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Linux Lab 3: Linux Bash Prompt

This lab focuses on user environment customization, shell management, and persistence. It covers critical concepts like changing user shells, managing environment variables, creating aliases, and customizing the bash prompt (PS1).

Table of Contents

Key Concepts

  • Shells: The program that interprets your commands. Common shells include bash (Bourne Again Shell) and sh (Bourne Shell).
  • Environment Variables: Dynamic values that affect the processes or shells on a computer (e.g., HOME, PATH, TERM, PS1).
  • Profile Files: Hidden files in a user's home directory (like .profile and .bashrc) that run every time a user logs in. They are used to make settings persistent.
  • Aliases: Custom shortcuts for longer commands.
  • PS1 (Prompt String 1): The environment variable that defines the appearance of your command line prompt.

Step-by-Step Walkthrough

1. Identifying the Default Shell

Question: What is the default shell for Bob? Answer: /bin/bash

Explanation: You can verify a user's default shell by looking at the /etc/passwd file, which stores essential user account information. 

grep bob /etc/passwd
# Output: bob:x:1000:1000::/home/bob:/bin/bash
The last field (/bin/bash) specifies the shell.

2. Changing the User Shell (usermod)

Task: Change Bob's shell from bash to sh. Command: sudo usermod -s /bin/sh bob

Explanation: * usermod: A command to modify a user account. * -s: The flag to change the login shell. * /bin/sh: The path to the new shell (Bourne Shell). * sudo: Required because modifying user accounts is an administrative task.

3. Terminal Environment Variables (TERM)

Question: What is the value of the environment variable TERM? Answer: xterm-256color

Explanation: The TERM variable tells the system what kind of terminal you are using so it knows how to display colors and handle cursor movements. * To check this value yourself, run: 

echo $TERM

4. Persistent Configuration (.profile)

Task: Understand how to make changes persistent.

Explanation: If you just type alias ll="ls -l" in your terminal, it only lasts until you close that window. To make it permanent ("persistent"), you must add the command to a startup script like ~/.profile or ~/.bashrc. * ~/.profile: Executed for login shells. Good for environment variables. * ~/.bashrc: Executed for non-login interactive shells. Ideally where aliases and prompts go, though .profile is often used for simplicity in these labs.

5. Creating Environment Variables

Task: Create a persistent environment variable PROJECT=MERCURY. Command: echo 'export PROJECT=MERCURY' >> ~/.profile

Explanation: * export: Makes the variable available to child processes (programs you run from the shell). * >>: Appends the line to the end of the file. (Using > would overwrite the entire file!)

6. Understanding the PATH Variable

Question: Which directory is NOT part of the PATH variable? Answer: /opt/caleston-code

Explanation: The PATH variable is a list of directories where the system looks for executable programs. * Standard directories include /usr/bin, /bin, /usr/sbin, /sbin, and /usr/local/bin. * /opt/caleston-code is a custom application directory and would not be in the system's default path unless manually added.

7. Creating Command Aliases

Task: Create a persistent alias named up that runs the uptime command. Command: echo 'alias up="uptime"' >> ~/.profile

Explanation: * Alias: A custom shortcut. Now, typing up will execute uptime. * Adding it to .profile ensures it works every time Bob logs in.

8. Customizing the Bash Prompt (PS1)

Task: Update Bob's prompt to display the date, user, host, and current directory. Format: [Wed Apr 22]bob@caleston-lp10:~$ Command: 

echo 'export PS1="[\$(date +%a\ %b\ %d)]\u@\h:\w\$ "' >> ~/.bashrc
source ~/.bashrc

Explanation: * PS1: The primary prompt string variable. * \u: Username (bob) * \h: Hostname (caleston-lp10) * \w: Working directory (~) * \$: Shell prompt symbol ($ for user, # for root) * $(date +%a\ %b\ %d): This is command substitution. It runs the date command with a specific format (Day Month Date) and puts the result into the prompt. * We escape the $ as \$ inside the echo command so that it writes the literal command into the file, rather than executing it right now and writing a static date. * source ~/.bashrc: This command reloads the configuration file immediately so you can see the change without logging out and back in.

Command Reference

Command Name Purpose Example
usermod User Modify Changes user account properties sudo usermod -s /bin/sh bob
grep Global Regular Expression Print Searches text/files for patterns grep bob /etc/passwd
echo Echo Prints text to screen or file echo $TERM
>> Append Redirect Adds output to the end of a file echo "text" >> file.txt
source Source Executes commands from a file in the current shell source ~/.bashrc
date Date Displays or sets the system date date +%Y-%m-%d
uptime Uptime Shows how long the system has been running uptime