DevOpsLinux Basics

Linux Basics

Master the Linux command line — the foundation of modern DevOps. Covers filesystem navigation, permissions, package management, process control, and shell scripting.

Filesystem Hierarchy

/bin — Essential user binaries
/etc — Configuration files
/var — Variable data (logs, databases)
/usr — User programs and libraries
/proc — Virtual filesystem for processes
/tmp — Temporary files (cleared on boot)
/home — User home directories

File Permissions

Permissions: rwx (read, write, execute) for Owner / Group / Others.

Octal	Owner	Group	Other	Use
755	rwx	r-x	r-x	Executable (common for scripts)
644	rw-	r--	r--	Regular file
600	rw-	---	---	Private file (SSH keys)
700	rwx	---	---	Private directory

Essential Commands

ls -laList all files with permissions, owner, size, date

cd /pathChange directory to specified path

pwdPrint working directory

grep 'pattern' fileSearch for pattern in file

awk '{print $1}' filePrint first column of each line

sed 's/old/new/g' fileReplace text across file

chmod 755 fileSet rwxr-xr-x permissions

chown user:group fileChange file owner and group

ps auxList all running processes

kill -9 PIDForce kill a process by PID

top / htopReal-time process monitoring

df -hDisk space usage in human-readable format

Package Management

Debian/Ubuntu (apt):
apt update && apt install nginx
RHEL/CentOS (yum/dnf):
yum install -y nginx
Arch (pacman):
pacman -S nginx
Alpine (apk):
apk add nginx

Process Management

ps aux — Snapshot of all processes
top — Real-time process viewer
kill -15 PID — Graceful termination
kill -9 PID — Force kill
systemctl start nginx — systemd service mgmt
journalctl -u nginx — View service logs
nice -n -10 ./app — Set process priority

Shell Scripting

#!/bin/bash
# Backup script with rotation
BACKUP_DIR="/var/backups"
SOURCE_DIR="/var/www"
RETENTION_DAYS=7

# Create timestamped backup
tar -czf "$BACKUP_DIR/backup-$(date +%Y%m%d).tar.gz" "$SOURCE_DIR"

# Remove backups older than retention period
find "$BACKUP_DIR" -name "*.tar.gz" -mtime +$RETENTION_DAYS -delete

# Log the operation
echo "[$(date)] Backup completed" >> /var/log/backup.log

Variables
NAME="value"; echo $NAME

Conditionals
if [ -f "$file" ]; then ...

Loops
for i in {1..10}; do ... done

Networking & Storage

Network Commands

ss -tuln — Listening ports
ip addr — IP configuration
curl -v http://... — HTTP debugging
nc -vz host 80 — Port connectivity

Disk & Filesystem

df -h — Disk usage
du -sh * — Directory sizes
fdisk -l — Partition table
mount /dev/sda1 /mnt — Mount filesystem

Interview Questions

Q1: What is the difference between a hard link and a symbolic link?

A hard link is a direct reference to the inode — it shares the same data blocks as the original file. Deleting the original does not remove the data. A symbolic link (symlink) is a pointer to the filename; if the original is deleted, the symlink breaks. Hard links cannot cross filesystems or link to directories.

Q2: Explain the Linux boot process.

1) BIOS/UEFI performs POST and loads the bootloader. 2) GRUB loads the kernel into memory. 3) Kernel initializes hardware and mounts the initial RAM disk (initramfs). 4) systemd (or init) starts as PID 1 and launches target units. 5) System services and getty are started, presenting the login prompt.

Q3: How do you troubleshoot a high CPU process in Linux?

Use `top` or `htop` to identify the offending PID. Check `strace -p PID` for system calls. Use `perf top` for profiling. For Java apps, use `jstack` to dump thread stacks. Common fixes include restarting the service, adding resource limits via systemd or ulimit, or optimizing the application code.

Q4: What is the role of cgroups and namespaces in Linux containers?

Namespaces provide isolation — they make each container see its own PID, network, mount, and UTS namespace. Cgroups (control groups) limit and account resource usage (CPU, memory, disk I/O). Together they form the foundation of Linux containerization used by Docker and Podman.