Linux Filesystem Hierarchy

operating system
Software that shares a computer system's resources (processor, memory, disk space, network
bandwidth, and so on) between users and the application programs they run. Controls access to the
system to provide security.
PAM
Pluggable Authentication Modules. A suite of shared libraries that determine how a user will be
authenticated. For example, conventionally UNIX users authenticate themselves by supplying a
password at the password prompt after they have typed their name at the login prompt. In many
circumstances, such as internal access to workstations, this simple form of authentication is
considered sufficient. In other cases, more information is warranted. If a user wants to log in to an
internal system from an external source, like the Internet, more or alternative information may be
required − perhaps a one−time password. PAM provides this type of capability and much more. Most
important, PAM modules allow you to configure your environment with the necessary level of
security.
PATH
The shell looks for commands and programs in a list of file paths stored in the PATH environment
variable. An environment variable stores information in a place where other programs and commands
can access it. Environment variables store information such as the shell that you are using, your login
name, and your current working directory. To see a list of all the environment variables currently
defined; type 'set' at the prompt. When you type a command at the shell prompt, the shell will look for
that command's program file in each directory listed in the PATH variable, in order. The first program
found matching the command you typed will be run. If the command's program file is not in a
directory listed in you PATH environment variable, the shell returns a "commands not found" error.
By default, the shell does not look in your current working directory or your home directory for
commands This is really a security mechanism so that you don't execute programs by accident. What
if a malicious user put a harmful program called ls in your home directory? If you typed ls and the
shell looked for the fake program in your home directory before the real program in the /bin directory,
what do you think would happen? If you thought bad things, you are on the right track. Since your
PATH doesn't have the current directory as one of its search locations, programs in your current
Linux Filesystem Hierarchy
Glossary
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directory must be called with an absolute path of a relative path specified as './program−name'. To see
what directories are part of your PATH enter this command: # echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/bin/X11