Essential Linux Command Cheat Sheet for System Administration

Although most of my work involves Java development, I frequently use Linux, especially on macOS with a dark terminal, and I have compiled a collection of useful Linux commands for quick reference.

Basic Operations

Linux shutdown, reboot

# Shutdown
shutdown -h now

# Reboot
shutdown -r now

View system and CPU information

# View kernel info
uname -a

# View kernel version
cat /proc/version

# View current environment variables
env

# View CPU info
cat /proc/cpuinfo

# Count logical CPUs and model names
cat /proc/cpuinfo | grep name | cut -f2 -d: | uniq -c

# Count physical CPUs and cores
cat /proc/cpuinfo | grep physical | uniq -c

# Check if CPU runs in 32‑bit or 64‑bit mode
getconf LONG_BIT

# Verify 64‑bit support
cat /proc/cpuinfo | grep flags | grep ' lm ' | wc -l

Create symbolic link

ln -s /usr/local/jdk1.8/ jdk

rpm related

# Check if a package was installed via rpm
rpm -qa | grep <package_name>

ssh key

# Generate ssh key
ssh-keygen -t rsa -C [email protected]

# Copy the content of id_rsa.pub to the remote server's ~/.ssh/authorized_keys (create the file if it does not exist; .ssh permission 700, authorized_keys permission 600)

Command alias

# Add alias in each user's .bash_profile
alias ll='ls -alF'

Synchronize server time

sudo ntpdate -u ntp.api.bz

Run command in background

# Background with nohup.out
nohup xxx &

# Background without any log
nohup xxx > /dev/null &

# Background with error output redirected to log
nohup xxx > out.log 2>&1 &

Force logout active user

# Force logout by TTY
pkill -kill -t <TTY>

Find command path

which <command>

View max file descriptors for a process

ulimit -n

Configure DNS

vim /etc/resolv.conf

nslookup, view domain routing

nslookup google.com

last, recent login info

# Show last 5 login accounts
last -n 5

Set static IP

ifconfig em1 192.168.5.177 netmask 255.255.255.0

View environment variables of a process

# Show environment of process with PID XXXXX
ps eww -p XXXXX

View process tree

ps auwxf

View process start path

cd /proc/<pid>
ls -all   # cwd shows the start directory

Add user and configure sudo

# Add user
useradd <username>
passwd <username>

# Grant sudo
vim /etc/sudoers   # Add a line: <username> ALL=(ALL) ALL

Kill all processes containing a name

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

Disk, File, Directory Operations

vim operations

# Global replace
:%s/x/y/g

# Move cursor to line start
0
# Move cursor to line end
$

# Jump to file end
Shift+g
# Jump to file start
gg

# Show line numbers
:set nu
# Hide line numbers
:set nonu

# Search forward
/xxx   # press n for next
# Search backward
?xxx

Edit read‑only file without switching user

:w !sudo tee %

View disk and directory information

# Show mounted filesystems
mount

# Show partition usage
df

# Show directory size recursively
du -H -h

# Show size of each item in current directory (non‑recursive)
du -sh *

wc command

# Count lines
wc -l filename

# Count words
wc -w filename

# Length of longest line
wc -L filename

# Count bytes
wc -c

Compression and Decompression

Compress

tar czvf xxx.tar   # compress directory
zip -r xxx.zip     # compress directory

Decompress

tar zxvf xxx.tar
tar zxvf xxx.tar -C /xxx/yyy/   # extract to specific folder
unzip xxx.zip

Change file ownership

chown eagleye.eagleye xxx.log

Copy, scp, mkdir

# Copy file
cp xxx.log

# Force overwrite
cp -f xxx.log

# Copy directory
cp -r src_dir dest_dir

# Remote copy
scp -P <ssh_port> [email protected]:/home/username/xxx /home/xxx

# Create nested directories
mkdir -p /xxx/yyy/zzz

# Batch create java and resources folders under test and main
mkdir -p src/{test,main}/{java,resources}

Compare two files

diff -u 1.txt 2.txt

Log output byte count (performance testing)

# Append a dot to log each iteration and monitor size
tail -f xxx.log | pv -bt

View and remove special characters

# Show special characters
cat -v xxx.sh

# Remove carriage return characters
sed -i 's/^M//g' env.sh

Convert file format

# Convert to Unix format
cat file.sh > file.sh_bak

# Create new file from stdin
cat > file1.sh

# Set encoding in vim
:set fileencodings=utf-8
:w
:set fileformat=unix

# Use dos2unix on mac
find . -name "*.sh" | xargs dos2unix

tee and redirect while displaying

awk '{print $0}' xxx.log | tee test.log

Network Related

Find process using a port

lsof -i:<port>

Get local IP address

/sbin/ifconfig -a | grep inet | grep -v 127.0.0.1 | grep -v inet6 | awk '{print $2}' | tr -d "addr:"

iptables

# Show status
service iptables status

# Block an IP
iptables -I INPUT -s <ip> -j DROP

# Unblock an IP
iptables -D INPUT -s <ip> -j DROP

# Open port 9090
/sbin/iptables -I INPUT -p tcp --dport 9090 -j ACCEPT

# Service control
/etc/init.d/iptables status
/etc/init.d/iptables start
/etc/init.d/iptables stop
/etc/init.d/iptables restart

nc (netcat) for TCP testing

# Send data to endpoint
nc 192.168.0.11 8000 < data.txt

# Listen and save incoming data
nc -l 8000 > received_data

# Keep listening
nc -lk 8000

tcpdump

# Capture TCP packets on port 12301
tcpdump -i em1 tcp port 12301 -s 1500 -w abc.pcap

traceroute

# Default UDP
traceroute -I www.163.com   # use ICMP
traceroute -M 3 www.163.com   # start from TTL 3
traceroute -p 8080 192.168.10.11

ss (socket statistics)

# List all listening ports
ss -l

# Show processes with sockets
ss -pl

# Show all TCP sockets
ss -t -a

# Show all UDP sockets
ss -u -a

# Show established SMTP connections
ss -o state established '( dport = :smtp or sport = :smtp )'

# Show established HTTP connections
ss -o state established '( dport = :http or sport = :http )'

# Show processes connecting to X server
ss -x src /tmp/.X11-unix/*

# Show socket statistics
ss -s

netstat

# Count connections per IP and total
netstat -n | awk '/^tcp/ {n=split($(NF-1),array,":");if(n<=2)++S[array[1]];else++S[array[4]];++s[$NF];++N} END {for(a in S){printf("%-20s %s
", a, S[a]);++I}printf("%-20s %s
","TOTAL_IP",I);for(a in s) printf("%-20s %s
",a, s[a]);printf("%-20s %s
","TOTAL_LINK",N);}'

Linux Performance Monitoring

top

Press uppercase F or O, then a‑z to sort by a column; press R to reverse the order.

dmesg

dmesg

iostat (disk I/O)

iostat -xz 1

free (memory usage)

free -m

sar (network throughput)

# Network device throughput
sar -n DEV 1

# TCP connection statistics
sar -n TCP,ETCP 1

vmstat (CPU, memory, I/O)

# Sample every 2 seconds, once
vmstat 2 1