Environment Variables
Environment variables allow storage of global settings for the shell or various other
programs. They are contextual but inheritable. For example, each process has its
own set of environment variables (they are contextual). Shells, like login shells, can
declare variables, which will be passed down to other programs they execute (they
are inheritable).
These variables can be defined system-wide in
/etc/profile
or per-user in
~/
.profile
but variables that are not specific to command line interpreters are better
put in
/etc/environment
, since those variables will be injected into all user sessions
thanks to a Pluggable Authentication Module (PAM) – even when no shell is executed.
3.3. The File System
3.3.1. The Filesystem Hierarchy Standard
As with other Linux distributions, Kali Linux is organized to be consistent with the Filesystem Hi-
erarchy Standard (FHS), allowing users of other Linux distributions to easily find their way around
Kali. The FHS defines the purpose of each directory. The top-level directories are described as
follows.
•
/bin/
: basic programs
•
/boot/
: Kali Linux kernel and other files required for its early boot process
•
/dev/
: device files
•
/etc/
: configuration files
•
/home/
: user’s personal files
•
/lib/
: basic libraries
•
/media/
: mount points for removable devices (CD/DVD-ROM, USB keys, and so on)
•
/mnt/
: temporary mount point
•
/opt/
: extra applications provided by third parties
•
/root/
: administrator’s (root’s) personal files
•
/run/
: volatile runtime data that does not persist across reboots (not yet included in the
FHS)
54
Kali Linux Revealed
•
/sbin/
: system programs
•
/srv/
: data used by servers hosted on this system
•
/tmp/
: temporary files (this directory is often emptied at boot)
•
/usr/
: applications (this directory is further subdivided into
bin
,
sbin
,
lib
according to
the same logic as in the root directory) Furthermore,
/usr/share/
contains architecture-
independent data. The
/usr/local/
directory is meant to be used by the administrator for
installing applications manually without overwriting files handled by the packaging system
(
dpkg
).
•
/var/
: variable data handled by services. This includes log files, queues, spools, and caches.
•
/proc/
and
/sys/
are specific to the Linux kernel (and not part of the FHS). They are used
by the kernel for exporting data to user space.
3.3.2. The User’s Home Directory
The contents of a user’s home directory are not standardized but there are still a few noteworthy
conventions. One is that a user’s home directory is often referred to by a tilde (“~”). That is useful
to know because command interpreters automatically replace a tilde with the correct directory
(which is stored in the HOME environment variable, and whose usual value is
/home/
user
/
).
Traditionally, application configuration files are often stored directly under your home directory,
but the filenames usually start with a dot (for instance, the
mutt
email client stores its configura-
tion in
~/.muttrc
). Note that filenames that start with a dot are hidden by default; the
ls
com-
mand only lists them when the -a option is used and graphical file managers need to be explicitly
configured to display hidden files.
Some programs also use multiple configuration files organized in one directory (for instance,
~/
.ssh/
). Some applications (such as the Firefox web browser) also use their directory to store a
cache of downloaded data. This means that those directories can end up consuming a lot of disk
space.
These configuration files stored directly in your home directory, often collectively referred to
as dotfiles, have long proliferated to the point that these directories can be quite cluttered with
them. Fortunately, an effort led collectively under the
FreeDesktop.org
2
umbrella has resulted
in the XDG Base Directory Specification, a convention that aims at cleaning up these files and
directories. This specification states that configuration files should be stored under
~/.config
,
cache files under
~/.cache
, and application data files under
~/.local
(or subdirectories thereof).
This convention is slowly gaining traction.
Graphical desktops usually have shortcuts to display the contents of the
~/Desktop/
directory (or
whatever the appropriate translation is for systems not configured in English).
2
https://www.freedesktop.org/
55
Chapter 3 — Linux Fundamentals
Finally, the email system sometimes stores incoming emails into a
~/Mail/
directory.
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