Some useful configuration files and directories in linux

Below are useful file in Linux. Every DBA must aware for daily operation.

File Name	Description
/etc/passwd	/etc/passwd file is used to keep track of every registered user that has access to a system. File that contains the following information: User name. Encrypted password. User ID number (UID)
/etc/hosts	hosts file is an operating system file that maps hostnames to IP addresses.
/etc/group	The /etc/group file contains basic group attributes. This is an ASCII file that contains records for system groups. Each record appears on a single line and is the following format: Name:Password:ID:User1,User2,...,Usern
/etc/issue	/etc/issue is a text file which contains a message or system identification to be printed before the login prompt
/etc/resolv.conf	The /etc/resolv.conf is resolver configuration file for Linux and UNIX like operating systems.The file /etc/resolv.conf file contains information that is read by the resolver routines the first time they are invoked by a process.
/etc/yum.conf	The main configuration file for yum is /etc/yum.conf.The /etc/yum.conf configuration file contains exactly one [main] section. You can add many additional options under the [main] section heading in /etc/yum.conf.
/var/log/yum.log	if you install packages using the yum command, this log stores all related information, which can be useful in determining whether a package and all components were correctly installed.
/etc/named.conf	The /etc/named.conf file is the configuration file for named.The /etc/named.conf file establishes the server as a master, slave, or cache-only name server.
/etc/exports	The /etc/exports file controls which file systems are exported to remote hosts and specifies options.The file /etc/exports contains a table of local physical file systems on an NFS server that are accessible to NFS clients. The contents of the file are maintained by the server's system administrator.
/etc/sudo.conf	/etc/sudo.conf configuration file are allowed to use the sudo command and the command is executed in the user's shell, not a root shell.
/etc/cups/snmp.conf	The snmp.conf file configures how the standard CUPS network backends (http, https, ipp, ipps, lpd, snmp, and socket) access printer information using SNMPv1 and is normally located in the /etc/cups directory.
/etc/cups/cupsd.conf	The /etc/cups/cupsd.conf file contains configuration directives that control how the server functions.
/etc/cups/subscriptions.conf	The subscriptions.conf file defines the local event notification        subscriptions that are active.
/etc/cups/printers.conf	/etc/cups/printers.conf file to store the list of available printers. This is the printer configuration file with entries for each print queue.
/etc/cups/client.conf	The client.conf file configures the CUPS client and is normally located in the /etc/cups or ~/.cups directory.
/etc/oracle-release	This shows you any files that might give a hint on the operating system version.
/etc/oraInst.loc	The Oracle inventory directory is used by the installer to keep track of all Oracle products installed on the computer. The inventory directory is stored in a file called oraInst.loc.
/etc/ntp.conf	The NTP program is configured using either the /etc/ntp.conf
/etc/sysctl.conf	As the /etc/sysctl.conf file is used to override default kernel parameter values  and therefore only contains a few of the possible parameters.
/home/.bash_history	The bash history is a log file that contains all commands that the user executed on the Linux shell.
/home/.bash_profile	The ~/.bash_profile file is a configuration file for configuring user environments. The users can modify the default settings and add any extra configurations in it.
/home/.bash_logout	The ~/.bash_logout file contains instructions for the logout procedure.
/home/.bash_login	The ~/.bash_login file contains specific settings that are executed when a user logs in to the system.
/var/log/maillog	Stores all logs related to mail servers
/var/log/messages
/var/log/cron	Stores all Crond-related messages (cron jobs), such as when the cron daemon initiated a job, related failure messages, etc.
/etc/shadow
/var/log/kern	Stores Kernel logs and warning data. This log is valuable for troubleshooting custom kernels as well.
/var/log/dmesg	Messages relating to device drivers. The command dmesg can be used to view messages in this file.
/var/log/faillog	Contains information all failed login attempts, which is useful for gaining insights on attempted security breaches, such as those attempting to hack login credentials as well as brute-force attacks.
/var/log/httpd/	A directory containing error_log and access_log files of the Apache httpd daemon
/var/log/boot.log	A repository of all information related to booting and any messages logged during startup.
/var/log/syslog or /var/log/messages	General messages, as well as system-related information. Essentially, this log stores all activity data across the global system.

List of useful directories:-

Directory	Description
/opt	According to the Filesystem Hierarchy Standard, /opt is for "the installation of add-on application software packages
/bin	All binaries needed for the boot process and to run the system in single-user mode, including essential commands such as cd, ls, etc.
/boot	contains files used for system startup including the kernel
/dev	Contains device files for all hardware devices on the system
/etc	Files used by application subsystems such as mail, the Oracle database, etc.
/etc/init.d	Contains various service startup scripts
/etc/profile.d	Holds application setup scripts run by /etc/profile upon login
/etc/rc.d	Contains subdirectories which contain run level specific scripts
/etc/rc.d/init.d	run level initialization scripts
/etc/rc.d/rc.d	Where ‘?’ is a number corresponding to the default run level. Contains symbolic links to scripts which are in /etc/rc.d/init.d. for services to be started and stopped at the indicated run level.
/etc/skel	Holds example dot files used to populate a new user’s home directory.
/etc/X11	Contains subdirectories and configuration files for the X Window system
/home	User home directories
/lib	Some shared library directories, files, and links
/mnt	The typical mount point for the user-mountable devices such as floppy drives and CDROM
/proc	Virtual file system that provides system statistics.  It doesn’t contain real files but provides an interface to runtime system information.
/sbin	Commands used by the super user for system administrative functions
/tmp	A standard repository for temporary files created by applications and users.
/usr	Directory contains subdirectories with source code, programs, libraries, documentation, etc.
/usr/bin	Contains commands available to normal users
/usr/bin/X11	X Window system  binaries
/usr/include	Holds include files used in C programs
/usr/share	Contains shared directories for man files, info files, etc.
/usr/lib	Library files searched by the linker when programs are compiled
/usr/local/bin	Common executable application files local to this system
/usr/sbin	Commands used by the super user for system administrative functions
/var	Administrative files such as log files, locks, spool files, and temporary files used by various utilities
/root	This is the home directory of root user and should never be confused with ‘/‘
/run	This directory is the only clean solution for early-runtime-dir problem.
/sys	Modern Linux distributions include a /sys directory as a virtual filesystem, which stores and allows modification of the devices connected to the system.