In this section, we will explore two fundamental concepts in Bash: redirection and piping. These techniques allow you to control the flow of data between commands and files, making your scripts more powerful and flexible.

Redirection is a method of changing the standard input/output devices when executing a command. By default, commands read input from the keyboard and write output to the terminal. Redirection allows you to change this behavior.

1. Output Redirection (> and >>)

   • >: Redirects the output to a file, overwriting the file if it exists.
   • >>: Redirects the output to a file, appending to the file if it exists.

2. Input Redirection (<)

   • <: Redirects input from a file to a command.

3. Error Redirection (2> and 2>>)

   • 2>: Redirects error messages to a file, overwriting the file if it exists.
   • 2>>: Redirects error messages to a file, appending to the file if it exists.

4. Combining Output and Error Redirection (&>)

   • &>: Redirects both standard output and error messages to a file.

• echo "Hello, World!" > output.txt: Writes "Hello, World!" to output.txt, overwriting the file if it exists.
• echo "This is a new line." >> output.txt: Appends "This is a new line." to output.txt.
• sort < unsorted_list.txt: Sorts the contents of unsorted_list.txt.
• ls non_existent_file 2> error.log: Redirects the error message from ls to error.log.
• ls non_existent_file &> combined_output.log: Redirects both the output and error message from ls to combined_output.log.

Piping is a method of passing the output of one command as input to another command. This is done using the pipe operator (|).

• ls -l | grep "txt": Lists files in long format and filters the output to show only lines containing "txt".
• cat file.txt | tr 'a-z' 'A-Z' | sort: Reads file.txt, converts all lowercase letters to uppercase, and sorts the output.

Task: Create a script that writes "Hello, Bash!" to a file named greeting.txt.

Solution:

Task: Create a script that attempts to list a non-existent file and redirects the error message to error.log.

Solution:

Task: Create a script that lists all files in the current directory, filters for .sh files, and sorts them alphabetically.

Solution:

• Overwriting Files: Be cautious with > as it overwrites files. Use >> to append if you don't want to lose existing data.
• Combining Redirection: Remember that &> redirects both standard output and error, which can be useful for logging.
• Piping Limitations: Pipes only pass standard output, not standard error. Use 2>&1 to combine them if needed.

Redirection and piping are powerful tools in Bash that allow you to control the flow of data between commands and files. By mastering these techniques, you can create more efficient and flexible scripts. In the next section, we will delve into scripting basics, starting with creating and running a script.