I/O and Redirection

I/O stands for input/output. Redirection allows us to direct input and output functions to specific files or folders using special commands or characters.

Standard Streams

Standard Input (stdin): This is the default source of input for a command, usually the keyboard. It’s represented by the number 0.
Standard Output (stdout): This is the default destination for output from a command, usually the terminal screen. It’s represented by the number 1.
Standard Error (stderr): This is the default destination for error messages from a command, also usually the terminal screen. It’s represented by the number 2.

Types of Redirection

1. Overwrite Redirection:

Overwrite redirection is useful when you want to save the output of a command to a file, replacing the file's existing content entirely.

“>” standard output – Redirects stdout to a file, overwriting the file if it exists

command > file.txt
ls > directory_list.txt
#lists the contents of the current directory and saves the output to directory_list.txt

“<” standard input – Redirects stdin from a file

command < file.txt
wc < hello.txt
#command counts the lines, words, and characters in hello.txt

2. Append Redirection:

Append the output to the file without overwriting the existing data.

“>>” standard output – Redirects stdout to a file, appending to the file if it exists

cat >> file.txt
echo "Hello again!" >> hello.txt
#adds “Hello again!” to the end of hello.txt

“<<” standard input

3. Merge Redirection:

Error Redirection – Save error messages to a file

2>” re-directs the error output to a file named “error.txt”

error message generated by “2” gets merged with the current output “1“.

Redirecting Both stdout and stderr

Overwrite a file with both outputs

Append both outputs to a file