Master Log Debugging with grep, tail, and less: Real‑World Tips

Introduction

A junior teammate struggled to find useful information in error logs, using only simple tail and grep commands that showed a single line without stack context. The author shares a set of practical commands to make log inspection fast and comprehensive.

Core Tool: grep

Scenario 1: View full stack trace

Java stack traces span multiple lines, so a plain grep "NullPointerException" only shows the first line. Use the -A (after) option to display following lines.

grep -A 50 "java.lang.NullPointerException" a.log

Pipe the result to less for paging and navigation.

grep -A 50 "java.lang.NullPointerException" a.log | less

Use ↑/↓ or Page Up/Down to scroll.

Press G to jump to the end of the file.

Search within less with /Exception.

Press q to quit.

Scenario 2: Real‑time monitoring

When an application is running, combine tail -f with grep -A to instantly see new matching lines and their surrounding context.

tail -f a.log | grep -A 50 "java.lang.NullPointerException"

Stop monitoring with Ctrl + C.

Add -i to ignore case.

Scenario 3: Search compressed logs

Log files are often rotated and compressed (e.g., .log.2025-07-02.gz). Use grep -H -A 50 for plain logs and zgrep for compressed ones.

grep -H -A 50 "java.lang.NullPointerException" *.log

zgrep -H -A 50 "java.lang.NullPointerException" *.gz

zgrep

works like grep but reads .gz files without manual decompression.

Scenario 4: Count occurrences

To quickly assess how frequent an exception appears, use the -c (count) option.

grep -c "java.lang.NullPointerException" a.log

grep -c "java.lang.NullPointerException" *.log

Other useful grep options

-B N

: show N lines before the match. -A N: show N lines after the match. -C N: show N lines of context (both before and after). -i: ignore case. -H: print the matching file name. -r: recursively search directories.