Detailed explanation of Linux environment variable configuration strategy

When customizing the installation of software, you often need to configure environment variables. The following lists various methods for configuring environment variables.

The environment for all the examples below is as follows:

• System: Ubuntu 14.0
• Username: uusama
• Need to configure MySQL environment variable path: /home/uusama/mysql/bin

Linux reads environment variables

Methods for reading environment variables:

• The export command displays all environment variables defined in the current system
• The echo $PATH command outputs the value of the current PATH environment variable

The effects of executing these two commands are as follows

uusama@ubuntu:~$ export
declare -x HOME="/home/uusama"
declare -x LANG="en_US.UTF-8"
declare -x LANGUAGE="en_US:"
declare -x LESSCLOSE="/usr/bin/lesspipe %s %s"
declare -x LESSOPEN="| /usr/bin/lesspipe %s"
declare -x LOGNAME="uusama"
declare -x MAIL="/var/mail/uusama"
declare -x PATH="/home/uusama/bin:/home/uusama/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
declare -x SSH_TTY="/dev/pts/0"
declare -x TERM="xterm"
declare -x USER="uusama"

uusama@ubuntu:~$ echo $PATH
/home/uusama/bin:/home/uusama/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

The PATH variable defines the search path for running commands, with colons separating different paths. Double quotes can be added or not when using export definitions.

Linux environment variable configuration method 1: export PATH

Use the export command to directly modify the value of PATH and configure MySQL to enter the environment variable:

export PATH=/home/uusama/mysql/bin:$PATH

# Or put PATH in front export PATH=$PATH:/home/uusama/mysql/bin

Note:

• Effective time: Immediately
• Validity period: Valid for the current terminal, invalid after the window is closed
• Scope of effect: Only valid for the current user
• Don't forget to add the original configuration to the configured environment variables, that is, the $PATH part, to avoid overwriting the original configuration

Linux environment variable configuration method 2: vim ~/.bashrc

Configure by modifying the ~/.bashrc file in the user directory:

vim ~/.bashrc

# Add export PATH=$PATH:/home/uusama/mysql/bin to the last line

Note:

• Effective time: It takes effect when you open a new terminal with the same user, or manually source ~/.bashrc
• Effective date: Permanent
• Scope of effect: Only valid for the current user
• If a subsequent environment variable loading file overwrites the PATH definition, it may not take effect.

Linux environment variable configuration method three: vim ~/.bash_profile

Similar to modifying the ~/.bashrc file, just add the new path at the end of the file:

vim ~/.bash_profile

# Add export PATH=$PATH:/home/uusama/mysql/bin to the last line

Note:

• Effective time: It takes effect when you open a new terminal with the same user, or manually source ~/.bash_profile
• Effective date: Permanent
• Scope of effect: Only valid for the current user
• If there is no ~/.bash_profile file, you can edit the ~/.profile file or create a new one

Linux environment variable configuration method 4: vim /etc/bashrc

This method is to modify the system configuration, which requires administrator privileges (such as root) or write privileges to the file:

# If the /etc/bashrc file is not editable, you need to modify it to be editable chmod -v u+w /etc/bashrc

vim /etc/bashrc

# Add export PATH=$PATH:/home/uusama/mysql/bin to the last line

Note:

• Effective time: It takes effect when you open a new terminal or manually source /etc/bashrc
• Effective date: Permanent
• Scope of effect: Valid for all users

Linux environment variable configuration method 5: vim /etc/profile

This method modifies the system configuration and requires administrator privileges or write privileges to the file, similar to vim /etc/bashrc:

# If the /etc/profile file is not editable, you need to modify it to be editable chmod -v u+w /etc/profile

vim /etc/profile

# Add export PATH=$PATH:/home/uusama/mysql/bin to the last line

Note:

• Effective time: It takes effect when a new terminal is opened, or when you manually source /etc/profile
• Effective date: Permanent
• Scope of effect: Valid for all users

Linux environment variable configuration method six: vim /etc/environment

This method is to modify the system environment configuration file, which requires administrator privileges or write privileges to the file:

# If the /etc/bashrc file is not editable, you need to modify it to be editable chmod -v u+w /etc/environment

vim /etc/profile

# Add export PATH=$PATH:/home/uusama/mysql/bin to the last line

Note:

• Effective time: It takes effect when you open a new terminal or manually source /etc/environment
• Effective date: Permanent
• Scope of effect: Valid for all users

Analysis of Linux environment variable loading principle

The above lists various configuration methods of environment variables, so how does Linux load these configurations? In what order are they loaded?

A specific loading order can cause environment variable definitions with the same name to be overwritten or ineffective.

Classification of environment variables

Environment variables can be simply divided into user-defined environment variables and system-level environment variables.

• User-level environment variable definition files: ~/.bashrc, ~/.bash_profile
• System-level environment variable definition files: /etc/bashrc, /etc/bash_profile, /etc/environment

In addition, in the user environment variables, the system will first read the ~/.bash_profile file. If the file does not exist, it will read ~/.bash_login. If the file does not exist either, it will read ~/.profile, and then read ~/.bashrc based on the contents of these files.

How to test the Linux environment variable loading order

In order to test the order in which environment variables are loaded for different files, we define the same environment variable UU_ORDER in the first line of each environment variable definition file. The value of this variable is its own value concatenated with the current file name.

The files that need to be modified are as follows:

• /etc/environment
• /etc/profile
• /etc/profile.d/test.sh, create a new file, no folders can be skipped
• /etc/bashrc, or /etc/bash.bashrc
• ~/.bash_profile, or ~/.profile
• ~/.bashrc

Add the following code to the first line of each file, and modify the content after the colon to the absolute file name of the current file.

export UU_ORDER="$UU_ORDER:~/.bash_profile"

After modification, save, open a new window, and then echo $UU_ORDER to observe the value of the variable:

uusama@ubuntu:~$ echo $UU_ORDER
$UU_ORDER:/etc/environment:/etc/profile:/etc/bash.bashrc:/etc/profile.d/test.sh:~/.profile:~/.bashrc

It can be inferred that the order in which Linux loads environment variables is as follows:

1. /etc/environment
2. /etc/profile
3. /etc/bash.bashrc
4. /etc/profile.d/test.sh
5. ~/.profile
6. ~/.bashrc

Detailed explanation of Linux environment variable file loading

From the above test, it can be easily concluded that the order in which Linux loads environment variables is as follows:

System environment variables -> User defined environment variables
/etc/environment -> /etc/profile -> ~/.profile

Open the /etc/profile file and you will find that the code in this file will load the /etc/bash.bashrc file, then check and load the .sh file in the /etc/profile.d/ directory.

# /etc/profile: system-wide .profile file for the Bourne shell (sh(1))
# and Bourne compatible shells (bash(1), ksh(1), ash(1), ...).

if [ "$PS1" ]; then
 if [ "$BASH" ] && [ "$BASH" != "/bin/sh" ]; then
  # The file bash.bashrc already sets the default PS1.
  # PS1='\h:\w\$ '
  if [ -f /etc/bash.bashrc ]; then
   . /etc/bash.bashrc
  fi
 else
  if [ "`id -u`" -eq 0 ]; then
   PS1='#'
  else
   PS1='$ '
  fi
 fi
fi

if [ -d /etc/profile.d ]; then
 for i in /etc/profile.d/*.sh; do
  if [ -r $i ]; then
   . $i
  fi
 done
 unset i
fi

Secondly, open the ~/.profile file and you will find that the ~/.bashrc file is loaded in it.

# if running bash
if [ -n "$BASH_VERSION" ]; then
  # include .bashrc if it exists
  if [ -f "$HOME/.bashrc" ]; then
  . "$HOME/.bashrc"
  fi
fi

# set PATH so it includes user's private bin directories
PATH="$HOME/bin:$HOME/.local/bin:$PATH"

It is not difficult to find from the code in the ~/.profile file that the /.profile file is only read once when the user logs in, while /.bashrc is read once each time the Shell script is run.

Some tips

You can customize an environment variable file, for example, define uusama.profile under a certain project, use export to define a series of variables in this file, and then add: source uusama.profile after the ~/.profile file. In this way, you can use the series of variables you defined in the Shell script every time you log in.

You can also use the alias command to define some command aliases, such as alias rm="rm -i" (double quotes are required), and add this code to ~/.profile. This way, every time you use the rm command, it is equivalent to using the rm -i command, which is very convenient.

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