Bash (Bourne Again Shell) is a powerful command-line interpreter commonly used in Linux and Unix environments. Below is a summary of essential Bash commands and scripting concepts.

• echo - Outputs text to the terminal. Example: `echo “Hello, World!”`
• pwd - Prints the current working directory.
• ls - Lists the contents of a directory. Example: `ls -l`
• cd - Changes the working directory. Example: `cd /home/user`
• cp - Copies files or directories. Example: `cp source.txt destination.txt`
• mv - Moves or renames files or directories. Example: `mv old.txt new.txt`
• rm - Removes files or directories. Example: `rm file.txt`
• touch - Creates an empty file or updates a file's timestamp. Example: `touch newfile.txt`

• Variable Definition: Assigns values to variables. Example: `name=“John”`
• Variable Usage: References variables using `$`. Example: `echo $name`
• User Input: Reads user input. Example:

```

  read name
  echo "Hello, $name!"
  ```

• if-else Statements - Executes commands based on conditions:

```

  if [ "$name" = "John" ]; then
    echo "Hello, John!"
  else
    echo "Who are you?"
  fi
  ```
* **[[bash:conditionals:case|case Statements]]** - Matches patterns to execute specific commands:
  ```
  case $fruit in
    apple) echo "This is an apple";;
    banana) echo "This is a banana";;
    *) echo "Unknown fruit";;
  esac
  ```

• for Loop - Iterates over a list of items:

```

  for i in 1 2 3; do
    echo "Number: $i"
  done
  ```
* **[[bash:loops:while|while Loop]]** - Repeats commands while a condition is true:
  ```
  count=0
  while [ $count -lt 5 ]; do
    echo "Count: $count"
    count=$((count + 1))
  done
  ```

• Defining Functions - Groups commands into reusable functions:

```

  greet() {
    echo "Hello, $1!"
  }
  greet "Alice"
  ```
* **[[bash:functions:return_values|Returning Values]]** - Functions can return status codes using `return`.

• Redirection - Redirects output to a file. Example: `echo “Text” > output.txt`
• Appending Output - Appends output to a file. Example: `echo “More text” » output.txt`
• Standard Input (stdin) - Redirects input from a file. Example: `cat < input.txt`
• Piping - Passes the output of one command as input to another. Example: `ls | grep “file”`

• File Testing - Tests file conditions. Example:

```

  if [ -f "file.txt" ]; then
    echo "File exists"
  fi
  ```
* Common file tests:
  * `-f` - Checks if a file exists and is regular.
  * `-d` - Checks if a directory exists.
  * `-r` - Checks if a file is readable.
  * `-w` - Checks if a file is writable.

• Background Processes - Runs a command in the background using `&`. Example: `sleep 10 &`
• jobs - Lists background jobs.
• kill - Terminates a process. Example: `kill 1234`

• Exit Status - The exit status of the last command can be accessed using `$?`.
• set -e - Stops script execution on any error.

• Single-line Comments - Starts with `#`. Example: `# This is a comment`
• Multi-line Comments - Use multiple `#` lines for multi-line comments.

This summary covers the fundamental concepts of Bash scripting. Each section can be expanded with detailed examples and explanations.