`/etc`

Table of Contents

Purpose of `/etc`

/etc is the central place for system-wide configuration files on a Linux system. Anything that controls how the system and services behave (for all users) is very likely to live here.

Key points:

It holds configuration, not user data.
Files here are typically plain text.
Changes often take effect on next service restart or next login.
Many tools have both:

system-wide config in /etc/...
per-user config in hidden files under ~ (like ~/.config/...)

You usually need sudo (or root) to edit files in /etc.

General Conventions in `/etc`

You’ll see a few common patterns:

Single config files: e.g. /etc/fstab, /etc/hosts
Config directories: e.g. /etc/ssh/, /etc/cron.d/
“d”-style drop-in directories:

Example: /etc/sudoers.d/, /etc/sysctl.d/, /etc/apt/sources.list.d/
Idea: one main file plus many small snippet files

Distribution-specific paths:

Debian/Ubuntu: /etc/apt/, /etc/default/
Fedora/RHEL: /etc/dnf/, /etc/yum.repos.d/
Arch: /etc/pacman.conf, /etc/pacman.d/

When unsure what a file does, use man (e.g. man fstab, man sshd_config) if available.

Key Areas Inside `/etc`

This is not a complete list, but covers the most commonly encountered parts for beginners.

System Identity and Network Basics

/etc/hostname

Contains the system’s hostname (a single name, e.g. my-laptop).
Used at boot and shown in your shell prompt on many systems.

/etc/hosts

Local mapping between hostnames and IP addresses.
Often contains entries like:

    127.0.0.1   localhost
    127.0.1.1   my-laptop

Useful for overriding DNS for specific names (e.g. for local testing).

/etc/resolv.conf

Defines DNS servers and related settings (which nameserver to query).
On modern systems, this may be managed automatically by network tools (e.g. systemd-resolved, NetworkManager), so direct edits may be overwritten.

User and Authentication Configuration

These files do not usually store plain-text passwords anymore, but they define user accounts and related policies:

/etc/passwd

List of user accounts, their home directories, shell, etc.
Readable by all users.

/etc/shadow

Encrypted password hashes and some password policy data.
Only readable by root for security reasons.

/etc/group

Group definitions and group membership.

/etc/login.defs

Default settings for user accounts (for tools like useradd).

/etc/sudoers and /etc/sudoers.d/

Rules for who can use sudo and how.
Must be edited with visudo to avoid syntax errors that could lock you out of admin access.

System-Wide Shell and Environment Settings

/etc/profile

System-wide shell initialization for login shells (typically sh/bash).

/etc/bash.bashrc (Debian/Ubuntu) or similar

Global configuration for interactive bash shells.

/etc/profile.d/

Directory with small scripts that adjust environment variables or behavior for all users (e.g. paths, language settings).

Per-user shell configuration typically lives in files like ~/.bashrc, ~/.profile, etc., which supplement or override these global settings.

Service and Daemon Configuration

Most system services keep their configuration under /etc, usually in subdirectories:

/etc/ssh/

Settings for the SSH client (ssh_config) and server (sshd_config).

/etc/cron.* (/etc/cron.d/, /etc/cron.daily/, etc.)

System-wide scheduled tasks (cron jobs).

/etc/systemd/

Systemd configuration snippets and overrides.
Often used for custom service configuration and drop-in files.

When installing new packages, you’ll often see new config files appear in /etc/ or /etc/<package-name>/.

Package Manager Configuration (Distribution-Specific)

Each distribution’s package manager keeps its configuration in /etc:

Debian/Ubuntu (APT):

/etc/apt/sources.list – main list of package repositories.
/etc/apt/sources.list.d/ – extra repository definitions as separate files.
/etc/apt/apt.conf.d/ – APT behavior snippets.

Fedora/RHEL (DNF/YUM):

/etc/dnf/dnf.conf – DNF main config.
/etc/yum.repos.d/ – repository definitions.

Arch Linux (pacman):

/etc/pacman.conf – pacman configuration.
/etc/pacman.d/ – extra configuration lists (like mirrors).

Editing these files controls where software comes from and how the package manager behaves.

Boot and Filesystem Configuration

/etc/fstab

Table of filesystems to mount at boot.
Each line describes a device/partition (or network share), mount point, and options.

/etc/mtab (or symlink to /proc/self/mounts)

List of currently mounted filesystems. More informational than edited by hand on modern systems.

Some bootloader-related configs (e.g. GRUB) also live under /etc:

/etc/default/grub (on many distros)

High-level GRUB options (boot timeout, default kernel options).
After editing, you usually run a command like update-grub or grub2-mkconfig depending on your distribution.

Application and Desktop Configuration (System-Wide)

System-wide defaults for various tools often live under /etc:

/etc/skel/

Template files for new user home directories.
Files here are copied into a new user’s home when the account is created.

/etc/X11/

X11 (traditional Linux graphical system) configuration files, if used.

/etc/pam.d/

Configuration for PAM (Pluggable Authentication Modules) controlling how authentication is performed for different services.

Many graphical or desktop-related defaults may also be placed here by packages.

Working Safely with `/etc`

Because /etc controls system behavior, changes can easily break things. A few safe practices:

Make backups before editing
For example:

  sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

Use appropriate tools

visudo for /etc/sudoers.
Distribution-specific helpers (e.g. update-grub) after changing certain files.

Check syntax when possible
Some daemons provide a “test config” mode, for example:

  sudo sshd -t

to validate /etc/ssh/sshd_config.

Restart or reload services after changes
Typically with systemctl (covered elsewhere), e.g.:

  sudo systemctl restart ssh

What Belongs in `/etc` (and What Does Not)

Belongs in /etc:

System-wide configuration generated by packages or administrators.
Text files that can be edited to change behavior.

Does not belong in /etc:

Large data files.
Logs (those go under /var/log).
User-specific settings (those go in each user’s home directory).

As a rule: if you’re changing how the system as a whole behaves, you’re probably editing something in /etc.

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