1️⃣ /etc — System Configuration Files

What it contains

/etc stores system-wide configuration files.

These files control how services, applications, and the OS behave.

Important files inside /etc

File	Purpose
/etc/passwd	List of system users
/etc/shadow	Encrypted user passwords
/etc/hosts	Local DNS mapping
/etc/resolv.conf	DNS server configuration
/etc/fstab	Filesystem mount configuration
/etc/ssh/sshd_config	SSH server configuration
/etc/nginx/	Nginx server config
/etc/systemd/	Service manager configs

Example

View DNS configuration:

cat /etc/resolv.conf

Example output:

nameserver 8.8.8.8
nameserver 1.1.1.1

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When developers use /etc

1️⃣ Configure services

Example: Change SSH port

/etc/ssh/sshd_config

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2️⃣ Configure web servers

Example:

/etc/nginx/nginx.conf
/etc/apache2/apache2.conf

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3️⃣ Add host mapping

Example:

/etc/hosts

Add:

127.0.0.1 myapp.local

Used for local development testing.

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2️⃣ /var — Variable Data (Logs, Runtime Data)

What it contains

/var stores data that changes frequently.

Examples:

• logs

• caches

• databases

• mail

• temporary runtime files

Important directories inside /var

Directory	Purpose
/var/log	System logs
/var/lib	Application data
/var/cache	Cached data
/var/spool	Queued jobs (mail, print)
/var/tmp	Temporary files

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Very important for developers

Most debugging happens here.

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Example: Check logs

cd /var/log
ls

Common logs:

Log	Use
/var/log/syslog	System events
/var/log/auth.log	Login attempts
/var/log/nginx/	Nginx logs
/var/log/apache2/	Apache logs

Example:

tail -f /var/log/syslog

Used for real-time debugging.

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DevOps Example

If a Docker container fails, you might check:

/var/log/docker.log

If Nginx returns 500 error

/var/log/nginx/error.log

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3️⃣ /bin — Essential User Commands

What it contains

/bin contains essential binary executable commands required by the system.

These commands are used by all users.

Examples inside /bin

Command	Use
ls	list files
cp	copy files
mv	move files
rm	remove files
cat	display file
echo	print text
mkdir	create directory
chmod	change permissions

Example:

ls /bin

Output example:

bash
cat
cp
ls
mkdir
rm
touch

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When developers use /bin

Mostly every day.

Example:

ls
cp
mv
rm
cat

These commands come from /bin.

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4️⃣ Quick Summary (Very Important for Interviews)

Directory	Stores	Used For
/etc	Configuration files	Change system/service configs
/var	Logs and dynamic data	Debugging and monitoring
/bin	Essential commands	Running basic Linux commands

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5️⃣ Real DevOps Example (Production Debugging)

Imagine your website is down.

Step 1 — Check service config

/etc/nginx/nginx.conf

Step 2 — Check logs

/var/log/nginx/error.log

Step 3 — Run command

systemctl restart nginx

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✅ Key idea

/etc → configuration
/var → logs and runtime data
/bin → commands

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4. /home — User Personal Directory

4

What /home stores

/home contains personal directories for each user.

Example:

/home/lav
/home/ubuntu
/home/devuser

Each user gets their own workspace here.

Example:

ls /home

Output:

lav
ubuntu
developer

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What files exist inside /home

Example:

/home/lav
 ├── Documents
 ├── Downloads
 ├── projects
 ├── .bashrc
 ├── .ssh
 └── .config

Important hidden files

File	Purpose
.bashrc	Shell configuration
.profile	Login environment
.ssh/	SSH keys
.gitconfig	Git configuration

Check hidden files:

ls -a

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Developer Use Cases

1️⃣ Store projects

Example:

/home/lav/projects/my-app

Developers store:

• code

• docker files

• git repositories

Example:

git clone https://github.com/project/app.git

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2️⃣ SSH keys for Git / servers

Location:

/home/lav/.ssh

Example:

id_rsa
id_rsa.pub
known_hosts

Used for:

• GitHub authentication

• server login

Example:

ssh user@server-ip

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3️⃣ Shell configuration

Example:

/home/lav/.bashrc

Used for:

• environment variables

• aliases

Example:

alias k=kubectl

Now you can run:

k get pods

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5. /usr — Installed Software and Libraries

What /usr stores

/usr contains most installed programs and libraries.

It stands for:

Unix System Resources

Most applications installed via apt go here.

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Important folders inside /usr

Directory	Purpose
/usr/bin	user commands
/usr/sbin	system admin commands
/usr/lib	libraries
/usr/local	manually installed software
/usr/share	documentation & shared files

Example:

ls /usr

Output:

bin
lib
local
sbin
share

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Developer Use Cases

1️⃣ Running programs

Most commands live here.

Example:

which python3

Output:

/usr/bin/python3

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2️⃣ Libraries used by applications

Example:

/usr/lib

Contains:

libssl.so
libcrypto.so
libpython.so

Used when programs need shared libraries.

Example:

ldd /usr/bin/docker

Shows libraries used by Docker.

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3️⃣ /usr/local for manual installs

If you install software manually:

Example:

/usr/local/bin

Example install:

/usr/local/bin/kubectl

DevOps engineers often install:

• kubectl

• terraform

• helm

Example:

sudo mv kubectl /usr/local/bin/

Now run:

kubectl get pods

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6. /tmp — Temporary Files

What /tmp stores

/tmp stores temporary files created by programs.

These files are:

• temporary

• automatically deleted after reboot

Example:

/tmp
 ├── tmpfile1
 ├── session123
 └── app-cache

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Example

Check contents:

ls /tmp

Create temporary file:

touch /tmp/testfile

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Developer Use Cases

1️⃣ Application temporary storage

Programs store runtime data here.

Example:

/tmp/app-temp-data

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2️⃣ Testing scripts

Developers test scripts safely.

Example:

nano /tmp/test-script.sh

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3️⃣ Large temporary downloads

Example:

wget file.tar.gz -P /tmp

After installing:

rm -rf /tmp/file.tar.gz

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4️⃣ Debugging programs

Some apps store logs here temporarily.

Example:

/tmp/docker-build.log

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4️⃣ Very Important DevOps Interview Summary

Directory	What it stores	Real use
/etc	configuration files	change service configs
/var	logs & runtime data	debugging
/bin	basic commands	run linux commands
/home	user files	projects & ssh keys
/usr	installed software	programs & libraries
/tmp	temporary files	runtime temporary storage

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5️⃣ Real DevOps Scenario

Example: Application is failing

Step 1 — Check config

/etc/nginx/nginx.conf

Step 2 — Check logs

/var/log/nginx/error.log

Step 3 — Check program binary

/usr/sbin/nginx

Step 4 — Check temporary runtime files

/tmp

Step 5 — Check user project

/home/lav/app

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✅ In DevOps debugging most used directories

/etc
/var/log
/home
/usr/local/bin
/tmp