Another way to do Arithmetic expansion

This is the third way to do arithmetic operations. First we looked at 'let' and 'expr' commands.
Now we use $ sign with two brackets.
example1:
echo $(( 5 * 6))

example2:
echo $(($((4 * 5)) + 300 ))

Redirect the standard error

There are three I/O streams available in Unix.
1. STDIN (0)
2. STDOUT (1)
3. STDERR (2)

To redirect the error to a file

ls -l doesnotexist 2>err.txt

Example:
ls -l exp 1>result.txt 2>error.txt
This command outputs the standard output into the file called result.txt and the standard error to the file called error.txt.

Redirection

To redirect the output to a file use '>' command
example :
ls -l > example.txt
'>' command replace the existing content of the file. If you want to append the output to with existing content, use '>>'
example:
ls -l >> example.txt
'<' command use to get the input from a file.
example:
cat < example.txt
This redirection command doesn't know where is the input come from.
For example if you use
wc -w < example.txt
It only outputs the number of words in the file.

If you use wc -w example.txt instead of that you will get the number of words and the name of the file.