SuSE and the File System Hierarchy Standard (FHS)
 

The root directory is the most important part of the whole system. During the boot process, this file system is mounted first and has to hold everything to bring the system to life. This requires certain things to be on this file system:
 

The subdirectory /bin contains commands that may be used by the system administrator, or users,and that request a single-user mode. It also contains commands that are used indirectly by scripts. There are no subdirectories in /bin. On a typical SuSE Linux system, /bin contains the commands shown in the following list.
 

arch            dd              gzip            netstat         su  
ash             df              hostname        nisdomainname   sync  
awk             dmesg           kill            passwd          tar  
basename        dnsdomainname   ksh             ping            tcsh  
bash            domainname      ln              ps              touch  
bash1           echo            login           pwd             true  
cat             egrep           ls              rm              umount  
chgrp           false           mail            rmdir           umssync  
chmod           fillup          mkdir           rpm             uname  
chown           fuser           mknod           sash            uncompress  
compress        gawk            mktemp          sed             ypdomainname  
cp              grep            more            sh              zcat  
csh             gtar            mount           sleep  
date            gunzip          mv              stty  

/boot contains everything that is required for the first stage of the boot process. The /boot directory stores data that is needed before the kernel begins executing user mode programs. It also holds backup copies of the boot records, sector map files, and other items that are not edited manually. Programs that access this data (mainly lilo) are located in /sbin, and their configuration files are in /etc.
 
On SuSE systems with release 6.0, the Linux kernel is also located in /boot. The Linux kernel makefile doesn't reflect this yet. The command make zlilo, which may be used to create a new kernel image and install it in the system, still places the kernel in the root directory. Placing it in /boot is a good idea on systems on which this directory is on a separate partition. In this case, the root file system may be located above the 1023 cylinder mark of the hard drive. Because of limitations in Intel PC BIOSes, the contents of /boot have to be below this mark in order to be accessible by the BIOS when the system is booted. If the whole root directory is below this mark, there is no need to have a separate partition for /boot, yet we highly recommend that you do, because the Linux Loader lilo won't work; it simply can't access the kernel image on the hard disc.
 

In spite of the fact that the FHS doesn't list the /cdrom directory, SuSE uses it to (temporarily) mount CD-ROMs. Every user on the system can do this by simply invoking the command mount /cdrom.

 
Device special files are located in /dev. We cover the most important facts about this directory earlier in Chapter 1.
 

The directory /etc is where all major configuration files are located. Applications that need more than one or two files have their own subdirectories in /etc. The kind of files found in /etc depends on the amount of software installed. Tables 2-5 , 2-6 , 2-7 , and 2-8 list some of most important files and directories in /etc along with a short description of their meaning.
 
 

The home directory of each individual user is located in /home. Depending on your system setup,you may want to sort users into several subdirectories within this hierarchy. For example, you could have /home/sales and /home/development as directories that put the homes of users relating to different departments in there.
 
SuSE creates home directories and places them in /home unless you instruct it to do otherwise.
 

The /lib directory contains the shared libraries needed to run the binaries in the root file system.It also contains the directories /lib/modules and /lib/YaST.
 
The /lib/modules directory contains the dynamic loadable modules for the Linux kernel. They are sorted by kernel version and functionality, as shown in Table 2-9 for kernel version 2.2.1.
 
 

The /mnt directory is provided for temporarily mounted file systems. It is not recommended that you use this directory for anything that is important to the system.

 
The /opt directory is reserved for installing add-on application software packages, such as Netscape, Applixware, KDE, and so on.
 
The directories /opt/bin, /opt/doc, /opt/include, /opt/info, /opt/lib, and /opt/man are reserved for local system administrator use. They are not created by default on SuSE systems. The directories /etc/opt and /var/opt are also not present in SuSE systems; however, they are recommended by the FHS for local configuration data and variable data needed by packages installed in the /opt hierarchy.
 
No other package files should exist outside the /opt, /var/opt, and /etc/opt hierarchies except for those package files that must reside in specific locations within the file system tree in order to function properly. For example, device lock files must be placed in /var/lock and devices must be located in /dev.
 

The traditional home directory for the root account is /. Many Linux systems still use the root directory for this purpose. However, there are good reasons to assign root its own directory.
 
Primarily, you want to keep as few files as possible in /, in order to keep it clean. Placing root's home into /home is not a good idea, because this directory may not exist on the root file system but may be mounted from another partition or even from another machine by using NFS. In case of a system crash or other emergency, root's home has to be on the root file system in order for the system administration to log on as root.
 
The FHS recommends that you use /root, so SuSE incorporated this issue and installed this directory to the superusers home directory.
 

The directory /sbin contains binaries essential for booting the system in addition to the binaries in /bin.
 
Users have read and execute permission for everything in /sbin except some setuid and setgid programs. The division between /bin and /sbin was not created for security reasons or to prevent users from seeing the operating system, but rather to provide a good partition between binaries that everyone uses and binaries that are used primarily for administration tasks. SuSE puts the commands shown in the following list into its /sbin directory.
 

SuSEconfig    dumpe2fs      insmod        modprobe    setserialbits  
YaST          e2fsck        ipfwadm       pack_cis    sfdisk  
activate      e2label       kbdrate       pcinitrd    shutdown 
agetty        fdisk         kerneld       pidof       sln  
badblocks     fsck          killall5      powerd      start-stop-daemon 
bg_backup     fsck.ext2     killproc      poweroff    startproc 
bg_restore    fsck.minix    ksyms         rarp        sulogin  
bootp         ftl_check     ldconfig      rcmysql     swapoff  
bootpc        ftl_format    lilo          rctab       swapon  
cardctl       genksyms      losetup       rdump       taper  
cardmgr       halt          lsmod         reboot      telinit  
cfdisk        hdparm        mingetty      restore     tune2fs  
checkproc     hwclock       mkdosfs       rmmod       udosctl  
conf.d        ide_info      mke2fs        rmt         umssetup 
ctrlaltdel    ifconfig      mkfs          route       umssync 
debugfs       ifport        mkfs.ext2     rrestore    update 
depmod        ifuser        mkfs.minix    runlevel    yast  
dump          init          mkfs.msdos    scsi_info   zast  
dump_cis      init.d        mkswap        setserial   

The /tmp directory is available for programs that require temporary files. Its permissions allow every user to write into this file system, yet no one can remove other users' files unless the owner grants permission.
 
Users should not assume that any files or directories in /tmp are preserved between invocations of the program.
 
The FHS added the recommendation that /tmp be cleaned at boot time on the basis of historical precedent and common practice, but it does not make it a requirement. SuSE does not implement any regular cleanup of /tmp in its system setup. The administrator has the option to add a corresponding command to the bootup scripts.

 
/usr is the second major section of the file system. The /usr hierarchy consists of shareable, read-only data, as you saw in Table 1-1. The /usr hierarchy should therefore be shareable between various hosts and should not be written to. Any information that is host specific or varies with time is stored elsewhere.
 
No large software packages should use a direct subdirectory under the /usr hierarchy. The only exception made by the FHS is for the X Window System because of considerable precedent and widely accepted practice. In contrast, SuSE installs some other third-party software in /usr. These are mostly commercial software packages that haven't adopted the FHS yet and are in /usr rather than /opt. Figure 2-2 shows the first level of the /usr hierarchy.
 

/usr/spool  -->  /var/spool  
/usr/tmp    -->  /var/tmp  
/usr/X11    -->  /usr/X11R6  
/usr/dict   -->  /usr/share/dict 

In the /usr/X11R6 hierarchy, you'll find the X Window System, version 11, release 6, and related files.
 
To simplify matters and make XFree86(TM) more compatible with the X Window System on other systems, the following symbolic links are present:
 

/usr/bin/X11      -->  /usr/X11R6/bin  
/usr/lib/X11      -->  /usr/X11R6/lib/X11  
/usr/include/X11  -->  /usr/X11R6/include/X11 

The /usr/bin directory is the primary directory of executable commands on the system. All binaries that don't belong to /bin, /sbin, or /usr/sbin can be found here.
 

The /usr/include directory holds all the system's general-use include files for the C and C++ programming languages. It contains several subdirectories for different compilers, languages, and libraries. See Figure 2-2. The subdirectory's presence strongly depends on the amount and kind of software you installed.
 

The /usr/lib directory contains object files, libraries, and internal binaries that are not intended to be executed directly by users or shell scripts.
 
All architecture-dependent data exclusively used by the application can be found in a single subdirectory of /usr/lib. For example, the perl5 subdirectory contains Perl5 modules and libraries. More examples can be seen in Figure 2-2. The figure should be interpreted as an example of what can be found in /usr/lib, because the subdirectories you find here depend on the software installed on the system. Traditionally, executable commands such as sendmail were placed in /usr/lib. The sendmail binaries are now placed by default in /usr/sbin, and a symbolic link remains to /usr/lib.
 

The /usr/local hierarchy is for use by the system administrator when installing software locally. It should be protected from overwriting when the system software is updated. It may be used for programs and data that are shareable among a group of hosts, but are not found in /usr.
 
The FHS requires that this directory always be empty after you first install the system. With SuSE Linux, two subdirectories are installed to /usr/local for system use. These are /usr/local/ftp and /usr/local/http, the document trees for the FTP and the HTTP (Web) server, respectively. Once again, the reasons for this are largely historical. The FHS does not provide recommendations as to where to place data such as this.
 
Other than the preceding requirements, the /usr/local hierarchy is free for use by the local system administrator to install software that doesn't come from the installation CDs. Beware that installing software into other parts of the file system may be affected by updates or patches of the distribution.
 

The /usr/sbin directory contains any nonessential binaries used exclusively by the system administrator. System administration programs that are required for system repair, system recovery, mounting /usr, or other essential functions are placed in /sbin. This directory also contains networking daemons, any nonessential administration tools, and binaries for noncritical server programs.
 
Locally installed system administration programs should be placed in /usr/local/sbin.
 

The /usr/share hierarchy is for all read-only architecture independent data files. The hierarchy is shareable among all architecture platforms of a given OS. For example, as soon as the Suse distribution is available for these processors, a site with i386, Alpha, and ARM platforms might maintain a single /usr/share directory that is centrally mounted. Figure 2-2 shows a typical /usr/share tree of a SuSE system.
 

The /usr/share/man subtree contains the manual pages. They are stored in a separate subdirectory for each section.
 

The /usr/src hierarchy contains the system source code. It typically contains the kernel source code in /usr/src/linux and the source of software packages in /usr/src/packages. Figure 2-3 shows this hierarchy.
 

The /var hierarchy contains variable data files. This includes spool directories and files, administrative and logging data, and transient and temporary files. Figure 2-4 shows the subdirectories in /var.
 

The /var/cache hierarchy is reserved for cached data from applications. Such data is locally generated as a result of time-consuming I/O or calculation. The application must be able to regenerate or restore the data. Unlike /var/spool, the cached files can be deleted without data loss. The data should remain valid between invocations of the application and rebooting the system.
 
Files located under /var/cache may be expired in an application-specific manner, by the system administrator, or both. The application should always be able to recover from manual deletion of files (generally because of a disk space shortage). No other requirements are made on the data format of the cache directories.
 

Lock files are stored within the /var/lock directory structure. These are (for example) device lock files, such as the serial device lock files. The naming convention used is LCK.., followed by the base name of the device. For example, to lock /dev/cua0, you would create the file /var/lock/LCK..cua0. The format used for device lock files is the HDB UUCP lock file format. The HDB format is to store the process identifier (PID) as a 10-byte ASCII decimal number, with a trailing newline. For example, if process 1230 holds a lock file, it contains the eleven characters: space, space,space, space, space, space, one, two, three, zero, and newline.
 
Then, anyone who wishes to use /dev/cua0 can read the lock file and act accordingly (all locks in /var/lock are world-readable). Some applications, such as Netscape, store private locks within their own data space.
 

The /var/log directory contains miscellaneous log files. Most logs are written to this directory oran appropriate subdirectory. Table 2-10 lists the most important log files contained in /var/log.  

The /spool/mail directory contains user mailbox files stored in the standard Unix mailbox format. Each user has a mailbox file in this directory. These files can be accessed by the users MUA (that is for example mutt, pine, or mailx), or by local daemons (like pop or imap) to deliver the mail to the user agent.
 
Following the terms of the FHS, these files should be in /var/mail. Because SuSE did not want to create incompatibilities with previous installations, it retained /var/spool/mail for mailboxes.
 

The /var/run directory contains system information files describing the system since it was booted. Files in this directory are cleared (removed or truncated as appropriate) at the beginning of the boot process.
 
Process identifier (PID) files are placed in /var/run. The naming convention for PID files is program-name.pid. For example, the crond PID file is named /var/run/crond.pid.
 
The file consists of the process identifier in ASCII-encoded decimal, followed by a newline character. For example, if crond was process number 25, /var/run/crond.pid will contain three characters: two, five, and newline.
 
The utmp file, which stores information about who is currently using the system, is also located in this directory.
 

The /var/spool directory contains data that is awaiting processing at a later date. Data in /var/spool represents work to be done in the future (by a program, user, or administrator); often, data is deleted after it has been processed. Table 2-11 lists some important subdirectories in /var/spool.
 
 

The /var/tmp directory is made available for programs that require temporary files or directories that are preserved between system reboots. Therefore, data stored in /var/tmp is more persistent than data in /tmp.
 
Files and directories located in /var/tmp are not deleted when the system is booted. The files in /tmp are also not deleted in SuSE systems. It is possible that they will be deleted in future releases.