Linux Environment Variables

Linux Environment Variables

A skill for managing Linux environment variables, including security hardening, proxy configuration, and system customization.

When to Use This Skill

Use this skill when you need to:

• Set or modify environment variables (global or local)
• Configure proxy settings for network tools
• Hide or control command history for security
• Manage SSL certificate paths
• Customize bash prompts (PS1)
• Debug variable inheritance issues
• Understand what common environment variables do

Core Concepts

Global vs Local Variables

Global variables are inherited by child processes:

export MYVAR="value"
echo $MYVAR  # Accessible in current session and children
unset MYVAR  # Remove the variable

Local variables only exist in the current shell:

LOCAL="value"
echo $LOCAL  # Only accessible here, not in child processes

Listing Variables

set           # All variables (including local)
env           # Exported variables only
printenv      # Same as env
cat /proc/$$/environ              # Current process env
cat /proc/$(python -c "import os; print(os.getppid())")/environ  # Parent process env

Common Environment Variables

Security & Hardening

Hiding Command History

Prevent commands from being saved to ~/.bash_history:

# Disable history file writing on session end
export HISTFILESIZE=0

# Disable in-memory history
export HISTSIZE=0

# Hide commands that start with a space
export HISTCONTROL=ignorespace

Usage: Commands prefixed with a space won't be saved:

$ echo "this saves"
$  echo "this does not save"  # Note the leading space

Proxy Configuration

Set HTTP/HTTPS proxies:

export http_proxy="http://10.10.10.10:8080"
export https_proxy="http://10.10.10.10:8080"

Set SOCKS proxy for all protocols:

export all_proxy="socks5h://10.10.10.10:1080"

Bypass proxy for specific hosts:

export no_proxy="localhost,127.0.0.1,.corp.local,10.0.0.0/8"

Note: Both lowercase and uppercase variants work (http_proxy/HTTP_PROXY).

SSL Certificate Configuration

Point tools to custom CA certificates:

export SSL_CERT_FILE=/path/to/ca-bundle.pem
export SSL_CERT_DIR=/path/to/ca-certificates

Customizing Your Prompt (PS1)

The PS1 variable controls your bash prompt appearance. Common elements:

• \u - username
• \h - hostname
• \w - current directory
• \$ - # for root, $ for users
• \! - command number
• \d - date
• \t - time

Example prompt:

export PS1="\u@\h:\w\$ "

Scripts

Use the bundled scripts for common tasks:

• scripts/list-variables.sh - Display all environment variables in a formatted table
• scripts/hide-history.sh - Configure history hiding for security
• scripts/set-proxy.sh - Configure proxy settings with validation

Troubleshooting

Variable not persisting

If a variable disappears after closing the terminal, add it to your shell config:

echo 'export MYVAR="value"' >> ~/.bashrc
source ~/.bashrc

Child process not seeing variable

Make sure you used export:

# Wrong - child won't see this
MYVAR="value"

# Right - child will inherit
export MYVAR="value"

PATH issues

Check if a command is in your PATH:

which command_name
hash -r  # Clear command hash cache

Best Practices

1. Use export for variables child processes need
2. Set HISTCONTROL=ignorespace for sensitive work
3. Validate proxy URLs before setting them
4. Document custom variables in your shell config
5. Use printenv to verify variables are set correctly
6. Be careful with PATH modifications - they affect all commands