tee

duplicate standard input

tee [-ai] [file …]

Copies standard input to standard output (unbuffered) as well as to file ….

Although file is optional it seems that without it only one output stream is produced.
With multiple files the same output is written to each.

`-a`	Append the output to the files rather than overwriting them.
`-i`	Ignore sigINT (usually `^c`)

Takes the default action for signals for example sigQUIT usualy ^\, except with -i

Examples

Send the echoed message both to stdout and to greetings.txt:

  $ echo "Hello" | tee greetings.txt
  Hello
  $ ls -log greetings.txt 
  -rw-r----- 1 6 Apr  1 16:33 greetings.txt

   echo "report length is:"
# use wc on stdout to count lines in ADDITION to writing the report to report.txt
   report | tee report.txt |wc -l   
   echo "view it?"
   read A
   if [ $A != 'y' ] ; then exit ;fi
   clear
   cat report.txt
   echo "\n\n\n\n\n\n"

   8739
   view it?
   n