103.1 Work on the command line

Introduction

This lesson is about feeling comfortable in the shell.

The main question is:

How do you move around, run commands, and control your shell environment without getting lost?

LPIC-1 assumes Bash.

What you should be able to do after this lesson:

• Run commands inside and outside PATH.
• Use environment variables.
• Read command history.
• Understand simple quoting.
• Use help tools such as man.

Big Idea: The Shell Is a Command Workspace

When you type a command, the shell decides:

• what command you meant
• where to find it
• how to expand variables
• how to treat quotes

That is why tiny details like $PATH and quoting matter.

Where Am I?

Print current directory

pwd

Example output:

/home/<user>/projects/lpic

What this shows:

• pwd prints the full path of the current working directory
• the exact path depends on the system and the current location

List files

ls
ls -la

Commands Inside and Outside PATH

Check what a command is

type cd
which ls

Example output:

cd is a shell builtin
/usr/bin/ls

What this shows:

• type understands shell builtins such as cd
• which shows where a normal executable command was found

Why both matter

• type understands shell builtins and aliases
• which looks for executable files in PATH

Run a command outside PATH

./script.sh
/usr/local/bin/mytool

One-Line Command Sequences

LPIC also expects comfort with simple one-line command sequences.

Examples:

pwd; ls -la
cd /tmp && ls

Simple idea:

• ; runs the next command after the previous one finishes
• && runs the next command only if the previous one succeeded

Environment Variables

Variables hold values the shell and programs can use.

Show environment

env | head
set | head

Show one variable

echo "$HOME"
echo "$PATH"

Sanitized example output:

/home/<user>
/home/<user>/.local/bin:/usr/local/bin:/usr/bin

What this shows:

• $HOME points to the home directory
• $PATH is the list of directories searched for commands

Create and export

EDITOR=nano
export EDITOR

Remove a variable

unset EDITOR

Simple rule:

• shell variable = local to the shell unless exported
• exported variable = also passed to child processes

Command History

The shell remembers commands.

Show history

history | tail

Rerun from history

• !! = previous command
• !123 = command number 123

History file

Common Bash history file:

~/.bash_history

Quoting: Very Important for Beginners

No quotes

The shell expands spaces and variables normally.

Single quotes

echo '$HOME'

This prints $HOME literally.

Double quotes

echo "$HOME"

This expands the variable but protects spaces better than no quotes.

Simple memory trick:

• single quotes = treat text more literally
• double quotes = still allow variable expansion

Help and Information

Manual pages

man ls
man bash

System information

uname -a

Practice Step by Step

1) Check how the shell sees a command

1. Run: type cd
2. Notice that cd is a shell builtin.
3. Then run: which ls
4. Notice that ls is found as a normal executable.

2) Create and export a variable

1. Run: MYVAR=hello
2. Check it: echo "$MYVAR"
3. Export it: export MYVAR
4. Confirm it appears in: env | grep MYVAR

Cheat Sheet

• pwd = print current directory
• ls -la = list files with details
• type = show how the shell understands a command
• which = find command in PATH
• env = show environment
• export = pass a variable to child processes
• unset = remove a variable
• history = show previous commands
• man = open manual pages
• single quotes = more literal text
• double quotes = allow variable expansion