When working with Linux systems, the Filesystem Hierarchy Standard (FHS) defines the purpose of each directory. Here's what you need to know about the key directories for application installation:

/var - Variable data files (logs, databases, spool files)
/opt - Optional application software packages
/usr/local - Locally installed software (not part of OS distribution)
/etc - Configuration files (not for application binaries)

The /opt directory is ideal for self-contained, third-party applications that don't follow standard Unix directory structure. For example:

# Typical /opt installation structure for Tomcat
/opt/apache-tomcat-9.0.54/
├── bin/
├── conf/
├── lib/
├── logs/ -> /var/log/tomcat9
├── webapps/

Advantages of /opt:

• Keeps all application files in one place
• Easier to remove entire applications
• Common practice for commercial software

While /var shouldn't contain application binaries, it's perfect for:

# Example of proper /var usage
/var/lib/tomcat9/ - Persistent application data
/var/log/tomcat9/ - Application log files
/var/cache/tomcat9/ - Cached data

Use /usr/local when compiling software from source that should be available system-wide:

# Typical ./configure command for source installation
./configure --prefix=/usr/local
make
sudo make install

This follows the standard Unix directory structure:

• Binaries in /usr/local/bin
• Libraries in /usr/local/lib
• Configuration in /usr/local/etc

For a Java application like Elasticsearch, here's how I would install it:

# Extract to /opt
sudo tar -xzf elasticsearch-7.15.0-linux-x86_64.tar.gz -C /opt

# Create symlink for easier version management
sudo ln -s /opt/elasticsearch-7.15.0 /opt/elasticsearch

# Configure data directories in /var
sudo mkdir -p /var/lib/elasticsearch
sudo chown elasticsearch:elasticsearch /var/lib/elasticsearch

Different distributions may have conventions:

• RHEL/CentOS: Prefers /opt for third-party software
• Debian/Ubuntu: Often uses /usr/share for application files
• Always check distro-specific guidelines

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The Linux Filesystem Hierarchy Standard defines specific purposes for each directory:

/var - Variable data (logs, spools, temp files)
/opt - Optional/add-on application software packages
/usr/local - Locally installed software (not from distribution packages)
/etc - Configuration files for system and applications

/opt is ideal for self-contained applications that don't follow standard Unix directory structure. Common use cases:

• Commercial software (Oracle Database, MATLAB)
• Large applications with all dependencies bundled
• Applications that may need multiple versions installed

Example Tomcat installation in /opt:

wget https://downloads.apache.org/tomcat/tomcat-10/v10.1.8/bin/apache-tomcat-10.1.8.tar.gz
sudo tar -xzvf apache-tomcat-10.1.8.tar.gz -C /opt
sudo ln -s /opt/apache-tomcat-10.1.8 /opt/tomcat

/var should be used for runtime data rather than application binaries. Typical scenarios:

• Application working directories
• PID files, lock files
• Log files (/var/log)
• Spool directories (/var/spool)

Example Java application runtime directory structure:

/var/lib/myapp/ - Application data
/var/log/myapp/ - Log files
/var/run/myapp/ - PID files
/var/cache/myapp/ - Cache files

While both are for locally installed software, they follow different conventions:

Directory	Structure	Best For
/opt	Application bundles all files in single directory	Large, self-contained apps
/usr/local	Follows standard Unix hierarchy (bin, lib, etc.)	Locally compiled software

/etc should only contain configuration files, not binaries or application data. Example Tomcat configuration:

/etc/tomcat10/server.xml - Main configuration
/etc/tomcat10/web.xml - Default web application settings
/etc/default/tomcat10 - Environment variables

For Java web applications:

• Install Tomcat in /opt/tomcat
• Place WAR files in /var/lib/tomcat/webapps
• Store logs in /var/log/tomcat

For system services:

• Place init scripts in /etc/init.d or /lib/systemd/system
• Runtime files in /var/run

For custom compiled software:

• ./configure --prefix=/usr/local
• Creates proper hierarchy under /usr/local

Here's how I typically structure a Java/Tomcat/PostgreSQL setup:

# PostgreSQL
/var/lib/postgresql/14/main - Data files
/etc/postgresql/14/main - Configuration

# Java
/usr/lib/jvm/java-11-openjdk-amd64 - JDK

# Tomcat
/opt/apache-tomcat-10.1.8 - Installation
/var/lib/tomcat/webapps - Deployed applications
/var/log/tomcat - Logs
/etc/tomcat - Configuration

/opt makes version management easier through symlinks:

/opt/tomcat-9.0.75
/opt/tomcat-10.1.8
/opt/tomcat -> /opt/tomcat-10.1.8

Update the symlink when switching versions while keeping configs in /etc/tomcat.

• /opt and /usr/local typically need root for installation
• /var directories often need specific user/group permissions
• Follow principle of least privilege for each directory

Distribution packages will automatically place files in correct locations:

# Ubuntu/Debian
apt install tomcat9 - Installs to FHS-compliant locations

# Manual install gives more control but requires proper placement