Learn how Linux file system works!
Linux System Admin Guide- What is Linux Operating System and how it works
Linux System Admin Guide- What are Directory Tree and Filesystem Hierarchy in Linux
Linux System Admin Guide- Introduction to Linux File Systems for System Admins
Linux System Admin Guide- Overview of Linux Virtual Memory and Disk Buffer Cache
Linux System Admin Guide- Best Practices for Monitoring Linux Systems
Linux System Admin Guide- Best Practices for Performing Linux Boots and Shutdowns
Linux System Admin Guide- Best Practices for Making and Managing Backup Operations
How Linux Operating System Memory Management works
Comprehensive Review of Linux Kernel Operating System Processes
Comprehensive Review of Linux File System Architecture and Management
What are mechanisms behind Linux Kernel task management
How Linux Kernel Sources and Functions work
Comprehensive look at how Linux Data Structures work
Introduction to Linux and Shell programming
Introduction to Linux System Administration
Comprehensive list of all Linux OS distributions
Comprehensive list of all special purpose Linux distributions
Comprehensive list of all secure Linux distributions for cybersecurity professionals
Contains variable data like system logging files, mail and printer spool directories, and transient and temporary files. Some portions of /var are not shareable between different systems. For instance, /var/log, /var/lock, and /var/run. Other portions may be shared, notably /var/mail, /var/cache/man, /var/cache/fonts, and /var/spool/news. Why not put it into /usr? Because there might be circumstances when you may want to mount /usr as read-only, e.g. if it is on a CD or on another computer. '/var' contains variable data, i.e. files and directories the system must be able to write to during operation, whereas /usr should only contain static data. Some directories can be put onto separate partitions or systems, e.g. for easier backups, due to network topology or security concerns. Other directories have to be on the root partition, because they are vital for the boot process. 'Mountable' directories are: '/home', '/mnt', '/tmp', '/usr' and '/var'. Essential for booting are: '/bin', '/boot', '/dev', '/etc', '/lib', '/proc' and '/sbin'.
If /var cannot be made a separate partition, it is often preferable to move / var out of the root partition and into the /usr partition. (This is sometimes done to reduce the size of the root partition or when space runs low in the root partition.) However, /var must not be linked to /usr because this makes separation of /usr and /var more difficult and is likely to create a naming conflict. Instead, link /var to /usr/var. Applications must generally not add directories to the top level of /var. Such directories should only be added if they have some system-wide implication, and in consultation with the FHS mailing list.
Directory containing backups of various key system files such as /etc/shadow, /etc/group, /etc/inetd.conf and dpkg.status. They are normally renamed to something like dpkg.status.0, group.bak, gshadow.bak, inetd.conf.bak, passwd.bak, shadow.bak
Is intended for cached data from applications. Such data is locally generated as a result of time-consuming I/O or calculation. This data can generally be regenerated or be restored. Unlike /var/spool, files here can be deleted without data loss. This data remains valid between invocations of the application and rebooting of the system. The existence of a separate directory for cached data allows system administrators to set different disk and backup policies from other directories in /var.
Locally-generated fonts. In particular, all of the fonts which are automatically generated by mktexpk must be located in appropriately-named subdirectories of /var/cache/ fonts.
A cache for man pages that are formatted on demand. The source for manual pages is usually stored in /usr/share/man/; some manual pages might come with a pre-formatted version, which is stored in /usr/share/man/cat* (this is fairly rare now). Other manual pages need to be formatted when they are first viewed; the formatted version is then stored in /var/man so that the next person to view the same page won't have to wait for it to be formatted (/var/catman is often cleaned in the same way temporary directories are cleaned).
Package specific cache data.
WWW proxy or cache data.
This directory will eventually hold system crash dumps. Currently, system crash dumps are not supported under Linux. However, development is already complete and is awaiting unification into the Linux kernel.
Data bank store.
Any variable data relating to games in /usr is placed here. It holds variable data that was previously found in /usr. Static data, such as help text, level descriptions, and so on, remains elsewhere though, such as in /usr/share/games. The separation of /var/games and /var/lib as in release FSSTND 1.2 allows local control of backup strategies, permissions, and disk usage, as well as allowing inter-host sharing and reducing clutter in /var/lib. Additionally, /var/games is the path traditionally used by BSD.
Holds dynamic data libraries/files like the rpm/dpkg database and game scores. Furthermore, this hierarchy holds state information pertaining to an application or the system. State information is data that programs modify while they run, and that pertains to one specific host. Users shouldn't ever need to modify files in /var/lib to configure a package's operation. State information is generally used to preserve the condition of an application (or a group of inter-related applications) between invocations and between different instances of the same application. An application (or a group of inter-related applications) use a subdirectory of /var/lib for their data. There is one subdirectory, /var/lib/misc, which is intended for state files that don't need a subdirectory; the other subdirectories should only be present if the application in question is included in the distribution. /var/lib/'name' is the location that must be used for all distribution packaging support. Different distributions may use different names, of course.
Variable data for local programs (i.e., programs that have been installed by the system administrator) that are installed in /usr/local (as opposed to a remotely mounted '/var' partition). Note that even locally installed programs should use the other /var directories if they are appropriate, e.g., /var/lock.
Many programs follow a convention to create a lock file in /var/lock to indicate that they are using a particular device or file. This directory holds those lock files (for some devices) and hopefully other programs will notice the lock file and won't attempt to use the device or file.
Lock files should be stored within the /var/lock directory structure. Lock files for devices and other resources shared by multiple applications, such as the serial device lock files that were originally found in either /usr/spool/locks or /usr/spool/uucp, must now be stored in /var/lock. The naming convention which must be used is LCK.. followed by the base name of the device file. For example, to lock /dev/ttyS0 the file LCK..ttyS0 would be created. The format used for the contents of such lock files must be the HDB UUCP lock file format. The HDB format is to store the process identifier (PID) as a ten byte ASCII decimal number, with a trailing newline. For example, if process 1230 holds a lock file, it would contain the eleven characters: space, space, space, space, space, space, one, two, three, zero, and newline.
Log files from the system and various programs/services, especially login (/var/log/wtmp, which logs all logins and logouts into the system) and syslog (/var/log/messages, where all kernel and system program message are usually stored). Files in /var/log can often grow indefinitely, and may require cleaning at regular intervals. Something that is now normally managed via log rotation utilities such as 'logrotate'. This utility also allows for the automatic rotation compression, removal and mailing of log files. Logrotate can be set to handle a log file daily, weekly, monthly or when the log file gets to a certain size. Normally, logrotate runs as a daily cron job. This is a good place to start troubleshooting general technical problems.
Record of all logins and logouts by normal users and system processes.
Log of all attempted bad logins to the system. Accessed via the lastb command.
Debugging output from various packages.
Kernel ring buffer. The content of this file is referred to by the dmesg command.
GDM log files. Normally a subset of the last X log file. See /var/log/xdm.log for mode details.
KDM log file. Normally a subset of the last X log file. See /var/log/xdm.log for more details.
Process accounting is the bookkeeping of process activity. The raw data of process activity is maintained here. Three commands can be used to access the contents of this file dump-acct, sa (summary of process accounting) and lastcomm (list the commands executed on the system).
Active user sessions. This is a data file and as such it can not be viewed normally. A human-readable form can be created via the dump-utmp command or through the w, who or users commands.
Log of all users who have logged into and out of the system. The last command can be used to access a human readable form of this file. It also lists every connection and run-level change.
XDM log file. Normally subset of the last X startup log and pretty much useless in light of the details the X logs is able to provide us with. Only consult this file if you have XDM specific issues otherwise just use the X logfile.
X startup logfile. An excellent resource for uncovering problems with X configuration. Log files are numbered according to when they were last used. For example, the last log file would be stored in /var/log/XFree86.0.log, the next /var/log/XFree86.9.log, so on and so forth.
The 'system' log file. The contents of this file is managed via the syslogd daemon which more often than not takes care of all log manipulation on most systems.
Contains user mailbox files. The mail files take the form /var/mail/'username' (Note that /var/mail may be a symbolic link to another directory). User mailbox files in this location are stored in the standard UNIX mailbox format. The reason for the location of this directory was to bring the FHS inline with nearly every UNIX implementation (it was previously located in /var/spool/mail). This change is important for inter-operability since a single /var/mail is often shared between multiple hosts and multiple UNIX implementations (despite NFS locking issues).
Variable data of the program packages in /opt must be installed in /var/opt/'package-name', where 'package-name' is the name of the subtree in /opt where the static data from an add-on software package is stored, except where superceded by another file in /etc. No structure is imposed on the internal arrangement of /var/opt/'package-name'.
Contains the process identification files (PIDs) of system services and other information about the system that is valid until the system is next booted. For example, /var/run/utmp contains information about users currently logged in.
Holds spool files, for instance for mail, news, and printing (lpd) and other queued work. Spool files store data to be processed after the job currently occupying a device is finished or the appropriate cron job is started. Each different spool has its own subdirectory below /var/spool, e.g., the cron tables are stored in /var/spool/cron/crontabs.
Temporary files that are large or that need to exist for a longer time than what is allowed for /tmp. (Although the system administrator might not allow very old files in /var/tmp either.)
Database for BIND. The Berkeley Internet Name Domain (BIND) implements an Internet domain name server. BIND is the most widely used name server software on the Internet, and is supported by the Internet Software Consortium, www.isc.org.
Database for NIS (Network Information Services). NIS is mostly used to let several machines in a network share the same account information (eg. /etc/passwd). NIS was formerly called Yellow Pages (YP).
The following directories, or symbolic links to directories, are required in /var for FSSTND compliance:
/var/cache Application cache data /var/lib Variable state information /var/local Variable data for /usr/local /var/lock Lock files /var/log Log files and directories /var/opt Variable data for /opt /var/run Data relevant to running processes /var/spool Application spool data /var/tmp Temporary files preserved between system reboots
Several directories are 'reserved' in the sense that they must not be used arbitrarily by some new application, since they would conflict with historical and/or local practice. They are:
/var/backups /var/cron /var/msgs /var/preserve
The following directories, or symbolic links to directories, must be in /var, if the corresponding subsystem is installed:
account Process accounting logs (optional) crash System crash dumps (optional) games Variable game data (optional) mail User mailbox files (optional) yp Network Information Service (NIS) database files (optional)
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