Unix file and directory organization

Note that there can be slight variations from one system to another.

You can read detailed information (source of most of the info on this page) about the FHS (Filesystem Hierarchy Standard) and/or FSSTND (Filesystem Standard) here:

http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/

The following sites contain further information on FHS:

Directories essential for booting: /bin, /boot, /dev, /etc, /lib, /proc, /sbin

/
/bin
/boot
/dev
/etc
/home
/lib
/lost+found
/media
/misc
/mnt
/net
/opt
/proc
/root
/sbin
/selinux
/srv
/sys
/tmp
/usr
var

`/`

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the root directory

the following directories are found in a Filesystem Standard (FSSTND v2.3 January 29, 2004) compliant root directory:

       /bin
       /boot      
       /dev       
       /etc       
       /lib       
       /media     
       /mnt       
       /opt       
       /sbin      
       /srv       
       /tmp       
       /usr       
       /var

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`/bin`

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essential command binaries for both system administrators and regular users.
binaries such as: bash, cp, grep, gzip, ls, tar

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`/boot`

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static files of the boot loader
GRUB directory
- used for dual-boot or multi-OS boot system setups
- GRUB will load a menu if it finds the menu.lst file (used for specifying which OS's are available to boot) here:
```
/boot/grub/menu.lst
```

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`/dev`

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device files

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`/etc`

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/etc is a very important directory, containing various configuration and initialization files.

configuration files as defined by tldp: "A 'configuration file' is defined as a local file used to control the operation of a program; it must be static and cannot be an executable binary."
configuration files found in this directory for programs such as: Apache, cron, rpm, yum, ssh, etc.
some interesting sub-directories in /etc:
- ```
/etc/X11
```
  configuration file directory for X Window System
- ```
/etc/cron.d
/etc/cron.daily
/etc/cron.weekly
/etc/cron.monthly
```
  contain scripts that the cron daemon executes on a temporal basis
- ```
/etc/httpd
```
  Apache configuration files
- ```
/etc/profile.d
```
  contains various shell scripts
- ```
/etc/rc.d
```
  files for control of system services and runlevels
  - ```
  /etc/rc.d/rc0.d
```
  specifies which services get started for runlevel 0
  - contains link files that point to various system service startup scripts in /etc/init.d or /etc/rc.d/init.d
  - all links start with either K or S:
    - K - this service will be killed if running, in this runlevel
    - S - this service will be started, in this runlevel
- ```
/etc/rc.d/rc1.d
```
    specifies which services get started for runlevel 1
  - ```
  /etc/rc.d/rc2.d
```
  specifies which services get started for runlevel 2
- ```
/etc/rc.d/rc3.d
```
    specifies which services get started for runlevel 3
  - ```
  /etc/rc.d/rc4.d
```
  specifies which services get started for runlevel 4
- ```
/etc/rc.d/rc5.d
```
    specifies which services get started for runlevel 5
  - ```
  /etc/rc.d/rc6.d
```
  specifies which services get started for runlevel 6
- ```
/etc/rc.d/init.d
```
  contains startup scripts for the daemons (or services) that are automatically started when the machine is booted
  - daemons such as: crond, denyhosts, httpd, ntpd, postfix, postgresql, sshd (see Daemons, Servers & Services WebHome for more info)
  - it is recommended that before you edit any scripts in this directory you should take a look at the man pages for: init, service, chkconfig, runlevel, inittab, and shutdown (see System Utilities WebHome for more info)

some interesting files in /etc:
- ```
/etc/bashrc
```
  system wide functions and aliases, determines the behavior of interactive shells
- ```
/etc/crontab
```
  cron configuration file
- ```
/etc/csh.cshrc
```
  csh configuration for all shell invocations
- ```
/etc/groups
```
  like /etc/passwd but for groups
- ```
/etc/inittab
```
  describes how the INIT process should set up the system in a certain run level
- ```
/etc/issue
```
  getty output prior to the login prompt - i.e. the system welcome message
- ```
/etc/mime.types
```
  listing of mime types and related extensions
- ```
/etc/motd
```
  "message of the day" - output message immediately after a successful login
- ```
/etc/passwd
```
  user database file with fields for username, home directory, other info for each user
- ```
/etc/php.ini
```
  php behavior settings file
- ```
/etc/profile
```
  system wide environment and startup programs
- ```
/etc/resolv.conf
```
  lists nameservers to use for host lookups
- ```
/etc/rsyslog.conf
```
  the main configuration file for the rsyslogd daemon which logs system messages - see man rsyslogd for more info
- ```
/etc/securetty
```
  identifies secure terminals from which root is allowed to log in. TLDP recommends that only virtual consoles are listed so that it is more difficult to gain root access via modem or network access.
- ```
/etc/shadow
```
  encrypted file which holds user passwords
- ```
/etc/shells
```
  list of trusted shells
- ```
/etc/services
```
  listing of internet services and ports used for each (i.e. ssh, http, ftp, etc.)
- ```
/etc/sudoers
```
  the sudoers file - lists users who can use sudo. See man sudoers for more info.

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`/home`

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user directories (but not root - see /root )

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`/lib`

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shared library images (C code library) needed to boot the system and run commands in the root filesystem (eg. /sbin and /bin)
kernel modules
```
/lib/modules
```

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`/lost+found`

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contains any recovered files after a system crash or power failure

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`/media`

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contains mount points for removable media (i.e. floppy, cdrom, etc.)

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`/misc`

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`/mnt`

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generic mount point where one can mount filesystems and/or devices

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`/net`

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`/opt`

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optional packages not part of default installation (applications like office, etc.) according to the 2004 standard. Our experience is that we see it rarely used.

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`/proc`

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files that provide status and statistics on system, kernel, and processes
it is a pseudo-filesystem which is used as an interface to kernel data structures
each running process has a directory in /proc that corresponds to it's PID from which you can extract info about the process - see man proc for more info

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`/root`

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root's home directory

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`/sbin`

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system programs (for system administration, etc.)
FSSTND states /sbin should contain only binaries essential for booting, restoring, recovering and/or repairing the system in addition to the binaries in /bin
programs like: service, ip, init, chkconfig
some "less important" system/admin programs can be found in /usr/sbin
locally installed system admin programs should be placed in /usr/local/sbin

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`/selinux`

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`/srv`

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site-specific data

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`/sys`

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`/tmp`

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temporary files
files in this directory should not be assumed to survive a restart or even invocations of any program

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`/usr`

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contains all user binaries, their documentation, libraries, header files, etc. - contains only static data (as opposed to /var which contains variable data)
some interesting subdirectories in /usr :
- ```
/usr/bin
```
  contains most binaries on the system (i.e. diff, emacs, gzip, g++, svn, etc.)
- ```
/usr/include
```
  header files for C programming language and header files used by various programs on the system
- ```
/usr/lib
```
  program libraries (collections of frequently used routines) - library of unchanging data files for programs
- ```
/usr/local
```
  locally installed software - reserved for the use of the local admin(s).
- ```
/usr/sbin
```
  system admin commands not needed on the root filesystem (i.e. most server programs such as: chrond, httpd, sshd, etc.)
- ```
/usr/share/doc
```
  miscellaneous documentation files
- ```
/usr/share/info
```
  GNU info documents
- ```
/usr/share/man
```
  manual pages
- ```
/usr/src
```
  a linux or Redhat or some such similar subdirectory contains the Linux kernel source, header files and docs
- ```
/usr/X11R6
```
  files for the X Window System

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`var`

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variable data dynamic files, such as: system log files, mail/printer spool directories, etc.
some interesting subdirectories or files:
- ```
/var/cache
```
  cached data from applications (usually stored from time-consuming I/O or calculations)
- ```
/var/lib
```
  application dynamic data libraries/files
- ```
/var/local
```
  variable data for local programs installed in /usr/local
- ```
/var/lock
```
  lock files for devices being used by a program
- ```
/var/log
```
  log files
  - ```
  /var/log/auth.log
```
  record of all logins and logouts by users and system processes
- ```
/var/log/btmp
```
    log of all attempted bad logins to the system (use lastb command)
  - ```
  /var/log/cron
```
  cron messages
- ```
/var/log/httpd
```
    apache server messages
    - ```
    /var/log/httpd/access_log
```
  log of accesses to the apache server
- ```
/var/log/httpd/error_log
```
      log of errors, very useful for debugging server operation
    - ```
    /var/log/httpd/rewrite_log
```
  helpful with debuggin mod_rewrite issues
- ```
/var/log/httpd/ssl_access_log
```
    - ```
    /var/log/httpd/ssl_error_log
```
- ```
/var/log/httpd/ssl_rewrite_log
```
  - ```
  /var/log/messages
```
  system logs
- ```
/var/mail
```
  user mailbox files
- ```
/var/named
```
  database for BIND (Berkeley Internet Name Domain), an Internet domain name server
- ```
/var/opt
```
  data of application installed in /opt (i.e. optional packages not part of the default installation)
- ```
/var/spool
```
  spool files for mail, news, printing (lpd), etc.
- ```
/var/tmp
```
  temporary files - can be held for longer period of time than in /tmp
- ```
/var/log/wtmp
```
  log of all users who logged into and out of the system (use last command)
- ```
/var/yp
```
  database for NIS (Network Information Services)

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-- RobertSajan - 20 May 2008