Linux Globbing — FULL Exercise Set

0️⃣ Setup (Run Once)

Create a safe practice directory.

mkdir ~/globbing-lab
cd ~/globbing-lab
touch file1.txt file2.txt file10.txt fileA.txt fileB.log fileC.log
touch app.log app1.log app2.log error.log
mkdir dir1 dir2 dirA

---

1️⃣ * — Match ANY number of characters

Concept

Matches zero or more characters

Exercises

ls *
ls *.txt
ls *.log
ls file*
ls app*

Expected understanding

• * expands before command runs
• Matches filenames, not file content

---

2️⃣ ? — Match EXACTLY ONE character

Concept

Matches one and only one character

Exercises

ls file?.txt
ls app?.log
ls dir?

Try and think

ls file??.txt

Why does file10.txt match?

---

3️⃣ [abc] — Match ONE character from a set

Concept

Matches any ONE character inside brackets

Exercises

ls file[12].txt
ls file[A-Z].txt
ls dir[12]

Practical DevOps usage

rm app[12].log

Deletes app1.log and app2.log

---

4️⃣ [a-z] — Match a RANGE

Concept

Alphabetical or numeric range

Exercises

ls file[a-z].txt
ls file[0-9].txt
ls dir[a-z]

---

5️⃣ [!abc] — NEGATION (NOT match)

Concept

Matches anything NOT in brackets

Exercises

ls file[!1].txt
ls file[!0-9].txt
ls *.log
ls [!a]*.log

---

6️⃣ {} — Brace Expansion (NOT globbing, but related)

⚠️ Happens before globbing

Exercises

touch {dev,stage,prod}.env
ls *.env

Multiple patterns at once:

ls file{1,2}.txt

---

7️⃣ Combine Globs (Real-world patterns)

Exercises

ls file[0-9]*.txt
ls app?.log
ls *[A-Z].*

---

8️⃣ Recursive Globbing ** (Advanced)

Enable first

shopt -s globstar

Create nested structure

mkdir -p logs/2024/app
touch logs/2024/app/app.log

Exercise

ls **/*.log

---

9️⃣ Hidden Files (.*)

Create hidden files

touch .env .gitignore .bashrc

Exercises

ls .*
ls -a

⚠️ * does NOT match hidden files by default

---

🔟 Globbing with Commands (Critical DevOps Skill)

Copy

cp *.log /tmp/

Delete safely

rm -i file*.txt

Count files

ls *.log | wc -l

---

1️⃣1️⃣ Quoting DISABLES Globbing (Important)

Exercise

echo *.log
echo "*.log"

Explain:

• Quotes prevent shell expansion

---

1️⃣2️⃣ When GLOB DOES NOT MATCH

Try

ls nomatch*

Enable safer behavior

shopt -s nullglob
ls nomatch*

---

1️⃣3️⃣ Exam / Interview Questions (Very Common)

Q1

What is the difference between * and ??

Q2

Why does rm *.log sometimes delete too much?

Q3

Why doesn’t ls * show .env?

Q4

Difference between {} and []?

---

1️⃣4️⃣ Mini Lab (Assignment)

1. Delete all .log files except error.log
2. List only files ending with numbers
3. Copy only file1.txt and file2.txt using ONE command
4. List everything except .txt files

---

1️⃣5️⃣ Real DevOps Usage Examples

kubectl logs pod-* 
aws s3 rm s3://bucket/logs/2024/*
scp app*.jar server:/opt/apps/

---

✅ Summary Table

Pattern	Meaning
*	Any characters
?	One character
[abc]	One from set
[a-z]	Range
[!abc]	Not in set
{a,b}	Expansion
**	Recursive
.*	Hidden files

find Command — ? Meaning & Exercises (Ubuntu)

1️⃣ What does ? mean in find?

👉 ? is a wildcard that matches EXACTLY ONE character
It is used inside -name or -iname.

Important:

• ? is evaluated by find, not by the shell
• It matches one character only

---

2️⃣ Setup (Run Once)

mkdir ~/find-lab
cd ~/find-lab
touch file1.txt file2.txt file10.txt fileA.txt fileB.log
mkdir dir1 dir2 dir10

---

3️⃣ Basic find with ?

Find files with ONE character after file

find . -name "file?.txt"

✔ Matches:

• file1.txt
• file2.txt
• fileA.txt

❌ Does NOT match:

• file10.txt (because 10 = two characters)

---

4️⃣ Compare ? vs * (Very Important)

Using ?

find . -name "file?.txt"

Using *

find . -name "file*.txt"

Difference:

• ? → exactly 1 character
• * → any number of characters

---

5️⃣ ? with directories

find . -type d -name "dir?"

✔ Matches:

• dir1
• dir2

❌ Does NOT match:

• dir10

---

6️⃣ Multiple ? characters

find . -name "file??.txt"

✔ Matches:

• file10.txt

Explanation:

• ?? = exactly two characters

---

7️⃣ Case-Insensitive search (-iname)

find . -iname "file?.txt"

Matches:

• fileA.txt
• filea.txt (if exists)

---

8️⃣ Real DevOps-style exercises

1️⃣ Find log files with one-digit version

find /var/log -name "app?.log"

---

2️⃣ Delete files carefully using ?

find . -name "file?.txt" -delete

⚠ Deletes only:

• file1.txt
• file2.txt
• fileA.txt

---

3️⃣ Count files matching ?

find . -name "file?.txt" | wc -l

---

4️⃣ Combine ? with type

find . -type f -name "file?.*"

---

9️⃣ Common Mistake (Interview Question)

❌ WRONG:

find . -name file?.txt

✔ CORRECT:

find . -name "file?.txt"

Why?

• Without quotes, shell may expand ? before find runs

---

🔟 Interview Questions

Q1: Difference between ? in ls and find?
Answer:

• ls → shell expands
• find → find evaluates pattern

Q2: Why does file10.txt not match file?.txt?
Answer:
Because ? matches exactly one character

---

✅ Summary

Pattern	Meaning
?	Exactly one character
??	Exactly two characters
*	Any number of characters

head and tail Commands — Full Explanation + Exercises

1️⃣ What is head?

👉 head shows the first lines of a file.

Default behavior

head file.txt

✔ Shows first 10 lines

---

2️⃣ What is tail?

👉 tail shows the last lines of a file.

Default behavior

tail file.txt

✔ Shows last 10 lines

---

3️⃣ Setup (Run Once)

mkdir ~/head-tail-lab
cd ~/head-tail-lab
seq 1 50 > numbers.txt

This file has 50 lines.

---

4️⃣ head Exercises

Show first 5 lines

head -n 5 numbers.txt

Show first 1 line

head -n 1 numbers.txt

Shortcut

head -5 numbers.txt

---

5️⃣ tail Exercises

Show last 5 lines

tail -n 5 numbers.txt

Show last line only

tail -n 1 numbers.txt

---

6️⃣ tail -f (VERY IMPORTANT for DevOps)

👉 Follow a file in real time

Terminal 1

tail -f numbers.txt

Terminal 2

echo "new log line" >> numbers.txt

You will see output instantly.

Used for:

• Logs
• Kubernetes
• Application debugging

---

7️⃣ tail -n +NUMBER (Start from line)

tail -n +10 numbers.txt

✔ Shows from line 10 till end

---

8️⃣ Combine head and tail

Get line 10 only

head -n 10 numbers.txt | tail -n 1

Get lines 10–15

head -n 15 numbers.txt | tail -n 6

---

9️⃣ Real DevOps Log Examples

View last 50 lines of syslog

tail -n 50 /var/log/syslog

Follow Docker logs

docker logs -f container_name

Follow Kubernetes pod logs

kubectl logs -f pod-name

---

🔟 Common Interview Questions

Q1

What is difference between tail and tail -f?

Answer:

• tail → static output
• tail -f → live streaming

---

Q2

How to get only the first line of a file?

head -n 1 file.txt

---

Q3

How to get last line only?

tail -n 1 file.txt

---

1️⃣1️⃣ Mini Lab (Homework)

1. Create a file with 100 lines
2. Print first 20 lines
3. Print last 15 lines
4. Show line number 50
5. Follow file while adding new lines

---

✅ Summary Table

Command	Purpose
head	First lines
head -n N	First N lines
tail	Last lines
tail -n N	Last N lines
tail -f	Follow live logs
tail -n +N	Start from line N

wc (Word Count) — Full Explanation + Exercises

1️⃣ What is wc?

👉 wc = word count
Used to count:

• lines
• words
• characters / bytes

---

2️⃣ wc Output Explained

wc file.txt

Output format:

LINES  WORDS  BYTES  filename

Example:

50  50  141  file.txt

---

3️⃣ Setup (Run Once)

mkdir ~/wc-lab
cd ~/wc-lab
seq 1 20 > numbers.txt
echo "hello linux devops world" > text.txt

---

4️⃣ wc -l — Line Count (Most Used)

wc -l numbers.txt

✔ Counts number of lines

---

5️⃣ wc -w — Word Count

wc -w text.txt

Counts words separated by spaces

---

6️⃣ wc -c — Byte Count

wc -c text.txt

Counts bytes (includes newline)

---

7️⃣ wc -m — Character Count

wc -m text.txt

Better for multi-byte characters (UTF-8)

---

8️⃣ Using wc with Pipes (Very Important)

Count files

ls | wc -l

Count log lines

tail -n 100 /var/log/syslog | wc -l

Count matching lines

grep error /var/log/syslog | wc -l

---

9️⃣ Real DevOps Use Cases

1️⃣ Count running pods

kubectl get pods | wc -l

2️⃣ Count files in directory

find . -type f | wc -l

3️⃣ Count users

cat /etc/passwd | wc -l

---

🔟 Common Mistakes

❌ Thinking lc exists
✔ Correct is:

wc -l

❌ Forgetting pipes

ls wc -l   # wrong
ls | wc -l # correct

---

1️⃣1️⃣ Mini Lab (Homework)

1. Count lines in /etc/passwd
2. Count words in a file you create
3. Count number of .log files
4. Count how many users have /bin/bash
5. Count errors in syslog

---

1️⃣2️⃣ Interview Questions

Q1: Difference between wc -c and wc -m
Answer:

• -c → bytes
• -m → characters

Q2: How to count output lines of a command?
Answer:
Use pipe:

command | wc -l

---

✅ Summary Table

Option	Meaning
wc	Lines, words, bytes
wc -l	Line count
wc -w	Word count
wc -c	Byte count
wc -m	Character count

File Size Determination — Exercises (Ubuntu)

1️⃣ Setup (Run Once)

mkdir ~/file-size-lab
cd ~/file-size-lab
echo "hello world" > small.txt
seq 1 1000 > numbers.txt
dd if=/dev/zero of=bigfile.bin bs=1M count=5
mkdir logs
cp numbers.txt logs/

---

2️⃣ ls -lh — Human-readable size (Most common)

Exercise

ls -lh

✔ Shows file sizes in:

• B
• KB
• MB
• GB

Questions

• Which file is the largest?
• What size is bigfile.bin?

---

3️⃣ du -h — Disk usage (Real storage used)

Exercise

du -h

File only

du -h small.txt

Directory size

du -h logs/

Summary only

du -sh logs/

✔ du = how much disk space is actually used

---

4️⃣ stat — Exact size in bytes (Interview favorite)

Exercise

stat numbers.txt

Look for:

Size: 3893

✔ Most accurate size info

---

5️⃣ wc -c — Size in bytes (Content-based)

Exercise

wc -c small.txt

✔ Counts bytes including newline

Compare with:

stat small.txt

---

6️⃣ Compare Commands (Important Concept)

Command	What it shows
ls -lh	Display size
du -h	Disk usage
stat	Exact metadata
wc -c	Content size

---

7️⃣ Find Files Bigger Than X Size

Bigger than 1MB

find . -type f -size +1M

Smaller than 1KB

find . -type f -size -1k

---

8️⃣ Sort Files by Size

Largest first

ls -lhS

Smallest first

ls -lhrS

---

9️⃣ Real DevOps Exercises

1️⃣ Find large log files

find /var/log -type f -size +100M

2️⃣ Check directory disk usage

du -sh /var/log

3️⃣ Monitor growing file

watch -n 1 ls -lh bigfile.bin

---

🔟 Common Mistakes (Interview)

❌ Using ls to check disk usage
✔ Use du for actual disk usage

❌ Confusing bytes vs blocks
✔ stat is the source of truth

---

1️⃣1️⃣ Mini Lab (Homework)

1. Create a file of 10MB
2. Show its size using 4 different commands
3. Find files larger than 1MB
4. Sort files by size
5. Check disk usage of a directory

---

1️⃣2️⃣ Interview Questions

Q1: Difference between ls -lh and du -h?
Answer:

• ls → file size
• du → disk blocks used

Q2: Which command gives exact size in bytes?
Answer:
stat

---

✅ Summary Cheat Sheet

ls -lh file
du -sh file_or_dir
stat file
wc -c file
find . -size +10M

Output Redirection Exercises — > and >>

1️⃣ What are > and >>?

Operator	Meaning
>	Redirect output and overwrite file
>>	Redirect output and append to file

---

2️⃣ Setup (Run Once)

mkdir ~/redirect-lab
cd ~/redirect-lab

---

3️⃣ > — Overwrite Redirection

Exercise 1

echo "Hello DevOps" > output.txt
cat output.txt

✔ File created

---

Exercise 2 (Overwrite)

echo "Linux is powerful" > output.txt
cat output.txt

❗ Previous content is deleted

---

4️⃣ >> — Append Redirection

Exercise 3

echo "Line 1" >> output.txt
echo "Line 2" >> output.txt
cat output.txt

✔ Content is added, not replaced

---

5️⃣ Real Difference Test (Very Important)

echo "START" > test.txt
echo "APPEND" >> test.txt
echo "OVERWRITE" > test.txt
cat test.txt

Expected result:

OVERWRITE

---

6️⃣ Redirect Command Output

Save ls output

ls > files.txt

Append more output

ls /etc >> files.txt

---

7️⃣ Redirect Errors (stderr)

Error without redirect

ls not_exist

Redirect error

ls not_exist 2> error.txt

Append error

ls not_exist 2>> error.txt

---

8️⃣ Redirect Both Output and Error

ls /etc not_exist > all.txt 2>&1

---

9️⃣ Discard Output (Production Trick)

ls not_exist > /dev/null
ls not_exist 2> /dev/null
ls not_exist > /dev/null 2>&1

---

🔟 Combine with wc, tail, grep

Count lines and save

ls /etc | wc -l > count.txt

Append logs

date >> app.log

---

1️⃣1️⃣ Real DevOps Examples

Log rotation simulation

echo "$(date) App started" >> app.log

Save command result in script

df -h > disk_report.txt

---

1️⃣2️⃣ Common Mistakes (Interview)

❌ Using > instead of >> → data loss
❌ Forgetting 2> for errors
❌ Overwriting log files accidentally

---

1️⃣3️⃣ Mini Lab (Homework)

1. Create report.txt using >
2. Append 5 lines using >>
3. Save ls /etc output
4. Save errors separately
5. Discard output using /dev/null

---

1️⃣4️⃣ Interview Questions

Q1: Difference between > and >>
Answer:

• > overwrites
• >> appends

Q2: How to redirect error only?
Answer:

2>

---

✅ Summary Cheat Sheet

command > file
command >> file
command 2> error.txt
command > all.txt 2>&1
command > /dev/null

tee Command — Exercises & Explanation

1️⃣ What is tee?

👉 tee reads from standard input and writes to:

• the screen
• and one or more files at the same time

Think of it as a splitter.

command | tee file.txt

---

2️⃣ Setup (Run Once)

mkdir ~/tee-lab
cd ~/tee-lab

---

3️⃣ Basic tee Exercise

Write output to screen AND file

echo "Hello DevOps" | tee output.txt

Check:

cat output.txt

---

4️⃣ tee -a (Append Mode)

Overwrite vs append

echo "Line 1" | tee output.txt
echo "Line 2" | tee -a output.txt

✔ -a = append
❌ Without -a = overwrite

---

5️⃣ tee vs > (Important Difference)

Using >

ls > files.txt

❌ No output on screen

Using tee

ls | tee files.txt

✔ Output shown AND saved

---

6️⃣ Multiple Files with tee

echo "Shared log line" | tee file1.log file2.log

---

7️⃣ tee in Pipelines

Count and save

ls | tee list.txt | wc -l

Analyze logs

grep error /var/log/syslog | tee errors.txt | wc -l

---

8️⃣ sudo + tee (VERY IMPORTANT)

Problem

sudo echo "text" > /root/test.txt   # fails

Solution

echo "text" | sudo tee /root/test.txt

Append with sudo

echo "more text" | sudo tee -a /root/test.txt

---

9️⃣ Real DevOps Use Cases

Save disk report

df -h | tee disk_report.txt

Capture install logs

sudo apt update | tee update.log

Kubernetes debugging

kubectl logs pod-name | tee pod.log

---

🔟 Common Mistakes

❌ Forgetting -a → overwrites logs
❌ Using > with sudo
❌ Thinking tee replaces pipes

---

1️⃣1️⃣ Mini Lab (Homework)

1. Save ls output and still see it
2. Append 5 lines to a log file
3. Write output to two files
4. Save error logs and count them
5. Write to a protected file using sudo

---

1️⃣2️⃣ Interview Questions

Q1: Why use tee instead of >?
Answer:
To see output and save it at the same time.

Q2: Why does sudo echo text > file fail?
Answer:
Redirection happens before sudo.

---

✅ Summary Cheat Sheet

command | tee file.txt
command | tee -a file.txt
command | tee file1 file2
command | sudo tee /path/file

uniq and grep — Exercises & Explanation

1️⃣ What is grep?

👉 grep searches for patterns in text.

Basic syntax:

grep PATTERN file

---

2️⃣ What is uniq?

👉 uniq removes or analyzes duplicate lines
⚠️ Important: uniq works only on adjacent lines → usually needs sort first.

---

3️⃣ Setup (Run Once)

mkdir ~/grep-uniq-lab
cd ~/grep-uniq-lab

cat <<EOF > logs.txt
error disk full
info service started
warning cpu high
error disk full
error network down
info service started
EOF

---

4️⃣ grep — Basic Exercises

Find lines containing error

grep error logs.txt

---

Case-insensitive search

grep -i ERROR logs.txt

---

Show line numbers

grep -n error logs.txt

---

Count matching lines

grep -c error logs.txt

---

Search multiple patterns

grep -E "error|warning" logs.txt

---

5️⃣ grep — Inverted Match (-v)

Show lines WITHOUT info

grep -v info logs.txt

Used a lot in log filtering.

---

6️⃣ grep with Pipes (Real Usage)

Find errors in last 100 log lines

tail -n 100 /var/log/syslog | grep error

Count errors

grep error logs.txt | wc -l

---

7️⃣ uniq — Basic (Important Rule)

❌ Wrong (no effect)

uniq logs.txt

Why?
👉 Duplicates are not adjacent

---

8️⃣ sort + uniq (Correct Way)

Remove duplicates

sort logs.txt | uniq

---

Count duplicate lines

sort logs.txt | uniq -c

Output example:

2 error disk full
2 info service started
1 warning cpu high

---

9️⃣ uniq Options (Very Important)

Show only duplicates

sort logs.txt | uniq -d

Show only unique (non-duplicate)

sort logs.txt | uniq -u

---

🔟 Combine grep + uniq

Unique error messages

grep error logs.txt | sort | uniq

---

Count unique errors

grep error logs.txt | sort | uniq -c

---

1️⃣1️⃣ Real DevOps Examples

Count unique IPs

grep "GET" access.log | awk '{print $1}' | sort | uniq -c

Unique Kubernetes errors

kubectl logs pod-name | grep error | sort | uniq -c

---

1️⃣2️⃣ Common Mistakes (Interview)

❌ Using uniq without sort
❌ Forgetting -i for case-insensitive search
❌ Using grep instead of grep -v

---

1️⃣3️⃣ Mini Lab (Homework)

1. Find all error lines
2. Count how many times each error appears
3. Show unique log messages
4. Exclude info messages
5. Find warnings OR errors

---

1️⃣4️⃣ Interview Questions

Q1: Why doesn’t uniq remove all duplicates?
Answer:
Because it works only on adjacent lines

---

Q2: Difference between grep error and grep -v error?
Answer:

• grep error → matches
• grep -v error → excludes

---

✅ Summary Cheat Sheet

grep pattern file
grep -i pattern file
grep -v pattern file
grep -c pattern file

sort file | uniq
sort file | uniq -c
sort file | uniq -d

PATH Environment Variable — Explanation & Exercises

1️⃣ What is PATH?

👉 PATH is a colon-separated list of directories.
When you type a command, Linux searches these directories in order.

Example:

echo $PATH

Output looks like:

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

---

2️⃣ Why PATH Matters (Real Meaning)

When you run:

ls

Linux actually checks:

/usr/local/sbin/ls
/usr/local/bin/ls
/usr/sbin/ls
/usr/bin/ls   ✅ found here

---

3️⃣ Setup (Run Once)

mkdir -p ~/path-lab/bin
cd ~/path-lab/bin

Create a custom command:

echo -e '#!/bin/bash\necho "Hello from custom command"' > hello
chmod +x hello

---

4️⃣ Command NOT in PATH

Try running:

hello

❌ Command not found

Run with full path:

~/path-lab/bin/hello

✔ Works

---

5️⃣ Temporarily Add to PATH

export PATH=$PATH:$HOME/path-lab/bin

Now run:

hello

✔ Works without full path

⚠️ This change lasts only for this terminal session

---

6️⃣ Check Where Command Comes From

which ls

which hello

Shows exact path of the command being used.

---

7️⃣ Order of PATH (VERY IMPORTANT)

Create another command:

echo -e '#!/bin/bash\necho "Fake ls command"' > ls
chmod +x ls

Run:

ls

Why didn’t it run your fake one?

👉 Because your directory is after /usr/bin

---

8️⃣ Put Directory FIRST in PATH (Danger Demo)

export PATH=$HOME/path-lab/bin:$PATH

Now:

ls

⚠️ Your fake ls runs!

👉 Security risk — interview favorite

---

9️⃣ Permanent PATH Change (User Only)

Edit:

nano ~/.bashrc

Add:

export PATH=$PATH:$HOME/path-lab/bin

Reload:

source ~/.bashrc

---

🔟 System-wide PATH (Admins)

/etc/profile
/etc/environment

⚠️ Requires sudo
⚠️ Affects all users

---

1️⃣1️⃣ PATH with Scripts (DevOps Use)

Script without path:

#!/bin/bash
aws s3 ls

If AWS CLI not in PATH → script fails

Fix:

which aws

Or use full path:

/usr/bin/aws s3 ls

---

1️⃣2️⃣ Common Mistakes (Interview)

❌ Overwriting PATH:

export PATH=/my/bin   # WRONG

✔ Correct:

export PATH=$PATH:/my/bin

❌ Putting . in PATH (security risk)

---

1️⃣3️⃣ Mini Lab (Homework)

1. Print your PATH
2. Create a custom command
3. Run it using full path
4. Add directory to PATH
5. Make change permanent
6. Explain PATH order

---

1️⃣4️⃣ Interview Questions

Q1: What is PATH?
Answer:
A list of directories where shell looks for commands

Q2: Why is PATH order important?
Answer:
First match wins → security impact

Q3: Difference between temporary and permanent PATH?
Answer:

• Temporary → export
• Permanent → .bashrc

---

✅ Summary Cheat Sheet

echo $PATH
export PATH=$PATH:/new/path
which command
source ~/.bashrc

Bash Variables & Environment Variables

Full Exercise Set for DevOps Engineers

---

1️⃣ Bash Variable (Local Variable)

What it is

• Exists only in current shell
• NOT available to child processes

Exercise

NAME="devops"
echo $NAME

❌ Space is not allowed:

NAME = devops   # wrong

---

2️⃣ Variable Scope Test (Very Important)

MYVAR="local"
bash
echo $MYVAR

❌ Output is empty
👉 Local variables do not pass to child shells

Exit:

exit

---

3️⃣ Environment Variable (export)

What it is

• Available to child processes
• Used heavily in DevOps tools

Exercise

export ENV="production"
echo $ENV

Test in child shell:

bash
echo $ENV
exit

✔ Works

---

4️⃣ Difference: Bash vs Env Variable

Type	Child shell
Local variable	❌ No
Environment variable	✅ Yes

---

5️⃣ List Variables

All shell variables

set

Only environment variables

env

or

printenv

---

6️⃣ Unset Variables

TEMP=123
unset TEMP
echo $TEMP

✔ Empty output

---

7️⃣ Read-only Variables

readonly VERSION="1.0"
VERSION="2.0"

❌ Permission denied

---

8️⃣ Command Substitution (VERY IMPORTANT)

Store command output in variable

DATE=$(date)
echo $DATE

Old style (still seen):

DATE=`date`

---

9️⃣ Variable Expansion Types

Default value

echo ${APP:-nginx}

Assign default

echo ${APP:=nginx}

Error if unset

echo ${APP:?APP is not set}

---

🔟 Positional Variables (Scripts)

Create script:

nano args.sh

#!/bin/bash
echo "Script: $0"
echo "First: $1"
echo "Second: $2"
echo "All: $@"

Run:

chmod +x args.sh
./args.sh dev prod

---

1️⃣1️⃣ Special Variables (INTERVIEW FAVORITE)

Variable	Meaning
$0	Script name
$1..$9	Arguments
$#	Number of args
$@	All args
$?	Exit code
$$	PID
$!	Last background PID

Exercise

ls
echo $?

---

1️⃣2️⃣ Global Environment Files (DevOps MUST KNOW)

File	Scope
~/.bashrc	User, interactive
~/.profile	Login shell
/etc/environment	System-wide
/etc/profile	System-wide

---

1️⃣3️⃣ Permanent Environment Variable

User-level

echo 'export APP_ENV=prod' >> ~/.bashrc
source ~/.bashrc

---

1️⃣4️⃣ Environment Variables in Scripts

#!/bin/bash
echo "Deploying to $APP_ENV"

Run:

APP_ENV=stage ./script.sh

---

1️⃣5️⃣ Inline Environment Variable (VERY COMMON)

DB_HOST=localhost DB_PORT=5432 ./app.sh

✔ Variable exists only for this command

---

1️⃣6️⃣ .env File (Real DevOps Pattern)

Create:

nano .env

DB_USER=admin
DB_PASS=secret

Load:

export $(cat .env | xargs)

Use:

echo $DB_USER

---

1️⃣7️⃣ PATH Variable (Critical)

echo $PATH
export PATH=$PATH:/custom/bin

⚠ Never overwrite PATH

---

1️⃣8️⃣ DevOps Tool Examples

Docker

export DOCKER_BUILDKIT=1

Kubernetes

export KUBECONFIG=~/.kube/config

AWS

export AWS_ACCESS_KEY_ID=xxx
export AWS_SECRET_ACCESS_KEY=yyy
export AWS_DEFAULT_REGION=us-east-1

---

1️⃣9️⃣ Security Best Practices (IMPORTANT)

❌ Never hardcode secrets:

DB_PASS=admin123

✔ Use env vars or secret managers

✔ .env → add to .gitignore

---

2️⃣0️⃣ Mini DevOps Lab (Homework)

1. Create local variable and test scope
2. Export env variable and test child shell
3. Create script using positional parameters
4. Use $? to check command success
5. Load variables from .env
6. Set permanent env variable
7. Use inline env variable

---

2️⃣1️⃣ Interview Questions (REAL)

Q: Difference between set and env
A:

• set → shell variables
• env → environment variables

Q: Why use env vars in DevOps?
A:
For configuration, secrets, portability

Q: How do you pass env vars to Docker/K8s?
A:
Environment variables or secret managers

---

✅ Final Cheat Sheet

VAR=value
export VAR=value
unset VAR
readonly VAR=value
VAR=$(command)
echo $?
env
set