grep — search for strings

💬 "grep command"

• Concept: Filters and displays only lines containing a specific string from files or command output. It is most commonly used in log analysis.
• Output structure: Prints only matching lines. If there is no match, nothing is displayed.
• Recommended output in practice: Use <code>grep -rn</code> when you need file names and line numbers, and <code>grep -c</code> when you only need to count matches.
• Left column: Not applicable — the original matching line is printed as-is.
• Right column: Not applicable — adding <code>-n</code> prepends line numbers, and <code>-r</code> prepends file paths.

3 essential command combinations

• grep "ERROR" app.log — extract only ERROR lines from a log
• grep -rn "listen" /etc/nginx — search for a string's location across an entire folder
• grep -c "ERROR" app.log — count only matching lines

There are also useful options to know.

• -i — does not distinguish case
• -v — show lines that do not match

💬 "Give me just one example used in real server work"

Scenario: Narrowing down error logs from the time of an incident.

Step 1: Extract error lines

Find ERROR with line numbers.

grep -n "ERROR" app.log

Step 2: Check the count

Count how many errors there are.

grep -c "ERROR" app.log

Step 3: View only recent entries

Check the last errors.

grep "ERROR" app.log | tail -n 20

Actual execution screen

Example screen of searching strings with grep -n and grep -rn.

!grep command execution result: matching log lines and configuration locations

Production server

Source (HTML): https://games.aidebatehub.com/programs/linux/logs/grep.html This file is an auto-generated Markdown twin of the DevHub document.