ETC is a folder which contain all your system configuration files in it. Then why the etc name?

“etc” is an English word which means etcetera i.e in layman words it is “and so on”. The naming convention of this folder is having some interesting history.

The history of /etc folder in Linux/Unix

In initial days of UNIX OS development there is a folder for each type of data like /bin folder for all your executable binaries
/boot folder for all booting related information.
/dev folder for all hardware devices attached to machine.
But people encountered a situation to keep some files which can be a config file or a data file or a socket file or some other files. So they implemented a folder to keep all these files in it and they named it as /etc(As said earlier etcetera). As time passed the meaning of this folder has changed but not the name “etc”. Now /etc folder means a central location for all your configuration files are located and this can be treated as nerve centre of your Linux/Unix machine.

Coming back to our /etc folder explanation, one post on this is not sufficient and the number files/folder in this directory will depend on the applications you install in it. Below are some list of files/folders commonly installed in most of Linux/Unix machines. I thought of posting this long time back almost three years back along with /bin, /boot, /dev but did not find a good way to approach this folder as number of files/folders are huge in number. Today I come up with a solution which I felt its bit easy for people who are new to Linux can understand without much fuss.

/etc folder content can be divided into following groups

1)Configuration tables(tab like crontab, fstab, mtab etc)
2)Running Configurations(rc like rc.local, rc1.d, bashrc, wgetrc etc)
3)Configuration files(conf/cfg like pam.conf, ntp.conf etc)
4)Deny/Allow files()
5)Other directories
6)Other Files

Please note that this is a special post which I will modify continuously based on readers inputs.

Table files (The ***TAB) in /etc folder in Linux

These are the files which end with tab or start with tab(which is short form of table) such as crontab, fstab, inittab etc. These files main intention is to keep a table of content for their corresponding settings. Below are some of the files which fall under this category.

/etc/crontab –For doing scheduling
/etc/inittab –Contains what are the run-levels, what system have to do at each run-level and default run-level info.
/etc/fstab –Contains file system mounting information and we can edit this file to mount file system permanently and delete mount points.
/etc/mtab –Contains all mounted device status like what devices are mounted, where they mounted and properties of mount points.
/etc/quotatab –Quota related table for mentioning different qutos
/etc/rwtab –When your computer comes back up, the root and any other system partitions will be mounted read-only. All the files and directories listed in /etc/rwtab will be mounted read-write on a tmpfs file system. You can add additional files and directories to rwtab to make them writeable after reboot.

Some other table files are as below.

/etc/anacrontab
/etc/crypttab
/etc/statetab
/etc/rwtab
/etc/mtab

Running Configuration files(rc) in /etc folder

These are again a type of config files which will force a service to start/stop, for a user to use specific environment etc.

/etc/bash.bashrc –System wide bash shell running configuration file.
/etc/inputrc — Global inputrc for libreadline
/etc/nanorc –Nano editor running configurations
rc0.d, rc1.d, rc2.d, rc3.d, rc4.d, rc5.d, rc6.d, rcS.d –running configurations like what services has to start and stop for each run level from zero to six. If you observe rcS.d this folder is same as rc1.d folder which means Single run level.
/etc/rc.local –This file contains commands which need to be executed after completing booting. we can edit this file and keep the commands which we want to execute at the time of booting.
vimrc –VIEW Improved editor system level configuration is stored here.
wgetrc –wget related running configurations.

Other RC files:

csh.cshrc
rc
rc.sysinit
nanorc
vimrc
virc

Config files in /etc folder

These are the main configuration files for many applications installed in your machine. These files are text files which can be edited with editors like VI editor so that applications works according to your requirement.

/etc/asound.conf –Configuration file for your speakers and audio devices.
/etc/dnsmasq.conf –DNS client related configuration file
/etc/exports –NFS share configuration file. In this file we will share our local folder to a specific IP address or network with different.
/etc/grub.conf –Main configuration file for grub boot loader
/etc/shadow –User login password hashes are stored here.
/etc/group –User groups information such as which user belongs to which group, what is GID etc.
/etc/passwd –User configurations such as user login name, shell, UID and GID are stored here.
/etc/my.cnf –Mysql configuration file.
/etc/mdadm.conf –Raid configuration file.
/etc/hosts –Hosts to IP address mapping file, This is mother of all name to IP matching files.
/etc/resolv.conf –DNS and domain client configuration file. With out proper DNS server in this file, we can not access internet.
/etc/rsyslog.conf –Remote syslog server configuration file.
/etc/securetty –Root login control file.
/etc/sestatus.conf –SELinux configuration file.

Other configuration files are as below

kdump.conf
krb5.conf
logrotate.conf
man.config
nsswitch.conf
ntp.conf
smartd.conf
sysctl.conf
yum.conf

Deny/Allow files in /etc folder

These are the files which contain what users are allowed/denied for particular service, what IP address are allowed/denied to access our services etc.

/etc/at.deny –Keep users in this file to deny at jobs execution.
/etc/at.allow –Keep users in this file to allow at jobs execution.
/etc/hosts.allow –TCP wrapper files for allowing services for a particular host/network.
/etc/hosts.deny –TCP wrapper files for allowing services for a particular host/network.

Directories in /etc folder

abrt/ -Useful for automatic bug reporting tool used by your vendor for reporting bugs.

acpi/ -The Advanced Configuration and Power Interface (ACPI) specification provides an open standard for device configuration and power management. This folder contain configuration files related to ACPI

alsa/ -The Advanced Linux Sound Architecture (ALSA) provides audio and MIDI functionality to the Linux operating system.
alternatives/ –A required folder/command to change default applications. Suppose you want to use emacs editor instead of vi editor then this folder contain those details.

cron.d/, cron.daily/, cron.deny, cron.hourly/, cron.monthly/, crontab, cron.weekly/ –Crontab related files and folders for effective management for scheduling in Linux.

default/ –Some of the common configuration files are located here.

Ok that’s enough, How many folders and files I should learn like this?

I want easiest and detailed way to learn this stuff. Is there any way to do that?

Yes, man pages will come for your rescue. Suppose you want to know more about passwd file then execute following command

man 5 passwd

aliases and aliases.db — Files which are related to Sendmail and postfix mail servers to mention alias

Other folders:

 

/etc/audit/
/etc/avahi/
/etc/bluetooth/
/etc/chkconfig.d/
/etc/cups/
/etc/dhcp/
/etc/fonts/
/etc/hal/
/etc/httpd/
/etc/init/
/etc/init.d/
/etc/logrotate.d/
/etc/modprobe.d/
/etc/ntp/
/etc/pam.d/
/etc/postfix/
/etc/ppp/
/etc/profile.d/
/etc/pulse/
/etc/rpm/
/etc/rwtab.d/
/etc/security/
/etc/selinux/
/etc/setuptool.d/
/etc/skel/
/etc/snmp/
/etc/sound/
/etc/ssh/
/etc/ssl/
/etc/statetab.d/
/etc/sysconfig/
/etc/terminfo/
/etc/X11/
/etc/xinetd.d/
/etc/yum/
/etc/yum.repos.d/

Other files in /etc folder

/etc/issue –This is the file which is displayed at every login session.
/etc/motd –Message of the day file which is shown after you login to any machine.
/etc/services –Available services and their corresponding port information is stored here
/etc/protocols –Available network protocols such as IGP, ISIS etc.
/etc/redhat-release –If yu find this file, that indicates you are working on Redhat based machines and it contain OS version and release details.
/etc/debian_version –If yu find this file, that indicates you are working on Debian based machines and it contain OS version and release details.
/etc/shells –All available/installed shells in your machine.
/etc/sudoers –Main sudo configuration file.

In our next post we will see /home directory.