Deep Dive into Shell Environment Variables

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Shell Environment Variables In-Depth: Loading Process Testing

Shell login environment execution script file syntax

Shell non-login environment variable execution script file syntax

         Shell Environment Variables In-Depth: Identifying Shell Environment Types

         Shell environment variables in-depth: switch the shell environment in detail

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Shell Environment Variables In-Depth: Loading Process Testing

Target

Understand the loading process test of Shell environment variables

Ability to know where environment variables should be configured

Introduction to Switching Shell Environments to Execute Script Files

When executing a script file, you can specify a specific shell environment to execute the script file. This is to switch the shell environment to execute the script.

Shell login environment execution script file syntax

sh/bash -l/--login 脚本文件

Meaning: First load the Shell login environment process to initialize the environment variables, and then execute the script file

Shell non-login environment variable execution script file syntax

bash # 加载Shell非登录环境
sh/bash 脚本文件 # 直接执行脚本文件

Meaning: First execute the process of loading the Shell non-login environment to initialize the environment variables, and then execute the script file

Test Case

need

The shell login environment will run /etc/profile

Shell non-login environment will run /.bashrc

Set the environment variable data in the /etc/profile and /current user/.bashrc files respectively, then output the environment variable data in the shell script file, and finally switch to different environments to execute the shell script file to observe and verify the operation of the above process

analyze

1. Clean up, clean up the VAR1 environment variable in the /etc/profile file to delete, and reload the configuration file

2. Edit /etc/profile, add environment variable VAR1=VAR1

3. Edit /root/.bashrc, add environment variable VAR2=VAR2

4. Create the demo1.sh file, read the environment variable data for printing

   # output environment variable VAR1 
   # output environment variable VAR2

5. Execute the demo1.sh script file in a shell non-login environment, and observe that only VAR2 is output, not VAR1

6. Execute the demo1.sh script file in the shell login environment, and observe that VAR2 and VAR1 will be output

demo

Edit /etc/profile file

gedit /etc/profile

Edit and add the following content, save and exit

 

In the root directory, edit the .bashrc file

gedit .bashrc

Edit and add the last 2 lines as follows, save and exit

Create the file demo1.sh

touch demo1.sh

Edit the file demo1.sh and add the following content

#!/bin/bash
echo $VAR1
echo $VAR2

Execute the script file directly

bash demo1.sh

Execute the script file directly, that is, neither the login shell environment variables nor the non-login shell environment variables are loaded

First load the non-login shell environment variables, and then execute the script file

bash
bash demo1.sh

 

 

Shell non-login environment will load 当前用户/.bashrcthe

So here will output the environment variable data of VAR2

First load the login shell environment variable, and then execute the script file

bash -l demo1.sh

 

The shell login environment etc/profileloads 当前用户/.bashrcthe file and environment variable data for

So here will output the environment variable data of VAR1 and VAR2

summary

1. The initial loading principle process of Shell environment variables

Classification	Initialize Environment Variables Process Execution File Order
Shell login environment initialization process	/etc/profile--》/etc/profile.d/*.sh--》~/.bash_profile--》~/.bashrc--》/etc/bashrc
Shell non-login environment initialization process	~/.bashrc--》/etc/bashrc--》/etc/profile.d/*.sh

2. So where are the environment variables defined? Difference between /etc/profile and /etc/bashrc?

The environment variables read by the executed shell script that needs to log in are configured in: /etc/profile, /current user/.bash_profile

The environment variables read by shell scripts executed by users who do not need to log in are configured in: /current user/.bashrc, /etc/bashrc

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Shell Environment Variables In-Depth: Identifying Shell Environment Types

Target

Understand how to identify shell login environments and non-login environments

grammar

Use $0 to recognize ambient syntax

echo $0

Output -bashstands for : shell login environment

output bashstands for : shell non-login environment

Note: This $0environment variable if used in a subshell (shell script file) outputs the filename of the shell script itself

bash command syntax

bash

Bash command: used to switch to Shell non-login environment

analyze

1. Log in directly to the system and output $0 for the shell login environment to observe the effect of the output information

2. Use the bash command to switch to the shell non-login environment and output $0 to observe the effect of the output information

3. Create the test.sh script file, edit and add the output $0, and execute the test.sh script file after the programming is saved to observe the effect of the output information

summary

1. How to identify the shell login environment and non-login environment?

$0 用于获取当前Shell环境的类型,  bash代表Shell非登录环境, -bash 代表Shell登录环境
# $0不可以在脚本文件中使用, 因为代表获取脚本文件名字

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Shell environment variables in-depth: switch the shell environment in detail

Target

Understand commands to switch shell environments

Introduction to switching shell environment commands

1. Direct login to load the shell login environment

2. su switches user to load Shell login and Shell non-login environment

3. Bash loads Shell login and Shell non-login environment

Switch environment mode: bash switch

Order

Syntax 1:

bash  # 加载【Shell非登录环境】

Syntax 2:

bash -l  Shell脚本文件 /  bash --login shell脚本文件
sh -l shell脚本文件 / sh --login shell脚本文件
# 先加载【Shell登录环境】然后运行指定Shell脚本文件

analyze

Use bash to execute the test.sh script file, and an error occurs indicating that the current environment is the shell non-login environment