Linux Command Line

# View current Shell
echo $SHELL

# View available Shells in the system
cat /etc/shells

# List directory contents
ls
ls -la          # Show detailed information
ls -lh          # Show in human-readable format
ls -R           # List recursively

# Change directory
cd /path/to/directory
cd ~            # Change to home directory
cd ..           # Change to parent directory

# Create directory
mkdir directory
mkdir -p dir1/dir2/dir3  # Create nested directories

# Delete file or directory
rm file.txt
rm -f file.txt  # Force delete
rm -r directory # Recursively delete directory

# Copy file or directory
cp source.txt destination.txt
cp -r source_dir destination_dir

# Move or rename file/directory
mv oldname.txt newname.txt
mv file.txt /path/to/directory/

# View file content
cat file.txt

# View with pagination
less file.txt
more file.txt

# View file head
head file.txt
head -n 10 file.txt  # View first 10 lines

# View file tail
tail file.txt
tail -n 10 file.txt  # View last 10 lines
tail -f file.txt     # View file changes in real-time

# View file type
file file.txt

# Search file content
grep "pattern" file.txt
grep -r "pattern" directory/  # Recursive search
grep -i "pattern" file.txt    # Ignore case
grep -n "pattern" file.txt    # Show line numbers

# View system information
uname -a

# View kernel version
uname -r

# View system distribution information
cat /etc/os-release
lsb_release -a

# View CPU information
cat /proc/cpuinfo
lscpu

# View memory information
free -h

# View disk space
df -h

# View directory size
du -h directory
du -sh directory  # View total size

# View system load
uptime
w

# View current logged-in users
who
whoami

# View system time
date
cal

# View network interfaces
ifconfig
ip addr

# View routing table
route -n
ip route

# Test network connection
ping google.com

# View network connections
netstat -tuln
ss -tuln

# View DNS configuration
cat /etc/resolv.conf

# View hostname
hostname
echo $HOSTNAME

# View local hostname resolution
cat /etc/hosts

# Download file
wget https://example.com/file.txt
curl -O https://example.com/file.txt

# View command history
history

# Use up/down arrows to browse history commands
# Press Ctrl+R to search history commands
# Execute the nth command in history
!n

# Execute previous command
!!

# Execute previous command starting with specific string
!string

# Clear command history
history -c

# Save command history to file
history > history.txt

# complete command
ls -la /u[TAB]  # Will complete to /usr

# complete filename
cat doc[TAB]    # Will complete to document.txt (if exists)

# Double tap Tab to show all possible completion options
ls /u[TAB][TAB] # Will show /usr, /var, etc.

# Create temporary alias
alias ll='ls -la'
alias la='ls -a'
alias cls='clear'

# View all aliases
alias

# Remove alias
unalias ll

# Create permanent alias (edit ~/.bashrc file)
echo "alias ll='ls -la'" >> ~/.bashrc
source ~/.bashrc  # Make alias effective

# View current working directory
pwd

# Quick directory switch
cd -

# Jump to previous directory
cd $OLDPWD

# Use pushd and popd to manage directory stack
pushd /path/to/dir1
pushd /path/to/dir2
popd
popd

# Redirect command output to file (overwrite)
echo "Hello World" > file.txt

# Append command output to file
 echo "Hello Again" >> file.txt

# Redirect standard error to file
error_command 2> error.txt

# Redirect both standard output and standard error to file
command > output.txt 2>&1
command &> output.txt

# Redirect input
cat < input.txt

# Null device (/dev/null) - used to discard output
command > /dev/null

# Here Document
cat > file.txt << EOF
Line 1
Line 2
Line 3
EOF

# Here String
cat <<< "Hello World"

# Pipe example
ls -la | grep "txt"  # List all .txt files

# Multiple pipe combination
ps aux | grep "python" | wc -l  # Count Python processes

# Common pipe commands
ls -la | sort -k 5 -nr  # Sort by file size

# Use xargs to pass pipe output as command arguments
find . -name "*.txt" | xargs cat  # View all .txt file contents
find . -name "*.tmp" | xargs rm  # Delete all .tmp files

# Use tee command to output to both terminal and file
ls -la | tee output.txt

# View all environment variables
env
export

# View specific environment variables
echo $HOME
echo $PATH
echo $USER
echo $SHELL

# Search environment variables
env | grep "PATH"
set | grep "HOME"

# Set temporary environment variable
MY_VAR="Hello"
export MY_VAR

# Set and export in the same line
export MY_VAR="Hello"

# Use environment variable
echo $MY_VAR

# Unset environment variable
unset MY_VAR

# Set permanent environment variable (edit ~/.bashrc file)
echo "export MY_VAR='Hello'" >> ~/.bashrc
source ~/.bashrc

# Or edit /etc/profile file (effective for all users)
sudo echo "export MY_VAR='Hello'" >> /etc/profile
source /etc/profile

# Create Shell script file
cat > hello.sh << 'EOF'
#!/bin/bash
# This is a simple Shell script
echo "Hello, World!"
echo "Current date: $(date)"
echo "Current user: $USER"
EOF

# Add execute permission to script
chmod +x hello.sh

# Run script
./hello.sh

# Or run with bash command
bash hello.sh

# Or run with sh command
sh hello.sh

#!/bin/bash

# Define variables
name="Linux"
age=10

# Use variables
echo "Hello, $name!"
echo "You are $age years old."

# Command substitution
current_dir=$(pwd)
echo "Current directory: $current_dir"

# Arithmetic operation
num1=10
num2=5
sum=$((num1 + num2))
echo "Sum: $sum"

# String operation
greeting="Hello, World!"
echo "Length: ${#greeting}"
echo "Substring: ${greeting:7:5}"

#!/bin/bash

# if statement
if [ $1 -gt 10 ]; then
    echo "Number is greater than 10"
elif [ $1 -eq 10 ]; then
    echo "Number is equal to 10"
else
    echo "Number is less than 10"
fi

# File test
if [ -f "file.txt" ]; then
    echo "File exists"
else
    echo "File does not exist"
fi

# String comparison
if [ "$USER" = "root" ]; then
    echo "You are root user"
else
    echo "You are not root user"
fi

#!/bin/bash

# for loop
for i in {1..5}; do
    echo "Number: $i"
done

# Iterate files
for file in *.txt; do
    echo "File: $file"
done

# while loop
count=1
while [ $count -le 5 ]; do
    echo "Count: $count"
    count=$((count + 1))
done

# until loop
count=1
until [ $count -gt 5 ]; do
    echo "Count: $count"
    count=$((count + 1))
done

#!/bin/bash

# Define function
greet() {
    echo "Hello, $1!"
}

# Call function
greet "Linux"

# Function with return value
add() {
    local sum=$(( $1 + $2 ))
    echo $sum
}

# Use function return value
result=$(add 10 20)
echo "Result: $result"

# Function arguments
print_args() {
    echo "Number of arguments: $#"
    echo "All arguments: $@"
    echo "First argument: $1"
    echo "Second argument: $2"
}

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