Linux Commands

Essential Linux command line operations.

File Operations

COMMAND	DESCRIPTION
ls	List directory contents
ls -la	List all files with details
ls -lh	List with human-readable sizes
cd /path	Change directory
pwd	Print working directory
mkdir dir	Create directory
mkdir -p path/to/dir	Create nested directories
rmdir dir	Remove empty directory
rm file	Remove file
rm -r dir	Remove directory and contents
rm -rf dir	Force remove directory
cp file1 file2	Copy file
cp -r dir1 dir2	Copy directory
mv file1 file2	Move/rename file
touch file	Create empty file

File Content

COMMAND	DESCRIPTION
cat file	Display file content
less file	View file with paging
head -n 10 file	View first 10 lines
tail -n 10 file	View last 10 lines
tail -f file	Follow file (log watching)
grep pattern file	Search in file
grep -r pattern dir	Search recursively
grep -i pattern file	Case-insensitive search
grep -v pattern file	Invert match
grep -n pattern file	Show line numbers
wc -l file	Count lines
wc -w file	Count words
wc -c file	Count characters
sort file	Sort lines
sort -r file	Reverse sort
sort -n file	Numeric sort
uniq file	Remove duplicate lines

File Permissions

COMMAND	DESCRIPTION
chmod 755 file	Set permissions (rwxr-xr-x)
chmod +x script.sh	Make script executable
chmod -R 755 dir	Recursive permission change
chown user:group file	Change owner
chown -R user:group dir	Recursive owner change
chgrp group file	Change group

Permission Codes

CODE	PERMISSION
7	rwx (read, write, execute)
6	rw- (read, write)
5	r-x (read, execute)
4	r-- (read only)
0	--- (no permission)

Process Management

COMMAND	DESCRIPTION
ps aux	List all processes
ps -ef	List processes in full format
top	Display system processes
htop	Interactive process viewer
kill PID	Terminate process
kill -9 PID	Force kill process
killall name	Kill all processes by name
pkill pattern	Kill processes matching pattern
pgrep pattern	Find process IDs
nohup command &	Run command in background
jobs	List background jobs
bg %1	Background job
fg %1	Foreground job

System Information

COMMAND	DESCRIPTION
uname -a	System information
uname -r	Kernel version
hostname	Hostname
df -h	Disk space usage
du -sh dir	Directory size
free -h	Memory usage
uptime	System uptime
who	Who is logged in
w	Who is logged in and what they're doing
date	Current date/time
cal	Calendar
dmesg	Kernel ring buffer messages

Network

COMMAND	DESCRIPTION
ip addr	Show IP addresses
ip link	Show network interfaces
ifconfig	Show network config (deprecated)
ping host	Ping host
traceroute host	Trace route to host
nslookup host	DNS lookup
dig host	DNS lookup (more detailed)
netstat -tuln	List listening ports
ss -tuln	List listening sockets
lsof -i :port	List processes using port
curl URL	Transfer data from URL
wget URL	Download file from URL
hostname -I	Show IP address

Disk & Filesystem

COMMAND	DESCRIPTION
df -h	Disk space (human-readable)
df -i	Disk inode usage
du -sh *	Size of all files/directories
du -h --max-depth=1	Size of top-level directories
mount	Show mounted filesystems
mount /dev/sdb1 /mnt	Mount filesystem
umount /mnt	Unmount filesystem
fdisk -l	List disk partitions
lsblk	List block devices
mkfs.ext4 /dev/sdb1	Format partition

Archive & Compress

COMMAND	DESCRIPTION
tar -cvf archive.tar files	Create tar archive
tar -xvf archive.tar	Extract tar archive
tar -czvf archive.tar.gz files	Create tar.gz
tar -xzvf archive.tar.gz	Extract tar.gz
zip -r archive.zip files	Create zip archive
unzip archive.zip	Extract zip archive
gzip file	Compress with gzip
gunzip file.gz	Decompress gzip

Find

COMMAND	DESCRIPTION
find . -name "pattern"	Find by name
find . -type f	Find files only
find . -type d	Find directories only
find . -size +100M	Find files > 100MB
find . -mtime -7	Find modified in last 7 days
find . -perm 755	Find with specific permissions
find . -exec command {} \;	Execute command on results

User Management

COMMAND	DESCRIPTION
whoami	Current user
id	User and group IDs
who	Logged in users
useradd username	Create user
userdel username	Delete user
usermod -aG group user	Add user to group
passwd username	Change password
groups	Show user groups
sudo -i	Switch to root

Environment Variables

COMMAND	DESCRIPTION
env	Display environment variables
export VAR=value	Set environment variable
echo $VAR	Display variable value
unset VAR	Unset variable
source ~/.bashrc	Reload bash configuration

Package Management

Debian/Ubuntu

COMMAND	DESCRIPTION
apt update	Update package list
apt upgrade	Upgrade packages
apt install package	Install package
apt remove package	Remove package
apt search package	Search for package
apt show package	Show package info

RHEL/CentOS

COMMAND	DESCRIPTION
yum update	Update packages
yum install package	Install package
yum remove package	Remove package
yum search package	Search for package

Systemd Services

COMMAND	DESCRIPTION
systemctl start service	Start service
systemctl stop service	Stop service
systemctl restart service	Restart service
systemctl status service	Show service status
systemctl enable service	Enable at boot
systemctl disable service	Disable at boot
systemctl list-units	List all units
journalctl -u service	View service logs

SSH

COMMAND	DESCRIPTION
ssh user@host	Connect to remote host
ssh -p port user@host	Connect on specific port
ssh-keygen -t ed25519	Generate SSH key
ssh-copy-id user@host	Copy SSH key to host
scp file user@host:/path	Copy file to remote

Useful Commands

Find large files

find . -type f -size +100M -exec ls -lh {} \;

Find old files

find . -type f -mtime +30

Count files in directory

find . -type f | wc -l

Monitor file changes

watch -n 1 ls -lh

Process using most CPU

ps aux --sort=-%cpu | head -n 10

Process using most memory

ps aux --sort=-%mem | head -n 10

Listen on port

nc -l 8080

Download with progress

wget --progress=bar:force URL

Parallelize downloads

aria2c -x 16 -s 16 URL

Best Practices

• Use tab completion
• Use man for command documentation
• Use aliases for frequently used commands
• Use sudo carefully
• Regularly update packages
• Monitor disk space
• Use log files for troubleshooting
• Keep backups of important files
• Use screen or tmux for long-running sessions
• Use version control for configuration files

TIP

Use history | grep "command" to find previously used commands.