execute permissions.

Important: Every directory requires Execute permissions otherwise it will be inaccessible.

Permissions can be given or revoked individually to each section (User, Group or Other) or to All. The chmod command is used for changing permissions. The format of the command is
chmod {section}{+ | -}{permission} filename

The section can be a for all sections, u for user, g for group and o for others.
A plus symbol adds permissions and a minus symbol removes permissions.
The permission can be r for read, w for write and x for execute.

Adding read permissions to all users	chmod a+r public_html
Removing read permissions from all users	chmod a-r private_files
Add execute permissions to a file for the user	chmod u+x bin_file
Remove write permission to a file for group members	chmod g-w read-only-file
Adding read and execute permissions to a directory to all users	chmod a+r public_html chmod a+x public_html

The following videos will further demonstrate using the chmod command

• This video, 100 seconds in length, will change the permissions for everybody, followed by changing the three types of permissions for others, then the group (Caution: large file. 534K)
  
  
• This video will demonstrate using chmod for the purposes of making your public_html directory accessible to all. This is crucial for creating your personal Web site at UIC. The mkdir command will also be used, as it is necessary to create the directory. (152K)

UNIX File System | File Properties | File Permissions | File Management | Directory Management

Displaying the contents of a file:	Use the cat command to display the contents of a file to the screen. If the file is larger than a screenful, it will scroll off the screen. Example: cat index.html
Displaying the contents of a file one screenful at a time	Use the more, page or less commands to display a file to the screen, one screenful at a time. Example: more index.html cat index.html | more
Copying a file to another file	Use the cp command to copy files either within the same directory, to another directory or from one directory to another directory. Example: cp index.html oldindex.html
Copying a file to another directory	Example: cp index.html public_html/index.html
Copying a file from one directory to another	Example: cp backup_html/myfile.html public_html/myfile.html
Copying an entire directory and its files to another directory	Example: cp -r backup_html public_html
Changing the name of a file	Use the mv command to change the name of a file. Example: mv index.html oldindex.html
Move a file from one directory to another directory.	Example: mv index.html public_html/index.html
Move an entire directory and its files to another directory	Example: mv -r new_html public_html
Delete a file	Use the rm command to delete a file. Example: rm index.html rm * will delete all files in the current directory rm -i *html will prompt you for each file before deleting. Here all files that end in html are being deleted.

UNIX File System | File Properties | File Permissions | File Management | Directory Management

Create a new directory	Use the mkdir command to create a new directory Example: mkdir tempdir
Delete an empty directory	Use the rmdir command to delete an empty directory Example: rmdir tempdir
Delete a directory and all the files it contains	Use the rm command with the -r option. Example: rm -r tempdir
Rename a directory	Use the mv command to change the name of a directory Example: mv tempdir temporary

UNIX File System | File Properties | File Permissions | File Management | Directory Management