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      • Environment Variables

      What are Environment Variables?

      Environment variables are dynamic values stored in the operating system that affect program behavior and how the system runs.

      Viewing Environment Variables

      # View all environment variables
$ env
$ printenv

# View specific variable
$ echo $PATH
$ printenv PATH

# Use set to view all variables (including Shell variables)
$ set

      Common Environment Variables

      VariableDescription
      PATHExecutable file search paths
      HOMEUser home directory
      USERCurrent username
      SHELLDefault shell
      PWDCurrent working directory
      OLDPWDPrevious working directory
      LANGLanguage setting
      TERMTerminal type
      EDITERDefault editor
      HOSTNAMEHostname
      PS1Command prompt
      LD_LIBRARY_PATHShared library search path

      Setting Environment Variables

      Temporary Setting

      # Set variable (only current Shell)
$ export MYVAR="value"

# Set variable and run command
$ MYVAR="value" command

# Unset variable
$ unset MYVAR

      Permanent Setting

      User-level Configuration

      Edit ~/.bashrc or ~/.profile:

      # ~/.bashrc
export MYVAR="value"
export PATH="$PATH:/new/path"
export EDITOR="vim"
      # Make configuration effective
$ source ~/.bashrc

      System-level Configuration

      # /etc/environment (recommended)
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/bin:/sbin:/bin"
LANG="en_US.UTF-8"

# /etc/profile (all users at login)
export JAVA_HOME="/usr/lib/jvm/java-11"

# /etc/profile.d/*.sh
# Create independent configuration files
$ sudo vim /etc/profile.d/myapp.sh
export MYAPP_HOME="/opt/myapp"

      PATH Variable

      PATH is one of the most important environment variables, defining the directories that Shell searches for commands.

      Viewing PATH

      $ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin

# Display one path per line
$ echo $PATH | tr ':' '\n'

      Modifying PATH

      # Add to end
$ export PATH="$PATH:/new/path"

# Add to beginning (priority search first)
$ export PATH="/new/path:$PATH"

# Permanent add (in ~/.bashrc)
export PATH="$PATH:$HOME/bin"
export PATH="$PATH:$HOME/.local/bin"

      PATH Search Order

      # View command location
$ which python
/usr/bin/python

# View all matches
$ which -a python
/usr/bin/python
/usr/local/bin/python

      Language and Region Settings

      # View current settings
$ locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
...

# Set language
export LANG="zh_CN.UTF-8"

# Set all regions
export LC_ALL="en_US.UTF-8"

# View available languages
$ locale -a

      Common Configuration Examples

      Development Environment

      # ~/.bashrc

# Java
export JAVA_HOME="/usr/lib/jvm/java-17"
export PATH="$PATH:$JAVA_HOME/bin"

# Go
export GOPATH="$HOME/go"
export PATH="$PATH:$GOPATH/bin"

# Node.js
export NVM_DIR="$HOME/.nvm"

# Python
export PYTHONPATH="$HOME/python/lib"

# Editor
export EDITOR="vim"
export VISUAL="vim"

      Proxy Settings

      export http_proxy="http://proxy.example.com:8080"
export https_proxy="http://proxy.example.com:8080"
export no_proxy="localhost,127.0.0.1,.example.com"

# Unset proxy
unset http_proxy https_proxy no_proxy

      Shell Variables vs Environment Variables

      # Shell variable (only current Shell)
myvar="value"

# Environment variable (passes to subprocesses)
export myvar="value"

# Verify
$ myvar="shell_var"
$ bash -c 'echo $myvar'    # empty

$ export myvar="env_var"
$ bash -c 'echo $myvar'    # env_var

      Summary

      This chapter introduced environment variables:

      • Viewing variables: env, printenv, echo
      • Setting variables: export, configuration files
      • PATH variable: Command search paths
      • Configuration files: ~/.bashrc, /etc/environment
      • Language settings: locale
      • Configuration examples: Development, proxy settings

      Environment variables are an important way to configure系统和应用程序。Mastering them will greatly improve your efficiency.

      Previous chapter: Shell Scripting Basics

      Next chapter: Practical Examples