1.3 Bash

Bash (Bourne Again Shell) is a command-line interpreter for Linux and other Unix-like systems. It allows users to interact with the operating system through typed commands and scripts. This tutorial will introduce you to the basics of Bash, including common commands, file manipulation, and scripting.

1. Basic Bash Commands

1.1 Navigating the Filesystem

pwd: Print the current working directory (shows where you are).
Terminal window
```
pwd
```
ls: List files and directories in the current directory.
- ls -l: List files with detailed information (permissions, file size, etc.).
- ls -a: Show all files, including hidden files (those starting with .).
Terminal window
```
ls
ls -l
ls -a
```

cd: Change directory.

cd /path/to/directory  # Go to a specific directory
cd ..                  # Go up one directory
cd ~                   # Go to the home directory

1.2 File Operations

cp: Copy files or directories.

cp file1.txt file2.txt      # Copy file1.txt to file2.txt
cp -r dir1/ dir2/           # Copy directory dir1 to dir2 (recursive)

mv: Move or rename files or directories.

mv oldfile.txt newfile.txt  # Rename file
mv file.txt /path/to/destination/  # Move file to a new directory

rm: Remove files or directories.

rm file.txt                # Remove a file
rm -r dir/                 # Remove a directory and its contents (recursive)
rm -f file.txt             # Force remove without confirmation

touch: Create an empty file or update the timestamp of an existing file.
Terminal window
```
touch newfile.txt  # Create an empty file named "newfile.txt"
```

1.3 Viewing File Contents

cat: Concatenate and display the contents of a file.
Terminal window
```
cat file.txt  # Display the content of file.txt
```
less: View large files one page at a time. You can scroll through the file.
Terminal window
```
less file.txt
```
head: Show the first 10 lines of a file.
Terminal window
```
head file.txt
```

tail: Show the last 10 lines of a file.

tail file.txt
tail -f file.txt  # Follow the file (useful for log files)

2. File Permissions and Ownership

chmod: Change file permissions.
- chmod 755 file.txt: Sets read/write/execute for owner, and read/execute for group and others.
- chmod +x script.sh: Adds executable permission to the script.
Permission notation:
- r = read
- w = write
- x = execute
Numeric representation:
- 7 = rwx (read, write, execute)
- 6 = rw-
- 5 = r-x
- 4 = r--
Terminal window
```
chmod 644 file.txt  # rw-r--r--
chmod 755 file.txt  # rwxr-xr-x
```

chown: Change file owner and group.

chown user:group file.txt  # Change owner and group

chgrp: Change group ownership.

chgrp group file.txt  # Change group ownership

3. Variables and Input

3.1. Setting and Using Variables

Define a variable:
Terminal window
```
VAR_NAME="Hello, world!"
```
Access the value of a variable:
Terminal window
```
echo $VAR_NAME  # Output: Hello, world!
```

Use variables in commands:

FOLDER="/home/user/Documents"
cd $FOLDER  # Change directory to /home/user/Documents

3.2. User Input

Read input from the user:

read -p "Enter your name: " USER_NAME
echo "Hello, $USER_NAME!"

4. Basic Bash Scripting

A Bash script is a text file containing a sequence of commands. It can automate tasks and execute commands in sequence.

4.1. Writing a Simple Script

Create a file with a .sh extension, e.g., hello.sh.
Add the following to the script:

#!/bin/bash
# This is a simple script
echo "Hello, World!"

Explanation:
- #!/bin/bash: This is called the shebang and tells the system which interpreter to use (in this case, Bash).
- echo: Prints text to the terminal.

Give the script execute permission:

chmod +x hello.sh

Run the script:

./hello.sh

4.2. Variables in Scripts

#!/bin/bash
# Script to greet the user
echo "Enter your name:"
read name
echo "Hello, $name!"

4.3. Conditional Statements

You can use if statements to make decisions in your script.

#!/bin/bash
echo "Enter a number:"
read num

if [ $num -gt 10 ]; then
    echo "The number is greater than 10"
else
    echo "The number is 10 or less"
fi

Explanation:
- -gt: Greater than.
- if [ $num -gt 10 ]: Checks if the value of num is greater than 10.

4.4. Loops

You can use for, while, and until loops in Bash scripts.

For Loop*

#!/bin/bash
for i in {1..5}; do
  echo "Iteration $i"
done

While Loop

#!/bin/bash
count=1
while [ $count -le 5 ]; do
  echo "Iteration $count"
  ((count++))  # Increment count
done

Until Loop*

#!/bin/bash
count=1
until [ $count -gt 5 ]; do
  echo "Iteration $count"
  ((count++))  # Increment count
done

4.5. Functions

Bash allows you to create reusable functions.

#!/bin/bash

greet() {
  echo "Hello, $1!"  # $1 is the first argument passed to the function
}

greet "Alice"
greet "Bob"

5. Advanced Bash Commands

grep: Search for patterns in files.

grep "pattern" file.txt  # Search for "pattern" in file.txt

find: Search for files in a directory hierarchy.

find /path/to/dir -name "*.txt"  # Find all .txt files in a directory

awk: Text processing tool.

awk '{print $1}' file.txt  # Print the first column of each line in file.txt

sed: Stream editor for basic text transformations.

sed 's/old/new/' file.txt  # Replace "old" with "new" in file.txt

tar: Archive files.

tar -czf archive.tar.gz directory/  # Create a compressed archive
tar -xzf archive.tar.gz            # Extract a compressed archive

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