Bash

Bash (Bourne Again SHell) is a widely used command language interpreter for the GNU operating system. It offers scripting capabilities, allowing for task automation, program execution, and more. Bash is a powerful tool for managing servers, automating tasks, and developing in a Linux environment.

Basic Bash Commands

Running Scripts

Execute a script:

bash script.sh

Make a script executable and run it:

chmod +x ./script.sh
./script.sh

File System Operations

Change directory:

cd /path/to/directory

Return to the home directory:

cd

Move back one directory:

cd ..

Copy files:

cp /path/to/file /path/to/destination

Move files:

mv /path/to/file /path/to/destination

List files in the current directory:

ls

List all files, including hidden ones:

ls -a

List files in a specific directory:

ls /path/to/directory

Create a new file:

touch file.txt

Create a new directory:

mkdir directory

Delete files:

rm file.txt

Delete a directory and its contents:

rm -r directory

Writing to Files

Append text to a file:

echo "Hello, World!" >> file.txt

Overwrite text in a file:

echo "Hello, World!" > file.txt

Bash Variables

Defining variables:

name="John"
age=30
is_admin=true

Accessing variables:

echo $name

String Manipulation

Convert to uppercase:

msg="hello"
echo ${msg^^}

Convert to lowercase:

msg="HELLO"
echo ${msg,,}

Replace text in a string:

msg="hello world"
echo ${msg/world/universe}

Arrays

Defining an array:

my_array=(apple banana cherry)

Accessing array elements:

echo ${my_array[0]} # Outputs 'apple'

Adding elements to an array:

my_array+=(date)

Associative Arrays (Dictionaries)

Defining an associative array:

declare -A user
user[name]="John"
user[age]=30

Accessing associative array elements:

echo ${user[name]}

Passing Arguments to Scripts

./script.sh arg1 arg2 arg3

Inside script.sh:

echo $1 # First argument
echo $2 # Second argument

System Variables

Commonly used system variables:

echo $USER          # Current username
echo $BASH_VERSION  # Bash version
echo $PWD           # Current directory
echo $HOME          # Home directory

Input and Output

Reading user input:

echo "Enter your name:"
read name
echo "Hello, $name"

Silent input (e.g., for passwords):

read -s password

Flow Control

If Statement

if [ condition ]; then
    # commands
elif [ other_condition ]; then
    # other commands
else
    # else commands
fi

Loops

While Loop

while [ condition ]; do
    # commands
done

For Loop

Loop over a range of numbers:

for i in {1..10}; do
    echo $i
done

Loop over a list:

for i in apple banana cherry; do
    echo $i
done

Functions

Defining a function:

function greet {
    echo "Hello, $1"
}

Calling a function with arguments:

greet John

Advanced Concepts

Process Control

Background a process:

long_running_command &

Kill a process:

kill [process_id]

Text Processing

Using grep to search inside files:

grep "pattern" file.txt

Using awk for pattern scanning and processing:

awk '/pattern/ { print $0 }' file.txt

Using sed for stream editing:

sed 's/old/new/g' file

.txt

Networking

Download files with wget:

wget [url]

Transfer files with scp:

scp file.txt user@remote:/path