• command1 | command2

  The output of one command is stored and then passed as the input for second command.

ps aux | less

• three channels
  • STDIN —— 0
  • STDOUT —— 1
  • STDERR —— 2
• output redirection
  • command > file : overwrite
  • command >> file : append
• input redirection
  • command < file
• error redirection
  • command 2> file
  • command 2>> file

echo "this should be in a file" 1> test.txt  # "1" can be ignored since it is the default
echo "this should be line 2" >> test.txt
mail -s "this is the subject" dave < message.txt
ls -alh thereisnosuchdir 2> err.txt

• ps [options]

  report a snapshot of the current processes.

Linux Sysadmin Basics 04 -- Shell Features -- Pipes and Redirection