Foundations
Linux is the base operating system for most DevOps workloads.
On this page
Use this page for fast recall. Use Full documentation when you want the complete end-to-end path.
A simple 5-step flow you can follow when using Linux in real work.
1) Setup
Install/run the tool and confirm version. Create a minimal config.
2) Small change
Do one small action end-to-end to prove the workflow.
3) Validate
Check output, logs, and status. Catch mistakes early.
4) Automate
Convert it into a repeatable script or pipeline step.
5) Productionize
Add safety: secrets, rollback, observability, and docs.
These are practical sequences you can copy into your own checklist or runbook.
Goal: Get from unknown outage to a clear hypothesis using evidence.
- Check service status and recent restarts (systemd).
- Inspect logs for the unit and system errors (journalctl, syslog).
- Check resource pressure (CPU, memory, disk).
- Check network listeners and upstream connectivity (ss, curl).
- Capture findings in notes and apply the smallest safe fix.
- Filesystem
- Permissions
- Processes
Learning path (high-level):
- Shell basics
- User and permission management
- Service operations
Check OS details
Command
uname -aList files
Command
ls -laFind current path
Command
pwdShort descriptions and practical intent. Search, filter, copy, and reuse.
Showing 30
uname -aKernel + OS information (fast environment check).
whoamiShow the current user (helps with permission issues).
pwdPrint current working directory.
ls -lahList files with human sizes + hidden files.
cd -Jump back to previous directory.
tree -L 2Show folder tree up to depth 2 (install `tree` if missing).
cat file.txtPrint a file.
less file.txtRead large files safely (search with `/`).
head -n 50 file.txtShow first N lines.
tail -n 50 file.txtShow last N lines.
tail -f /var/log/syslogFollow logs live (Debian/Ubuntu).
grep -R "needle" .Recursive text search in current folder.
rg "needle" .Fast recursive search (ripgrep).
find . -type f -name "*.log"Find files by pattern.
chmod 644 file.txtSet permissions (rw-r--r--) for a file.
chmod -R u+rwX,go-rwx secret_dirMake dir private for owner.
chown -R user:group /pathChange ownership recursively.
idShow uid/gid and groups for current user.
ps aux | headList processes (classic view).
topInteractive process viewer (CPU/mem).
free -hMemory usage summary.
df -hDisk usage by filesystem.
du -sh * | sort -hFind large folders/files in current directory.
ss -lntpShow listening TCP ports and processes.
curl -I https://example.comFetch HTTP headers (quick health check).
dig example.comDNS lookup.
systemctl status nginxCheck service status (systemd).
systemctl restart nginxRestart a service.
journalctl -u nginx -n 200 --no-pagerShow last logs for a systemd unit.
sudo -lShow allowed sudo commands (permission debugging).
Small blocks you can drop into your terminal, config, or runbook.
Find what is listening on a port
Use this when you get 'address already in use'.
bash
sudo ss -lntp | rg ":8080"Basic service log triage
bash
sudo systemctl status nginx
sudo journalctl -u nginx -n 200 --no-pagerWhen things break, follow this order to stay calm and move fast.
- If a command fails: confirm user identity, groups, and file permissions.
- If a service is down: `systemctl status`, then read logs: `journalctl -u <unit>`.
- If disk is full: `df -h` then locate large paths with `du -sh` and logs.
- If a port is unavailable: `ss -lntp` (or `lsof -i :PORT`) to find the process.
The common mistakes that slow people down when using Linux.
- Copy-pasting commands without understanding inputs/outputs and side effects.
- Not documenting defaults (ports, paths, credentials) and then getting stuck in prod.
- Skipping logs and metrics when troubleshooting; always collect evidence first.
Do these tasks in order. You will feel the tool instead of just reading about it.
- Create a folder tree in `~/lab` and practice `pwd`, `ls -lah`, `cd -`, and `find` to locate files.
- Create a file, change permissions with `chmod`, and confirm with `ls -l`.
- Use `journalctl` or `/var/log/*` to find a failing service error line.
Use these to test if you truly understand the basics (and can explain them clearly).
- Explain file permissions and how `chmod 644` differs from `chmod 755`.
- How do you find which process is listening on port 8080?
- What’s the difference between `systemctl` and `service`?