File System

Files on a Linux server are stored in a tree of directories, also called folders. This is how a complete path to a file is written:

/home/wilma/data/file.txt

where:

• the first ‘/’ points to the root of the tree
• ‘/home/wilma’ is Wilma’s “home” directory (see below)
• ‘/home/wilma/data’ is the directory, or dirname, of the file
• ‘file.txt’ is the name of the file itself, called its basename
• ‘.txt’ is the “extension” of the file that tells you what kind of file it is

Special-case paths and symbols

Certain file paths on Linux have special purposes, sometimes with shortcut symbols. Many are only important for system administrators, but you need to know you have a home directory for your use:

/home/<your_username>

that can be accessed with the following universal shortcut without needing to know the actual path:

~

Another shortcut to know about refers to the parent of the current working directory, i.e., “move up one level on the directory tree”:

..

whereas, the following refers to the current working directory itself:

.

File access permissions

On a shared server, you don’t want just anybody reading your files! Linux servers use a “owner/group/other” + “read/write/execute” method for determining who can do what to a given file or directory.

The kinds of people being granted permissions, in order of precedence, are:

• owner a.k.a. ‘user’ = generally the person who created the file
• group = the name of a group of users to which a person belongs
• other = everyone who is not ‘user’ or someone in ‘group’

The kinds of permissions being granted are:

• read = view the contents of a directory or file
• write = create or modify a directory or file
• execute = run a program or move into a directory

The following shows a common way this information is communicated:

drwxrw-r-- userName groupName myDirectory
-rwxrw-r-- userName groupName myFile

which is split apart and explained as follows:

# d=directory owner group other  owner group       directory
  d           rwx   rw-   r--    wilma flintstones myDirectory

# -=file      owner group other  owner group       file
  -           rwx   rw-   r--    wilma flintstones myFile

Thus, the two lines refer to one directory and one file, respectively. Each grants read+write+execute permissions to the owner, who is ‘wilma’, only read+write to people in the ‘flintstones’ group, and only read to everyone else.