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6. Linux Advanced Commands

6.4 Pipes, Redirection and Here-Documents

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Also in the AWS course: this lesson is the same as 4.4 Pipes and redirection in the AWS course, which is the main version of this page.

Pipe mhanje relay race – ek command cha output pudhchya command cha input. Chhote commands jodun motha kaam – hech Linux che saundarya aahe.

Symbol Meaning Example
| (pipe) Output of left command becomes input of right command ps aux | grep nginx
> Redirect output to file (overwrite) echo "hi" > a.txt
>> Append output to file date >> log.txt
< Take input from file mysql -​u root -​p mydb < backup.​sql
2> Redirect errors find / -​name x 2> errors.​txt
2>&1 Send errors to same place as output ./​script.​sh > out.​log 2>&1
&> Output and errors together (bash) cmd &> all.log
/dev/null "Black hole" — discard cmd > /​dev/​null 2>&1
tee Write to file and screen echo "x" | sudo tee /​etc/​file
&& / || Run next only if success / failure sudo nginx -​t && sudo service nginx reload

Why sudo tee instead of sudo echo > file?

In sudo echo "text" > /etc/file, the redirection > is done by your shell (not root), so it fails with Permission denied. Pipe the text into sudo tee /etc/file instead (or sudo tee -a to append).

Here-documents (used a lot in this book)

A here-doc writes multiple lines into a file in one command — perfect for creating config files by copy-paste. Everything between <<'EOF' and the line containing only EOF is written to the file:

sudo tee /tmp/hello.txt > /dev/null <<'EOF'
Line one
Line two with $HOME not expanded because 'EOF' is quoted
EOF
cat /tmp/hello.txt

Copy-pasting here-docs

Paste the whole block at once, including the final EOF line. The closing EOF must be at the very start of the line with nothing after it.

Ravindra Bagale's Tip

If you confuse > and >>, an important file becomes empty – many students have wiped a config this way. > means overwrite; >> means append to the end. If in doubt, use >> or take a backup first.

Practice task

Save the output of ls -la /etc to a file, append today's date to it, and write a two-line config file into /tmp using sudo tee and a here-doc.

Extended teaching notes

Why this lesson matters

Use this lesson to strengthen systems you own: understand the risk, practise safely in a lab, then write and apply the fix.

Apply the fix across stacks

Same class of bug, same idea in every language.

Java / Spring Boot

Parameterise queries, encode output, CSRF tokens, limited Actuator, dependency scanning.

ASP.NET Core

EF Core / parameterised SQL, Razor encoding, HTTPS, Data Protection.

Node.js / Express

helmet, parameterised SQL, npm audit, avoid eval and shell concat.

Python / Django / Flask / FastAPI

ORM, autoescape, pip-audit, Bandit; never shell=True with user input.

PHP / Laravel

PDO/Eloquent, Blade escaping, composer audit.

Go / Ruby

database/sql placeholders / ActiveRecord; html/template / ERB escaping; govulncheck / Brakeman.

nginx / Apache / Tomcat

Hide versions, TLS, security headers, least-open ports.

MySQL / PostgreSQL / MongoDB / Redis

Private networks, auth on, least-privilege users, no 0.0.0.0/0.

AWS / Azure / GCP

Roles not long-lived keys, private data stores, cloud audit logs on.

Mini case study

Suppose we are on a purple-team exercise for an Indian mid-size company (sample data). Red notes a technique from this lesson against a lab clone. Blue must show the control that stops it, the log that would have fired, and the ticket that hardens production. That is the standard this course trains you for.

Defender's worksheet (do this today)

  1. Write the lesson title, date and the exact lab target you are allowed to use.
  2. List three owned systems (VMs, apps, or cloud accounts) where this lesson applies.
  3. Run one safe check from the lesson on a lab system only. Paste the command and output.
  4. Write a two-sentence fix you would give a teammate.
  5. Name one related public advisory (CERT-In, CISA, NVD, or a vendor bulletin) with its date.

Indian workplace (sample data)

Suppose we are a junior analyst at Infosys, TCS, Wipro or a fintech SOC in Bengaluru/Pune. Turn this lesson into one checklist item with an owner, a due date and a place to store evidence (ticket + screenshot).

Global practice

Compare your notes with OWASP, CIS Benchmarks, NIST CSF/IR guidance or a cloud Well-Architected security question. Write five lines on what overlaps.

Everyday analogy

Explain the risk and the fix to a family member without jargon. If they can repeat the warning, you understand it well enough to teach juniors.

Key terms

  • Control: a safeguard (patch, config, process, detection).
  • Finding: a weakness backed by evidence.
  • Remediation: the fix plus how you verified it.
  • Scope: systems you have written permission to touch.
  • Least privilege: only the access the task needs.
  • Defence in depth: several layers so one failure does not end the story.

Common mistakes

  • Skipping written permission "just for a quick scan".
  • Practising on hostel, office or public targets without authorisation.
  • Collecting tool output without writing the fix.
  • Copying attack steps onto production to "verify". Defenders verify with versions, configs and patches.
  • Publishing screenshots that contain secrets, cookies or internal hostnames.

Recap

  • Evidence, fix, owner, date.
  • Own lab only.
  • Fix > spectacle.