Redirection operators redirects the output/input of a script (or file) to another script (or file) as input/output.

• "<", "0<": STDIN operator (file, keyboard, mouse, ...)
  
  
• ">", "1>": STDOUT operator (file, monitor, printer, ...)
  
  
• "2>": STDERR operator (file, monitor, printer, ...)
  
  
• "&>": STDOUT & STDERR operator (file, monitor, printer, ...)
  

STDIN: standard input
STDOUT: standard output
STDERR: standard error

STDOUT and STDERR are two different streams of output.

• "<" STDIN redirection operator.
  
  Search the word "foo" in the file "file1":
  

  $ grep foo < file1

• "0<" STDIN redirection operator (same as "<").
  
  Search the word "foo" in the file "file1":
  

  $ grep foo 0< file1

• The use of the STDIN redirection operator "<" or "0<" is optional, as it's the default operator in the absence of an explicit redirection operator.
  
  Search the word "foo" in the file "file1":
  

  $ grep foo file1

• You can also use the redirection operator "<<" that allow you to type a text that can be use a standard input for a command.
  The syntax for using this operator is as follow: command <<FLAG <ENTER> text <ENTER> ... FLAG<ENTER>
  The word flag will act as an indicator that you you have ended typing your text.
  
  Sort the text entered in the standard input (between <<end and end):
  

  $ sort <<end
  > aaa
  > ccc
  > bbb
  > end
  aaa
  bbb
  ccc

• ">" STDOUT redirection operator
  
  Write the result of the comand "ls -al" in the file "file1":

  $ ls -al > file1

• "1>" STDOUT redirection operator (same as ">")
  
  Write the result of the comand "ls -al" in the file "file1":
  

  $ ls -al 1> file1

• ">>" STDOUT redirection operator + append
  
  Append the result of the comand "ls -al" in the file "file1":
  

  $ ls -al >> file1

• "1>>" STDOUT redirection operator + append (same as >>)
  
  Append the result of the comand "ls -al" in the file "file1":
  

  $ ls -al 1>> file1

• "2>" STDERR redirection operator
  
  Write the error (ls: -: No such file or directory) of the comand "ls -" in the file "file1":
  

  $ ls - 2> file1

• "2>>" STDERR redirection operator + append
  
  Append the error (ls: -: No such file or directory) of the comand "ls -" in the file "file1":
  

  $ ls - 2>> file1

• "1>&2" redirect STDOUT to the same "stream" as STDERR
  
  Write/append the result of the command "ls -al" to the same "stream" as STDERR ("file1"):
  

  $ ls -al 2> file1 1>&2
  
  $ ls -al 2>> file1 1>&2

• "2>&1" redirect STDERR to the same "stream" as STDOUT
  
  Write/append the error (ls: -: No such file or directory) of the command "ls -" to the same "stream" as STDOUT ("file1"):

  $ ls - > file1 2>&1
  
  $ ls - >> file1 2>&1
  
  $ ls - 1> file1 2>&1
  
  $ ls - 1>> file1 2>&1

Redirect errors to /dev/null:

$ ls - > file1 2>/dev/null

"&>" both STDOUT and STDERR redirection operator.

• Write the result of the comand "ls -al" in the file "file1":
  

  $ ls -al &> file1

• Write the error (ls: -: No such file or directory) of the comand "ls -" in the file "file1":

  $ ls - &> file1

pipe operator redirects the output of a command (on the left of the pipe) as input to the next command (on the right of the pipe).

# "|" pipe operator
$ ps -ef | grep java

tee command redirects the output of a script to files and STDOUT.

$ ls -1 | tee files
file1
file2
file3

$ tee file1_copy < file1
file1 content