A Quick Tour Of the Linux Filesystem

LinuxSA - June 1997 Meeting
Matthew Tippett
(mtippett@tne.net.au)

Introduction

• The filesystem standardised in the FSSTND:
  • Why standardise?
  • What the FSSTND covers

• What files are classified as:
  • Static and variable
  • Shareable and unshareable

• Where files are located:
  • Configuration files
  • Administrator binaries
  • User binaries
  • System libraries
  • /usr hierachy
  • /var hierachy

Why Standardise?

• No Standards in the Variants of Unix
  • No standard layout
  • Historical layouts
    • BSD
    • System V
    • SunOS

• Lack of regularity
  • Difficult for newcomers to Linux
    • especially from non-UNIX backgrounds
  • Experienced UNIX users found navigating difficult

• Incompatibilities for developers and users
  • Symbolic link workarounds becoming the norm
  • Movements between distributions very difficult

The Linux File System Structure (FSSTND)

• Developed through consultation with Linux developers, administrators, programmers and users

• Defines locations for various files
  • /sbin directories
  • /bin directories
  • /etc directories
  • /tmp directories

• Defines uses of sections of the filesystem
  • Root partition
  • /usr partition
  • /var partition

• Provides guidelines on where files should be placed
  • Do I place this in /usr/sbin or /usr/bin?

The Filesystem

• The UNIX filesystem
  • Hierachical structure
  • Consistent treatment of data
  • Protection of file data

• Orthogonal categories of files
  • Shareable vs Unshareable
  • Variable vs Static

• Examples applying these categories

  	Shareable	Unshareable
  Static	/usr	/etc /boot
  Variable	/var/spool/mail /var/spool/news	/var/run /var/lock

Shareable vs Unshareable

• Distinction allows more efficient use of resources

• Shareable data
  • Can be shared between multiple machines
  • Not machine dependant
  • Examples:
    • Common post-boot binaries (/usr)
    • News and mail spool directories

• Unshareable Data
  • Data useful to local machine only
  • Data meaningless on another machine
  • Can be shared on a per-machine basis
  • Examples:
    • Local configuration information (/etc)
      
    • Lock and pid files (/var/run and /var/lock)
      

Static vs Variable

• Distinction allows protection and partioning of data
  • Administrator can make decision about what to mount read/write and what to mount read-only

• Static data
  • Won't change without super-user interaction
  • Doesn't need write access
  • Examples:
    • /usr hierachy
      
    • /etc hierachy
      

• Variable Data
  • Changes with user interaction
  • Changes due to system operation
  • Definately needs write access
  • Examples:
    • /tmp
    • /var hierachy
    • /home hierachy
    • Local spool files (mail and news)

The Root Directory

• The base of the system, enough to:
  • Boot the system
  • Mount /usr and other parts of the filesystem
  • Repair a corrupt filesystem
  • Recover data from system backups

• Issues about the root filesystem
  • Small size less prone to errors in system crash
  • Some programs may be statically linked
  • Has a number of sub-directories
  • No new root directory entries needed
  • Contains generally unshareable data
  • Contains both variable and static data

The Root Directory (cont.)

/bin
• Essential user binaries

/boot

• Static boot files

/dev

• Entries for the device special files

/etc

• Configuration files and directories
• /etc/skel & /etc/X11

/home

• User home directories

The Root Directory (cont.)

/lib
• Essential shared libraries
• Kernel modules

/proc

• Kernel information
• Process information

/sbin

• Essential system binaries
• Binaries for root and power users
• Historically in /etc

/tmp

• Storage of temporary files
• Data may be cleared out frequently

The /usr Hierachy

• A major part of filesystem, containing only:
  • Shareable data
  • Read-only (static) data
  • Non-essential (post-boot) files

• Issues about the /usr Hierachy
  • X Window System sits in /usr
  • No other large package should be placed directly in /usr
  • /usr can be shared between multiple machines
  • A small number of symbolic links to /var permissible for compatibility

The /usr Hierachy (cont.)

/usr/X11R6 and /usr/X386
• X Window System, version 11 release 6 and 5 respectively
• On /usr for compatibility only

/usr/bin

• Most user binaries

/usr/doc

• Miscellaneous documentation

/usr/etc

• Network wide configuration files

/usr/games

• Games and "Educational" binaries

/usr/include

• Headers for C programs

The /usr Hierachy (cont.)

/usr/lib
• Libraries
• Package Directories

/usr/local

• Locally installed packages

/usr/man

• Online manual pages

/usr/sbin

• System administration binaries

/usr/share

• Architecture-independent data

/usr/src

• Source code

The /var Hierachy

• A major part of filesystem, containing:
  • A mixture of shareable and unshareable data
  • Mostly variable data

• Issues about the /var Hierachy
  • Exists primarily to allow a read-only /usr
  • Some portions of /var are used network wide
  • Other portions definately local information only
  • May be a separate partition, or placed on / or /usr/var

The /var Hierachy (cont.)

/var/adm
• Obsolete, link to /var/log or remove

/var/catman

• Locally formatted manual pages

/var/lib

• Application state files for /usr/lib files

/var/local

• Variable storage for /usr/local

/var/lock

• Various lock files

/var/log

• System log files

The /var Hierachy (cont.)

/var/named & /var/nis
• DNS and NIS naming services files

/var/preserve

• Crash storage for vi and ed

/var/run

• Information (pid, utmp) on the active system

/var/spool

• Used for spooling to UNIX subsystems (mail, news, at, lp)

/var/tmp

• Temporary storage, with longer "life"

Conclusion

• Where to look for programs and data

• Where packages should be installed

• Where and how partitions should be mounted

• Demystified why things work sometimes, and not others

More Information

• Linux Filesystem Structure (FSSTND) v1.2

• Soon to be released FHS

Questions