Essential Linux & Java Debugging Tools Every Engineer Should Know

1. Introduction

In daily work we often encounter difficult problems; various tools play a crucial role in solving them. This note serves both as a personal reference and a shared resource for colleagues.

2. Linux Command Tools

2.1 tail

Commonly used tail -f for real‑time log monitoring.

tail -300f shopbase.log   # show last 300 lines and follow

2.2 grep

grep forest f.txt                     # search in a single file</code><code>grep forest f.txt cpf.txt            # search in multiple files</code><code>grep 'log' /home/admin -r -n          # recursive search</code><code>cat f.txt | grep -i shopbase          # case‑insensitive</code><code>grep 'shopbase' /home/admin -r -n --include *.{vm,java}   # include specific extensions</code><code>grep 'shopbase' /home/admin -r -n --exclude *.{vm,java}   # exclude specific extensions</code><code>seq 10 | grep 5 -A 3                # show 3 lines after match</code><code>seq 10 | grep 5 -B 3                # show 3 lines before match</code><code>seq 10 | grep 5 -C 3                # show 3 lines around match</code><code>cat f.txt | grep -c 'SHOPBASE'      # count occurrences

2.3 awk

Basic usage:

awk '{print $4,$6}' f.txt</code><code>awk '{print NR,$0}' f.txt cpf.txt</code><code>awk '{print FNR,$0}' f.txt cpf.txt</code><code>awk '{print FNR,FILENAME,$0}' f.txt cpf.txt</code><code>awk '{print FILENAME,"NR="NR,"FNR="FNR,"$"NF"="$NF}' f.txt cpf.txt</code><code>echo 1:2:3:4 | awk -F: '{print $1,$2,$3,$4}'

Pattern matching:

awk '/ldb/ {print}' f.txt                     # match ldb</code><code>awk '!/ldb/ {print}' f.txt                    # not match ldb</code><code>awk '/ldb/ && /LISTEN/ {print}' f.txt        # match both</code><code>awk '$5 ~ /ldb/ {print}' f.txt               # match column 5

Built‑in variables: NR: total record number (line count) across all input files. FNR: record number within the current file. NF: number of fields in the current record.

2.4 find

sudo -u admin find /home/admin /tmp /usr -name *.log          # search multiple directories</code><code>find . -iname *.txt                                         # case‑insensitive name</code><code>find . -type d                                            # list directories</code><code>find /usr -type l                                         # list symbolic links</code><code>find /usr -type l -name "z*" -ls                         # detailed info of links</code><code>find /home/admin -size +250000k                         # files larger than 250 MB</code><code>find /home/admin -perm 777 -exec ls -l {} ;               # find by permission</code><code>find /home/admin -atime -1                                 # accessed within 1 day</code><code>find /home/admin -ctime -1                                 # status changed within 1 day</code><code>find /home/admin -mtime -1                                 # modified within 1 day</code><code>find /home/admin -amin -1                                 # accessed within 1 minute</code><code>find /home/admin -cmin -1                                 # status changed within 1 minute</code><code>find /home/admin -mmin -1                                 # modified within 1 minute

2.5 pgm

Batch query logs from vm-shopbase:

pgm -A -f vm-shopbase 'cat /home/admin/shopbase/logs/shopbase.log.2017-01-17|grep 2069861630'

2.6 tsar

Alibaba’s open‑source collection tool for historical and real‑time system metrics.

tsar               # view recent day metrics

tsar --live        # real‑time metrics (default 5‑second refresh)

tsar -d 20161218   # view data of a specific day (up to ~4 months)

tsar --mem</code><code>tsar --load</code><code>tsar --cpu   # can be combined with -d for daily view

2.7 top

Use top together with other commands to investigate JVM issues. ps -ef | grep java; top -H -p <pid> Convert thread IDs from decimal to hex and use jstack for detailed analysis.

2.8 Other useful commands

netstat -nat|awk '{print $6}'|sort|uniq -c|sort -rn   # show connection states

3. Troubleshooting Tools

3.1 btrace

btrace is a powerful production‑environment tracing tool.

@OnMethod(clazz = "java.util.ArrayList", method="add", location = @Location(value = Kind.CALL, clazz = "/.*/", method = "/.*/"))
public static void m(@ProbeClassName String probeClass, @ProbeMethodName String probeMethod, @TargetInstance Object instance, @TargetMethodOrField String method) {
    if (getInt(field("java.util.ArrayList", "size"), instance) > 479) {
        println("check who ArrayList.add method:" + probeClass + "#" + probeMethod + ", method:" + method + ", size:" + getInt(field("java.util.ArrayList", "size"), instance));
        jstack();
        println();
        println("===========================");
        println();
    }
}
</code><code>@OnMethod(clazz = "com.xxxxxxx.sellerhome.transfer.biz.impl.C2CApplyerServiceImpl", method="nav", location = @Location(value = Kind.RETURN))
public static void mt(long userId, int current, int relation, String check, String redirectUrl, @Return AnyType result) {
    println("parameter# userId:" + userId + ", current:" + current + ", relation:" + relation + ", check:" + check + ", redirectUrl:" + redirectUrl + ", result:" + result);
}

More details at https://github.com/btraceio/btrace .

3.2 Greys

Features overlapping with btrace: sc -df xxx: show class details, source location, classloader. trace class method: display method execution time breakdown.

3.3 javOSize

Allows on‑the‑fly bytecode modification for quick logging.

3.4 JProfiler

Previously used for many issues; now Greys and btrace cover most cases.

https://www.ej-technologies.com/products/jprofiler/overview.html

4. Java “Seven Swords” (Key Tools)

4.1 jps

sudo -u admin /opt/xxxxx/java/bin/jps -mlvV

4.2 jstack

sudo -u admin /opt/taobao/install/ajdk-8_1_1_fp1-b52/bin/jstack 2815

sudo -u admin /opt/taobao/install/ajdk-8_1_1_fp1-b52/bin/jstack -m 2815

4.3 jinfo

sudo -u admin /opt/taobao/install/ajdk-8_1_1_fp1-b52/bin/jinfo -flags 2815

4.4 jmap

Two main uses:

Inspect heap:

sudo -u admin /opt/taobao/install/ajdk-8_1_1_fp1-b52/bin/jmap -heap 2815

Dump heap:

sudo -u admin /opt/taobao/install/ajdk-8_1_1_fp1-b52/bin/jmap -dump:live,format=b,file=/tmp/heap2.bin 2815

sudo -u admin /opt/taobao/install/ajdk-8_1_1_fp1-b52/bin/jmap -dump:format=b,file=/tmp/heap3.bin 2815

Combine with zprofiler and btrace for deeper analysis.

sudo -u admin /opt/taobao/install/ajdk-8_1_1_fp1-b52/bin/jmap -histo 2815 | head -10

4.5 jstat

sudo -u admin /opt/xxx/install/ajdk-8_1_1_fp1-b52/bin/jstat -gcutil 2815 1000

4.6 jdb

Used for pre‑release debugging; attach to remote JVM on port 8000:

sudo -u admin /opt/taobao/java/bin/jdb -attach 8000.

4.7 CHLSDB

sudo -u admin /opt/taobao/java/bin/java -classpath /opt/XXXXX/java/lib/sa-jdi.jar sun.jvm.hotspot.CLHSDB

More details at http://rednaxelafx.iteye.com/blog/1847971 .

5. Other Useful Utilities

5.1 dmesg

When a Java process disappears without trace, dmesg can reveal OOM‑killer actions. sudo dmesg | grep -i kill | less Search for keywords such as oom_killer:

[6710782.021013] java invoked oom-killer: gfp_mask=0xd0, order=0, oom_adj=0, oom_score_adj=0
[6710782.070639] ? oom_kill_process+0x68/0x140
[6710782.257588] Task in /LXC011175068174 killed as a result of limit of /LXC011175068174
[6710784.698347] Memory cgroup out of memory: Kill process 215701 (java) score 854 or sacrifice child
[6710784.707978] Killed process 215701, UID 679, (java) total-vm:11017300kB, anon-rss:7152432kB, file-rss:1232kB

OOM killer monitors memory usage; when the system is about to run out, it selects the highest‑scoring process and terminates it to protect the machine.

Convert dmesg timestamps to real time:

date -d "1970-01-01 UTC $(date +%s)-$(cat /proc/uptime|cut -f 1 -d' ')+12288812.926194|bc seconds"

6. New Skill – RateLimiter

To finely control QPS (e.g., limit to 400 requests per second), use Guava’s RateLimiter. See details at http://ifeve.com/guava-ratelimite .