143 lines
4.5 KiB
Markdown
143 lines
4.5 KiB
Markdown
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# Linux Environment Variables
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## Global variables
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The **global variables** will be **inherited** by **child processes**.
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You can create a Global variable for your current session doing:
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```bash
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export MYGLOBAL="hello world"
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echo $MYGLOBAL #Prints: hello world
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```
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This variable will be accessible by your current sessions and its child processes.
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You can **remove** a variable doing:
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```bash
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unset MYGLOBAL
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```
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## Local variables
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The **local variables** can only be **accessed** by the **current shell/script**.
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```bash
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LOCAL="my local"
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echo $LOCAL
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unset LOCAL
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```
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## List current variables
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```bash
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set
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env
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printenv
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cat /proc/$$/environ
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cat /proc/`python -c "import os; print(os.getppid())"`/environ
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```
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## Persistent Environment variables
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#### **Files that affect behavior of every user:**
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* _**/etc/bash.bashrc**_ ****: This file is read whenever an interactive shell is started \(normal terminal\) and all the commands specified in here are executed.
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* _**/etc/profile and /etc/profile.d/\***_**:** This file is read every time a user logs in. Thus all the commands executed in here will execute only once at the time of user logging in.
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* **Example:**
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`/etc/profile.d/somescript.sh`
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```bash
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#!/bin/bash
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TEST=$(cat /var/somefile)
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export $TEST
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```
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#### **Files that affect behavior for only a specific user:**
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* _**~/.bashrc**_ **:** This file behaves the same way _/etc/bash.bashrc_ file works but it is executed only for a specific user. If you want to create an environment for yourself go ahead and modify or create this file in your home directory.
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* _**~/.profile, ~/.bash\_profile, ~/.bash\_login**_**:** These files are same as _/etc/profile_. The difference comes in the way it is executed. This file is executed only when a user in whose home directory this file exists, logs in.
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**Extracted from:** [**here**](https://codeburst.io/linux-environment-variables-53cea0245dc9) **and** [**here**](https://www.gnu.org/software/bash/manual/html_node/Bash-Startup-Files.html)\*\*\*\*
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## Common variables
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From: [https://geek-university.com/linux/common-environment-variables/](https://geek-university.com/linux/common-environment-variables/)
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* **DISPLAY** – the display used by **X**. This variable is usually set to **:0.0**, which means the first display on the current computer.
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* **EDITOR** – the user’s preferred text editor.
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* **HISTFILESIZE** – the maximum number of lines contained in the history file.
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* **HISTSIZE -** Number of lines added to the history file when the user finish his session
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* **HOME** – your home directory.
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* **HOSTNAME** – the hostname of the computer.
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* **LANG** – your current language.
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* **MAIL** – the location of the user’s mail spool. Usually **/var/spool/mail/USER**.
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* **MANPATH** – the list of directories to search for manual pages.
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* **OSTYPE** – the type of operating system.
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* **PS1** – the default prompt in bash.
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* **PATH -** stores the path of all the directories which holds binary files you want to execute just by specifying the name of the file and not by relative or absolute path.
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* **PWD** – the current working directory.
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* **SHELL** – the path to the current command shell \(for example, **/bin/bash**\).
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* **TERM** – the current terminal type \(for example, **xterm**\).
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* **TZ** – your time zone.
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* **USER** – your current username.
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## Interesting variables for hacking
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### **HISTFILESIZE**
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Change the **value of this variable to 0**, so when you **end your session** the **history file** \(~/.bash\_history\) **will be deleted**.
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```bash
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export HISTFILESIZE=0
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```
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### **HISTSIZE**
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Change the **value of this variable to 0**, so when you **end your session** any command will be added to the **history file** \(~/.bash\_history\).
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```bash
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export HISTSIZE=0
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```
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### http\_proxy
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The processes will use the **proxy** declared here to connect to internet through **http**.
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```bash
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export http_proxy="http://10.10.10.10:8080"
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```
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### https\_proxy
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The processes will use the **proxy** declared here to connect to internet through **https**.
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```bash
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export https_proxy="http://10.10.10.10:8080"
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```
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### PS1
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Change how your prompt looks.
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**I have created** [**this one**](https://gist.github.com/carlospolop/43f7cd50f3deea972439af3222b68808) \(based on another, read the code\).
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Root:
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Regular user:
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One, two and three backgrounded jobs:
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One background job, one stopped and last command dind't finish correctly:
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