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Linux — Environment

Short & clear revision notes · 6 topics

01 · What is the Environment?

The environment in Linux is defined by environment variables — character strings that hold values like numbers, text, filenames, or device names. Some are set by the system, some by you, and some by the shell itself.

TEST="Unix Programming"   # set a variable (no $ sign)
echo $TEST                # access it (use $ prefix)
# Output: Unix Programming

💡 Variables are set without $ but accessed with $. They retain their values until you exit the shell.

02 · Shell Initialisation — Startup Files

When you log in, the shell reads two files in order to set up your environment:

Login
  │
  ├── 1. /etc/profile      (system-wide — applies to ALL users)
  │
  └── 2. ~/.profile        (personal — applies to YOU only)
  │
  └── Shell prompt $ appears
File Controlled by Purpose
/etc/profile System administrator Shell config required by all users
~/.profile You Your personal customisations

💡 No error is shown if either file is missing — the shell simply skips it. This applies to all Bourne-type shells. bash and ksh also use additional files.

Minimum things to configure in .profile

  • The type of terminal you are using
  • Directories where the shell should look for commands (PATH)
  • Variables affecting the look and feel of your terminal

03 · Key Environment Variables

TERM — Terminal Type

TERM=vt100

Sets what kind of terminal you're using. Usually auto-detected, but can be set manually if output looks garbled.

PATH — Command Search Path

PATH=/bin:/usr/bin

When you type a command, the shell searches each directory in PATH (separated by :) left to right. If not found anywhere:

$ hello
hello: not found

PS1 — Primary Prompt

The characters shown as your command prompt. Fully customisable:

PS1='=>'           # changes prompt to =>
PS1="[\u@\h \w]\$"  # shows username@hostname current-dir

PS2 — Secondary Prompt

Shown when a command is incomplete and the shell is waiting for more input. Default is >.

$ PS2="secondary prompt->"
$ echo "this is a
secondary prompt-> test"

04 · PS1 Escape Sequences

Use these inside PS1 to display live info in your prompt:

Escape What it shows
\t Current time — HH:MM:SS
\d Current date — Weekday Month Date
\n Newline
\s Current shell environment
\W Working directory (short name only)
\w Full path of working directory
\u Current user's username
\h Hostname of the current machine
\# Command number — increments with each new command
\$ Shows # if root, $ if normal user

💡 Add your PS1 setting to ~/.profile so it applies automatically every time you log in.

05 · Important Environment Variables

Variable What it holds
HOME Home directory of the current user — default target for cd
PATH Colon-separated list of directories the shell searches for commands
PWD Current working directory (updated by cd)
TERM Display/terminal type
LANG Default system locale (e.g. pt_BR = Brazilian Portuguese)
TZ Timezone — e.g. GMT, AST
UID Numeric user ID of the current user
RANDOM Generates a random integer (0–32767) each time it's referenced
SHLVL Increments each time a new bash instance starts
DISPLAY Identifier for the display X11 programs should use
IFS Internal Field Separator — used by the parser for word splitting
LD_LIBRARY_PATH Directories the dynamic linker searches for shared libraries

Viewing variable values

echo $HOME       # /root
echo $TERM       # xterm
echo $PATH       # /usr/local/bin:/bin:/usr/bin:/home/amrood/bin
echo $DISPLAY    # (blank if no display set)

06 · Quick Reference

Setting a variable:
  NAME="value"          # no spaces around =, no $ sign

Accessing a variable:
  echo $NAME            # use $ prefix to read value

Making a variable available to child processes:
  export NAME           # export it to the environment

Common prompt customisation (add to ~/.profile):
  PS1="[\u@\h \w]\$ "  # user@host current-dir $

Check your PATH:
  echo $PATH

Add a directory to PATH:
  PATH=$PATH:/new/dir
  export PATH

Linux Environment Notes — for revision use