hacktricks/linux-hardening/linux-environment-variables.md
Carlos Polop 43da32d5b8 a
2024-02-07 05:06:18 +01:00

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Linux Environment Variables

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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 users 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 users 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:

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