Essential Linux Shell Commands for System Monitoring and Troubleshooting

Key System Monitoring Commands

The following commands help administrators quickly assess the health and performance of a Linux server. find -type f -size 0 -exec rm -rf {} \; Delete all zero‑byte files. PS -e -o "%C : %p : %z : %a" | sort -k5 -nr List processes sorted by memory usage (largest first). ps -e -o "%C : %p : %z : %a" | sort -nr List processes sorted by CPU utilization.

grep -r -a jpg /data/cache/* | strings | grep "http:" | awk -F'http:' '{print "http:"$2;}'

Print URLs of cached JPEG files.

netstat -n | awk '/^tcp/ {++S[$NF]} END {for(a in S) print a, S[a]}'

Show the number of HTTP connections and their TCP states. sed -i '/Root/s/no/yes/' /etc/ssh/sshd_config Replace no with yes in the Root line of sshd_config.

ps aux | grep mysql | grep -v grep | awk '{print $2}' | xargs kill -9

Kill all MySQL processes. killall -TERM mysqld Gracefully terminate the MySQL daemon. ls /etc/rc3.d/S* | cut -c 15- List services started at run‑level 3 (demonstrates cut usage).

cat << EOF
+--------------------------------------------------------------+
|       === Welcome to Tunoff services ===                |
+--------------------------------------------------------------+
EOF

Show how to output multiple lines using a here‑document.

cd /usr/local/mysql/bin
for i in *
do ln /usr/local/mysql/bin/$i /usr/bin/$i
done

Create symbolic links for all MySQL binaries in /usr/bin.

ifconfig eth0 | grep "inet addr:" | awk '{print $2}' | cut -c 6-

Extract the IP address of eth0. free -m | grep "Mem" | awk '{print $2}' Show total memory size in megabytes. cat /proc/loadavg Display the three‑minute load average values. mpstat 1 1 Show CPU usage statistics; check that %idle is not too low. df -h Report disk space usage in human‑readable format. du -cks * | sort -rn | head -n 10 Find the ten largest files or directories in the current partition. iostat -x 1 2 Show detailed I/O statistics; verify that %util does not exceed 100%. sar -n DEV Display network traffic statistics (rx/tx bytes per second). netstat -i Check for network errors such as drops, collisions, or carrier problems.

lsof -i :80 | grep -v "ID" | awk '{print "kill -9",$2}' | sh

Kill processes listening on port 80.

ps -eal | awk '{ if ($2 == "Z") {print $4}}' | kill -9

Remove zombie processes.

tcpdump -c 10000 -i eth0 -n dst port 80 > /root/pkts

Capture 10,000 packets destined for port 80 for later analysis.

# less pkts | awk {'printf $3"
"'} | cut -d. -f 1-4 | sort | uniq -c | awk {'printf $1" "$2"
"'} | sort -n -t\ +0

Sort captured IP addresses by occurrence count. netstat -anp | grep php-cgi | grep ^tcp | wc -l Count active php-cgi processes.

chkconfig --list | awk '{if ($5=="3:on") print $1}'

List services configured to start at run‑level 3. kudzu --probe --class=network Show network card model information.

Common Regular Expressions

Below is a curated list of useful regular expressions for text processing.

Match Chinese characters: [\u4e00-\u9fa5] Match double‑byte characters (including Chinese): [^\x00-\xff] Match blank lines: \n\s*\r Match HTML tags: <(\S*?)[^>]*>.*?</\1>|<.*? /> Trim leading/trailing whitespace: ^\s*|\s*$ Validate email address: \w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)* Match URLs: [a-zA-Z]+://[^\s]* Validate usernames (5‑16 characters, letters, numbers, underscore, start with a letter): ^[a-zA-Z][a-zA-Z0-9_]{4,15}$ Match Chinese phone numbers: \d{3}-\d{8}|\d{4}-\d{7} Match QQ numbers (starting from 10000): [1-9][0-9]{4,} Match Chinese postal codes: [1-9]\d{5}(?!\d) Match Chinese ID numbers (15 or 18 digits): \d{15}|\d{18} Match IPv4 addresses: \d+\.\d+\.\d+\.\d+ Match positive integers: ^[1-9]\d*$ Match negative integers: ^-[1-9]\d*$ Match integers (positive or negative): ^-?[1-9]\d*$ Match non‑negative integers (including zero): ^[1-9]\d*|0$ Match non‑positive integers (including zero): ^-[1-9]\d*|0$ Match positive floating‑point numbers: ^[1-9]\d*\.\d*|0\.\d*[1-9]\d*$ Match negative floating‑point numbers: ^-([1-9]\d*\.\d*|0\.\d*[1-9]\d*)$ Match any floating‑point number: ^-?([1-9]\d*\.\d*|0\.\d*[1-9]\d*|0?\.0+|0)$ Match alphabetic strings: ^[A-Za-z]+$ Match uppercase strings: ^[A-Z]+$ Match lowercase strings: ^[a-z]+$ Match alphanumeric strings: ^[A-Za-z0-9]+$ Match word characters (letters, digits, underscore):

^\w+$