Detect if shell script is running through a pipe

How do I detect from within a shell script if its standard output is targetting a terminal or if it's piped to another process? (Case in point: I'd like to add escape codes to colorize output, but only when run interactively, but not when piped, similarly to what ls --color does.)

From stackoverflow hisham_hm

• Pure bash:

  if [ -t 1 ] ; then echo terminal; fi
  

  returns "terminal" and

  (if [ -t 1 ] ; then echo terminal; fi) | echo
  

  returns nothing.

  ---

  The -t flag is described in the man pages as

  -t fd True if file descriptor fd is open and refers to a terminal.

  and I take advantage of the usual file descriptor assignments

  0:     stdin  
  1:     stdout  
  2:     stderr
  

  From dmckee

• The command test (builtin in bash), has an option to check if a file descriptor is a tty.

  if [ -t 1 ]; then
      # stdout is a tty
  fi
  

  See "man test" or "man bash" and search for "-t"

  From Beano

• You don't mention which shell you are using, but in Bash, you can do this:

  #!/bin/bash
  
  if [[ -t 1 ]]; then
      # stdout is a terminal
  else
      # stdout is not a terminal
  fi
  

  From Dan Moulding