1. Agenda Basic Shell Operations Standard Input / Output / Error Redirection of Standard Input / Output / Error ( >, >>, <, <<) Using Pipes (|)

2. Attention These PowerPoint notes are based on Chapter 5 of textbook “A Practical Guide to Linux” Read pages 94 to 104

3. Unix Philosophy Recall the Unix philosophy to do one thing well. Unix has thousands of commands (utilities) to perform specific tasks. Although these specific commands may not be powerful alone, a Unix tool called “redirection” can be used to achieve very powerful results.

4. Unix Philosophy In order to understand the Unix tool of redirection, first understand what can be redirected, and then how it can be redirected. Redirection Terms: Standard Input (STDIN) Standard Output (STDOUT) Standard Error (STDERR)

5. What to Redirect There are generally 3 major things in Unix that can be redirected: Standard Input Data read from a file or terminal (eg. keyboard) Standard Output Data (output) as a result of a command or program executed. Standard Error Error message as a result of improper syntax of command or factors that lead to failure of task.

6. Directing Standard Input  < Redirects standard input from a file to a Unix command. Example:  mail username@learn < myfile

7. Redirecting Standard Output  1> or >Redirects standard output to a file. (Will delete or “overwrite” any existing contents in the file). Example: ls > listing.txt  1>> or >> Redirects standard output to a file but adds to the bottom of file’s existing contents. Example: cat work >> things_to_do

8. Redirecting Standard Error  2> Redirects the standard error to a file. This can be used to write error messages to a file for later reference. 2>> appends error message to bottom of existing file.  Example: cat a b c 2> error_file (Note: if file “b” does not exist, error message is redirected to file called “error_file”.)

9. Pipes ( | ) Pipes are used to send the standard output of one command into another command as the standard input. Pipes have the advantage of modifying standard output to achieve a task without having to create temporary files (which need to be removed later).

10. Pipes ( | )  Consider the following command:  who | sort | more  Result: whosortmore STD OUTPUT from who STD OUTPUT from sort STD INPUT for sort STD INPUT for more STD OUTPUT from more

11. Pipes ( | ) Compare using pipes to transferring water from Lake Ontario to your home: The amount of water (STD INPUT) being sucked into pipes from Lake Ontario is huge. The water is redirected through a series of pipes and processes and filters until it reaches your home as a small stream of water from your tap (STD OUTPUT).