Linux Environment Variables

Global variables

The global variables will be inherited by child processes.

You can create a global variable for your current session doing:

export MYGLOBAL="hello world"
echo $MYGLOBAL #Prints: hello world

This variable will be accessible by your current sessions and its child processes.

You can remove a variable doing:

unset MYGLOBAL

Local variables

The local variables can only be accessed by the current shell/script.

LOCAL="my local"
echo $LOCAL
unset LOCAL

List current variables

set
env
printenv
cat /proc/$$/environ
cat /proc/`python -c "import os; print(os.getppid())"`/environ

Common variables

From: https://geek-university.com/linux/common-environment-variables/

• DISPLAY – the display used by X. This variable is usually set to :0.0, which means the first display on the current computer.
• EDITOR – the user’s preferred text editor.
• HISTFILESIZE – the maximum number of lines contained in the history file.
• HISTSIZE – Number of lines added to the history file when the user finish his session
• HOME – your home directory.
• HOSTNAME – the hostname of the computer.
• LANG – your current language.
• MAIL – the location of the user’s mail spool. Usually /var/spool/mail/USER.
• MANPATH – the list of directories to search for manual pages.
• OSTYPE – the type of operating system.
• PS1 – the default prompt in bash.
• 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.
• PWD – the current working directory.
• SHELL – the path to the current command shell (for example, /bin/bash).
• TERM – the current terminal type (for example, xterm).
• TZ – your time zone.
• USER – your current username.

Interesting variables for hacking

HISTFILESIZE

Change the value of this variable to 0, so when you end your session the history file (~/.bash_history) will be deleted.

export HISTFILESIZE=0

HISTSIZE

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).

export HISTSIZE=0

http_proxy & https_proxy

The processes will use the proxy declared here to connect to internet through http or https.

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

SSL_CERT_FILE & SSL_CERT_DIR

The processes will trust the certificates indicated in these env variables.

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

PS1

Change how your prompt looks.

This is an example

Root:

Regular user:

One, two and three backgrounded jobs:

One background job, one stopped and last command didn't finish correctly: