Essential Linux Commands for System Monitoring and Regex Toolkit

Essential Linux System Monitoring Commands

The following one‑line commands help you quickly assess the health 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 cached under /data/cache.

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

Show concurrent HTTP requests and their TCP connection counts. 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
killall -TERM mysqld
kill -9 `cat /usr/local/apache2/logs/httpd.pid`

Terminate MySQL processes. ls /etc/rc3.d/S* | cut -c 15- Show services started at runlevel 3.

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

Display a multi‑line banner using a heredoc.

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.

ifconfig eth0 | grep "inet addr:" | awk '{print $2}' | cut -c 6-
# or
ifconfig | grep 'inet addr:' | grep -v '127.0.0.1' | cut -d: -f2 | awk '{print $1}'

Retrieve the server's IP address. free -m | grep "Mem" | awk '{print $2}' Show total memory size (in MB). cat /proc/cpuinfo | grep -c processor Count logical CPU cores. cat /proc/loadavg Display current CPU load averages. mpstat 1 1 Show detailed CPU statistics. free Show overall memory usage; alternatively cat /proc/meminfo. vmstat 1 5 Monitor swap usage and activity. df -h Check disk space usage. du -cks * | sort -rn | head -n 10 Find the ten largest files or directories. iostat -x 1 2 Inspect disk I/O load; verify that %util does not exceed 100%. sar -n DEV Monitor network traffic (rxbyt/s, txbyt/s). netstat -i Check for network errors; cat /proc/net/dev provides detailed stats.

netstat -an | grep -E "^(tcp)" | cut -c 68- | sort | uniq -c | sort -n

Count active network connections. ps aux | wc -l Show total number of processes (alert if >250). vmwtat 1 5 Display runnable processes (compare against 4× CPU count). top -id 1 Identify abnormal processes. who | wc -l Count logged‑in users (alert if >50).

# cat /var/log/rflogview/*errors
grep -i error /var/log/messages
grep -i fail /var/log/messages

Search system logs for errors or failures. dmesg View kernel logs for hardware or driver issues. date Check the current system time. lsof | wc -l Count open file descriptors.

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

Terminate zombie processes.

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

Capture 10,000 packets targeting port 80 for analysis.

# less pkts | awk '{printf $3"
"}' | cut -d. -f1-4 | sort | uniq -c | awk '{printf $1" "$2"
"}' | sort -n -t+ 0

Sort duplicate IPs from the capture. 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 set to start automatically. kudzu --probe --class=network Detect network card model.

Common Regular Expressions

Useful patterns for validation and text processing.

[\u4e00-\u9fa5]               # Match Chinese characters

[^\x00-\xff]                 # Match double‑byte characters (including Chinese)

\s*\r                     # Match blank lines

<(\S*?)[^>]*>.*?</\1>|<.*? />   # Match HTML tags (limited)

^\s*|\s*$                   # Trim leading/trailing whitespace

\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*   # Match email addresses

[a-zA-Z]+://[^\s]*           # Match URLs

^[a-zA-Z][a-zA-Z0-9_]{4,15}$   # Valid username (5‑16 chars, starts with letter)

\d{3}-\d{8}|\d{4}-\d{7}   # Match Chinese telephone numbers

[1-9][0-9]{4,}               # Match Tencent QQ numbers

[1-9]\d{5}(?!\d)           # Match Chinese postal codes

\d{15}|\d{18}               # Match Chinese ID numbers

\d+\.\d+\.\d+\.\d+      # Match IPv4 addresses

^[A-Za-z]+$                  # Only letters

^[A-Z]+$                    # Only uppercase letters

^[a-z]+$                    # Only lowercase letters

^[A-Za-z0-9]+$               # Letters and numbers

^\w+$                      # Letters, numbers, underscore