Master Log Searching with grep, tail, and less: Real‑World Tips for Developers
This guide walks you through practical command‑line techniques—using grep, tail, less, and related options—to efficiently locate Java exceptions in log files, view surrounding context, monitor live logs, search compressed archives, and quickly count occurrences, all without opening files manually.
Preface
A new colleague struggled with log inspection, using naive commands like tail -f a.log | grep "java.lang.NullPointerException" that only displayed a single matching line, making it hard to see stack traces.
Formal Teaching
The core tool is the grep command. Below are several practical scenarios.
Scenario 1: View full exception stack
Java stack traces span multiple lines, so grep "NullPointerException" shows only the first line. Use the -A (after) option to display the matching line plus N following lines:
# Show the matching line and the next 50 lines
grep -A 50 "java.lang.NullPointerException" a.logIf the output scrolls too fast, pipe it to less for paging:
grep -A 50 "java.lang.NullPointerException" a.log | lessIn the less view you can:
Use ↑/↓ or Page Up/Down to scroll.
Press G to jump to the end of the file.
Enter /Exception to search forward.
Press q to quit.
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