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Linux Filesystem Structure

There is a certain way that the root filesystem should be structured on a Linux system.

These notes are about the purpose of the various directories on a Linux system.

Main source of information is the Filesystem Hierarchy Specification (FHS) from the Linux Foundation.

/ (root)

The root directory contains everything.
All files on a system are in either root or a subdirectory of root.

/dev

The /dev directory contains device files.

These are special files that are either character special files, block special files, or pipe special files.

This is where you'd find devices like keyboards and mice, TTYs (pseudo-terminals), and other special files that are meant to store, retrieve, or generate data.

A few examples:

  • /dev/null: A character special file used to discard data.
  • /dev/zero: A character special file used to generate zeroes indefinitely.
  • /dev/random: A character special file used to generate random values.

The list goes on.

/bin

The /bin (binaries) directory stores essential user command binaries, which should be available to all users.

The /bin directory has no subdirectories.

There are tools that are required to be in /bin, as specified by the Linux Foundation.

Command Description
cat Utility to concatenate files to standard output
chgrp Utility to change file group ownership
chmod Utility to change file access permissions
chown Utility to change file owner and group
cp Utility to copy files and directories
date Utility to print or set the system data and time
dd Utility to convert and copy a file
df Utility to report filesystem disk space usage
dmesg Utility to print or control the kernel message buffer
echo Utility to display a line of text
false Utility to do nothing, unsuccessfully
hostname Utility to show or set the system's host name
kill Utility to send signals to processes
ln Utility to make links between files
login Utility to begin a session on the system
ls Utility to list directory contents
mkdir Utility to make directories
mknod Utility to make block or character special files
more Utility to page through text
mount Utility to mount a filesystem
mv Utility to move/rename files
ps Utility to report process status
pwd Utility to print name of current working directory
rm Utility to remove files or directories
rmdir Utility to remove empty directories
sed The `sed' stream editor
sh POSIX compatible command shell
stty Utility to change and print terminal line settings
su Utility to change user ID
sync Utility to flush filesystem buffers
true Utility to do nothing, successfully
umount Utility to unmount file systems
uname Utility to print system information

These can be symlinked to a different location as long as they're linked to the correct command.

If these programs exist on the system, they also must be placed in /bin:

Command Description
csh The C shell
ed The `ed' editor
tar The tar archiving utility
cpio The cpio archiving utility
gzip The GNU compression utility
gunzip The GNU uncompression utility
zcat The GNU uncompression utility
netstat The network statistics utility
ping The ICMP network test utility

These are all optional.

/usr

The /usr directory contains all the executables and libraries that the user should have access to.

This directory is not required by the Linux Foundation's File Hierarchy Specification.

/usr will contain a few subdirectories:

  • /usr/bin: Basic user-executable binaries (like /bin)
  • /usr/sbin: More binaries for admins.
  • /usr/lib: The system libraries needed for binaires.

/usr/sbin

The /sbin (usually symlinked to /usr/sbin) directory stores binaries that are used by administrators.

/opt

The /opt (optional) directory is usually used for self-contained third-party applications.

Self-contained meaning that the application's files aren't spread across the rest of the filesystem (e.g., no files in /etc, /var, or other system dirs).

/var

This contains variable data files.
Files that are subject to change.

The files in this directory are meant to be persistent.

/var/lib

This is where applications' essential libraries are stored.

/boot

The /boot directory contains the essential files required during the operating system's boot process.

It contains everything required for the boot process with the exception of config files that aren't needed at boot time or needed by the map installer.

The config files not needed at boot time go in /etc.

Includes:

  • Static files of the bootloader (e.g., GRUB).

    • All the binaires needed by the bootloader to boot a file go in /sbin (system binaries directory, usually symlinked to /usr/sbin).

  • The kernel (the kernel can alternatively sometimes be located in the / [root] directory).

  • The initrd image (initial RAM disk image), used for loading kernel modules needed at boot time.

These files are used before the kernel can access system files.

/etc

The /etc (et cetera) directory contains primarily configuration files.

There should be no binaries here. There can be scripts, though.

There are some subdirectories that should be here (only the first one is mandatory):

  • /etc/opt: Stores config files for /opt.
  • /etc/X11: Stores config files for the X window manager.
    • Optional.
  • /etc/sgml: Stores config files for SGML (Standard Generalized Markup Language).
    • Optional.
  • /etc/xml: Stores config files for XML (eXtensible Markup Language).
    • Optional.

This is also where you'd find config files for system daemons, like the SSH daemon (in /etc/ssh).

/srv

The /srv (service) directory is not on all Linux machines by default.
This directory is used for data that is being served by the system (e.g., Samba, NFS).

/home

This stores the home directory for users.

This directory doesn't have to be on a system per the FHS. User account home directories can be stored elsewhere.

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