Unix/Linux Directory Abbreviations Explained: Decoding /etc, /opt and Other Key System Paths


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In Unix systems, the /etc directory name originally stood for "et cetera" (Latin for "and other things"). This reflects its purpose as the catch-all location for system configuration files that didn't belong elsewhere. Some examples:

# Common /etc files
/etc/passwd
/etc/hosts
/etc/ssh/sshd_config

The /opt directory (short for "optional") stores add-on application software packages, typically those not managed by the system's package manager. Commercial software often installs here:

# Typical /opt structure
/opt/google/chrome
/opt/oracle/java

While the original meanings remain, modern Linux distributions follow the Filesystem Hierarchy Standard (FHS):

  • /etc: Host-specific system-wide configuration files
  • /opt: Static application software packages

Example of querying these directories programmatically in Python:

import os

# List contents of /etc
etc_files = os.listdir('/etc')
print(f"/etc contains {len(etc_files)} configuration files")

# Check if software exists in /opt
if os.path.exists('/opt/myapp'):
print("Application found in /opt")

When writing installation scripts or system utilities, follow these conventions:

# Bash example for config file handling
CONFIG_DIR="/etc/myapp"
if [ ! -d "$CONFIG_DIR" ]; then
sudo mkdir -p "$CONFIG_DIR"
sudo chmod 755 "$CONFIG_DIR"
fi

For package maintainers, the FHS specifies that /opt should contain one subdirectory per package, following the format /opt/<provider>/<package>.

For completeness, here are some other common Unix directory abbreviations:

Directory Meaning Contents
/usr Unix System Resources Read-only user utilities
/bin Binaries Essential command binaries
/var Variable Files that change frequently
/tmp Temporary Temporary files

Understanding these conventions helps when debugging issues or designing system software. The standardized locations make scripts and tools more portable across Unix-like systems.

Every Unix/Linux user encounters these fundamental directories:

/
├── bin    # Essential command binaries
├── usr    # User programs and utilities
└── sbin   # System binaries

Contrary to popular belief, /etc doesn't stand for "electronic configuration." It originates from early Unix at Bell Labs where it was literally "et cetera" - the catch-all directory for miscellaneous system files.

Today, it's the primary location for system configuration files:

/etc/
├── passwd     # User account information
├── hosts      # Hostname to IP mappings  
├── fstab      # Filesystem table
└── nginx/     # Web server configs

Your guess about /opt being "optional" is correct. This directory holds:

  • Third-party applications
  • Large software packages
  • Self-contained programs

Example structure:

/opt/
├── google/
│   └── chrome/  # Chrome browser files
└── jetbrains/
    └── toolbox/ # JetBrains IDE manager

When writing scripts that interact with these directories:

#!/bin/bash

# Read from /etc
if [[ -f "/etc/os-release" ]]; then
    source /etc/os-release
    echo "Running $NAME $VERSION"
fi

# Check for /opt programs
if [[ -d "/opt/google/chrome" ]]; then
    echo "Chrome is installed in /opt"
fi

While these directories remain standard, some modern systems use alternatives:

  • /usr/local/etc for local configurations
  • /usr/local/opt in Homebrew (macOS)
  • AppImage/Snap packages that don't require /opt