Day 04 — Linux Basic Commands for DevOps Engineers

🎯 Goal

Build strong command-line fundamentals to navigate systems, inspect files, and operate Linux servers confidently.

These are the commands you will use every single day in DevOps and backend engineering.

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🧠 Mental Model

Linux CLI is your API to the OS.

Commands → Combined → Solve real problems

Each command:

• Does one thing well
• Can be combined with others
• Works on text streams

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1️⃣ Navigation & Directory Management

pwd — Print Working Directory

pwd

What it does:
Shows your current location in the filesystem.

Why it matters:

• Always know where you are before running commands
• Prevent accidental file deletion

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ls — List Files

ls
ls -l
ls -lh
ls -a

What it does:
Lists files and directories.

Options:

Option	Meaning
-l	Detailed list (permissions, size, owner)
-h	Human-readable sizes
-a	Show hidden files

Why it matters:

• Inspect directories
• Check permissions and file sizes

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cd — Change Directory

cd /etc
cd ~
cd ..

What it does:
Moves you between directories.

Why it matters:

• Navigate system structure quickly

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mkdir — Create Directory

mkdir test
mkdir -p parent/child

What it does:
Creates directories.

Why it matters:

• Create project structure
• -p creates nested directories safely

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Delete Directories

rmdir test
rm -r test
rm -rf test

What it does:

• rmdir → removes empty directory
• rm -r → recursive delete
• rm -rf → force delete

⚠️ Dangerous command — use carefully.

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2️⃣ File Creation & Manipulation

touch — Create File

touch file.txt

What it does:
Creates empty file or updates timestamp.

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cp — Copy Files

cp file.txt copy.txt
cp -r dir1 dir2

What it does:
Copies files/directories.

Why it matters:

• Backup files
• Duplicate configs

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mv — Move / Rename

mv file.txt new.txt
mv file.txt /tmp/

What it does:
Moves or renames files.

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3️⃣ Viewing File Content

cat — Display File

cat file.txt

What it does:
Prints file content to terminal.

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zcat — Read Compressed Files

zcat file.gz

What it does:
Reads .gz files without extracting.

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less / more

less file.txt
more file.txt

What it does:
View large files page by page.

👉 less is preferred (scrollable).

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head / tail

head file.txt
tail file.txt
tail -f logfile.log

What it does:

• head → first lines
• tail → last lines
• tail -f → live log monitoring

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4️⃣ Text Processing Commands

wc — Count

wc file.txt
wc -l file.txt

What it does:
Counts lines, words, bytes.

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cut — Extract Columns

cut -d ":" -f1 /etc/passwd

What it does:
Extracts specific fields.

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sort — Sort Data

sort file.txt
sort -n numbers.txt

What it does:
Sorts lines alphabetically or numerically.

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diff — Compare Files

diff file1.txt file2.txt

What it does:
Shows differences between files.

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tee — Output + Save

echo "hello" | tee file.txt

What it does:
Writes output to file AND terminal.

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5️⃣ Links (Important Concept)

Hard Link

ln file.txt hardlink.txt

What it does:
Creates another reference to same file.

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Soft Link (Symlink)

ln -s file.txt symlink.txt

What it does:
Creates pointer to file.

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6️⃣ System Utilities

clear

clear

Clears terminal screen.

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7️⃣ Editing Files — vi

vi file.txt

Modes:

Mode	Action
i	Insert
Esc	Exit insert
:w	Save
:q	Quit
:wq	Save + Quit

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🧪 Day-4 Lab

pwd
ls -l
cd /tmp
 
mkdir testdir
cd testdir
touch file1.txt
echo "hello world" > file1.txt
 
cp file1.txt file2.txt
mv file2.txt renamed.txt
 
cat file1.txt
less file1.txt
head file1.txt
tail file1.txt
 
wc file1.txt
cut -d " " -f1 file1.txt
sort file1.txt
 
ln file1.txt hardlink.txt
ln -s file1.txt symlink.txt
 
diff file1.txt renamed.txt
 
cd ..
rm -r testdir

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🧠 Key Takeaways

• CLI is your main interface
• Files & logs are core to debugging
• Commands are composable
• Small tools → powerful workflows

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✅ Outcome

You can now:

• Navigate Linux systems confidently
• Manage files and directories
• Inspect file contents
• Perform basic data processing
• Understand links and editing

Next → Advanced Linux Commands 🚀