D3 Tips and Tricks v4

Wednesday, 30 March 2016

Linux Directory Structure

The following post is a section of the book 'Just Enough Linux'.  The entire book can be downloaded in pdf format for free from Leanpub or you can read it online here.
Since this post is a snapshot in time. I recommend that you download a copy of the book which is updated frequently to improve and expand the content.

Linux Directory Structure

To a new user of Linux, the file structure may feel like something at best arcane and in some cases arbitrary. Of course this isn’t entirely the case and in spite of some distribution specific differences, there is a fairly well laid out hierarchy of directories and files with a good reason for being where they are.
We are frequently comfortable with the concept of navigating this structure using a graphical interface similar to that shown below, but to operate effectively at the command line we need to have a working knowledge of what goes where.
Linux Directories
The directories we are going to describe form a hierarchy similar to the following;
Directory Hierarchy
For a concise description of the directory functions check out the cheat sheet. Alternatively their function and descriptions are as follows;


The / or ‘root’ directory contains all other files and directories. It is important to note that this is not the root users home directory (although it used to be many years ago). The root user’s home directory is /root. Only the root user has write privileges for this directory.


The /bin directory contains common essential binary executables / commands for use by all users. For example: the commands cdcpls,pingps. These are commands that may be used by both the system administrator and by users, but which are required when no other filesystems are mounted.


The /boot directory contains the files needed to successfully start the computer during the boot process. As such the /boot directory contains information that is accessed before the Linux kernel begins running the programs and process that allow the operating system to function.


The /dev directory holds device files that represent physical devices attached to the computer such as hard drives, sound devices and communication ports as well as ‘logical’ devices such as a random number generator and /dev/null which will essentially discard any information sent to it. This directory holds a range of files that strongly reinforces the Linux precept that Everything is a file.


The /etc directory contains configuration files that control the operation of programs. It also contains scripts used to startup and shutdown individual programs.


The /etc/cron.d/etc/cron.hourly/etc/cron.daily/etc/cron.weekly/etc/cron.monthly directories contain scripts which are executed on a regular schedule by the crontab process.


The /rc0.d/rc1.d/rc2.d/rc3.d/rc4.d/rc5.d/rc6.d/rcS.d directories contain the files required to control system services and configure the mode of operation (runlevel) for the computer.


Because Linux is an operating system that is a ‘multi-user’ environment, each user requires a space to store information specific to them. This is done via the /home directory. For example, the user ‘pi’ would have /home/pi as their home directory.


The /lib directory contains shared library files that supports the executable files located under /bin and /sbin. It also holds the kernel modules (drivers) responsible for giving Linux a great deal of versatility to add or remove functionality as needs dictate.


The /lost+found directory will contain potentially recoverable data that might be produced if the file system undergoes an improper shut-down due to a crash or power failure. The data recovered is unlikely to be complete or undamaged, but in some circumstances it may hold useful information or pointers to the reason for the improper shut-down.


The /media directory is used as a directory to temporarily mount removable devices (for example, /media/cdrom or /media/cdrecorder). This is a relatively new development for Linux and comes as a result of a degree of historical confusion over where was best to mount these types of devices (/cdrom/mnt or /mnt/cdrom for example).


The /mnt directory is used as a generic mount point for filesystems or devices. Recent use of the directory is directing it towards it being used as a temporary mount point for system administrators, but there is a degree of historical variation that has resulted in different distributions doing things different ways (for example, Debian allocates /floppy and /cdrom as mount points while Redhat places them in/mnt/floppy and /mnt/cdrom respectively).


The /opt directory is used for the installation of third party or additional optional software that is not part of the default installation. Any applications installed in this area should be installed in such a way that it conforms to a reasonable structure and should not install files outside the /opt directory.


The /proc directory holds files that contain information about running processes and system resources. It can be described as a pseudo filesystem in the sense that it contains runtime system information, but not ‘real’ files in the normal sense of the word. For example the/proc/cpuinfo file which contains information about the computers cpus is listed as 0 bytes in length and yet if it is listed it will produce a description of the cpus in use.


The /root directory is the home directory of the System Administrator, or the ‘root’ user. This could be viewed as slightly confusing as all other users home directories are in the /home directory and there is already a directory referred to as the ‘root’ directory (/). However, rest assured that there is good reason for doing this (sometimes the /home directory could be mounted on a separate file system that has to be accessed as a remote share).


The /sbin directory is similar to the /bin directory in the sense that it holds binary executables / commands, but the ones in /sbin are essential to the working of the operating system and are identified as being those that the system administrator would use in maintaining the system. Examples of these commands are fdiskshutdownifconfig and modprobe.


The /srv directory is set aside to provide a location for storing data for specific services. The rationale behind using this directory is that processes or services which require a single location and directory hierarchy for data and scripts can have a consistent placement across systems.


The /tmp directory is set aside as a location where programs or users that require a temporary location for storing files or data can do so on the understanding that when a system is rebooted or shut down, this location is cleared and the contents deleted.


The /usr directory serves as a directory where user programs and data are stored and shared. This potential wide range of files and information can make the /usr directory fairly large and complex, so it contains several subdirectories that mirror those in the root (/) directory to make organisation more consistent.


The /usr/bin directory contains binary executable files for users. The distinction between /bin and /usr/bin is that /bin contains the essential commands required to operate the system even if no other file system is mounted and /usr/bin contains the programs that users will require to do normal tasks. For example; awkcurlphppython. If you can’t find a user binary under /bin, look under /usr/bin.


The /usr/lib directory is the equivalent of the /lib directory in that it contains shared library files that supports the executable files for users located under /usr/bin and /usr/sbin.


The /usr/local directory contains users programs that are installed locally from source code. It is placed here specifically to avoid being inadvertently overwritten if the system software is upgraded.


The /usr/sbin directory contains non-essential binary executables which are used by the system administrator. For example cron anduseradd. If you can’t locate a system binary in /usr/sbin, try /sbin.


The /var directory contains variable data files. These are files that are expected to grow under normal circumstances For example, log files or spool directories for printer queues.


The /var/lib directory holds dynamic state information that programs typically modify while they run. This can be used to preserve the state of an application between reboots or even to share state information between different instances of the same application.


The /var/log directory holds log files from a range of programs and services. Files in /var/log can often grow quite large and care should be taken to ensure that the size of the directory is managed appropriately. This can be done with the logrotate program.


The /var/spool directory contains what are called ‘spool’ files that contain data stored for later processing. For example, printers which will queue print jobs in a spool file for eventual printing and then deletion when the resource (the printer) becomes available.


The /var/tmp directory is a temporary store for data that needs to be held between reboots (unlike /tmp).

The post above (and heaps of other stuff) is in the book 'Just Enough Linux' that can be downloaded for free (or donate if you really want to :-)).

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