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Tutorial5: Redirection

Revision as of 06:15, 24 January 2020 by Msaul (talk | contribs) (Piping (Using Pipes))

REDIRECTION: STANDARD INPUT / STANDARD OUTPUT / STANDARD ERROR


Main Objectives of this Practice Tutorial

  • Define the terms Standard Input (stdin), Standard Output (stdout), and Standard Error (stderr)
  • Understand the purposes of redirection symbols >, >>, 2>, 2>>, | and define their purpose in terms of redirection
  • Understand the purpose of the additional file manipulation commands: cut, tr, and wc
  • Define the term pipeline command and explain how a pipeline command works in terms of redirection
  • Define the term filter and how it relates to redirection using pipeline commands
  • Understand how to use the semicolon symbol ";" to issue multiple Unix / Linux commands
  • Understand how to use grouping using the symbols "( )" and how it is used to issue multiple Unix / Linux commands


Tutorial Reference Material

Course Notes
Linux Command / Shortcut Reference
YouTube Videos
Course Notes:


Redirection

Multiple Commands

Redirection Filters Brauer Instructional Videos:

KEY CONCEPTS

Redirection (Standard Input, Standard Output, Standard Error)

... standard streams are preconnected input and output communication channels between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called standard input (stdin), standard output (stdout) and standard error (stderr). Originally I/O happened via a physically connected system console (input via keyboard, output via monitor), but standard streams abstract this. When a command is executed via an interactive shell, the streams are typically connected to the text terminal on which the shell is running, but can be changed with redirection or a pipeline.

Reference: https://en.wikipedia.org/wiki/Standard_streams

 
The standard input (stdin) symbol that describes where a Unix/Linux command receives input

Standard input (stdin) is a term which describes from where a command receives input.
This would apply only to Unix/Linux commands that accept stdin input (like cat, more, less, sort, grep, head, tail, etc.).
With the examples below, standard input is being sent into the command from the text file.

Examples:

tr 'a-z' 'A-Z' < words.txt
cat < abc.txt
sort < xyz.txt


 
The standard input (stdin) symbol with one greater than sign overwrites existing file content with command output
 
The standard input (stdin) symbol with two greater than signs add command's output to bottom of existing file's contents.

Standard output (stdout) describes where a command sends it's output.
In the examples below, output from a command is sent to the monitor, unless it is sent to a regular file.
We will discuss another redirection technique called pipelines later.

Examples:

ls -l
ls -l > detailed-listing.txt
ls /bin >> output.txt

 
The standard error (sterr) symbol with one greater than sign overwrites existing file content with command's error message.
 
The standard error (stderr) symbol with two greater than signs add command's error message to bottom of existing file's contents.

Standard Error (stderr) describes where a command sends it's error messages.
In the examples below, error messages from a command are sent to a file or are redirected to a file that acts as a "garbage can".

Examples (remember, Unix/Linux is case sensitive and issuing PWD in caps will generate an error message):

PWD
PWD 2> error-message.txt
PWD 2 >> error-messages.txt
PWD 2> /dev/null


Additional File Manipulation Commands

There are some additional regular file manipulation commands that you can use with redirection
(in addition to the other regular file manipulation commands introduced in week 2). These commands are displayed in the table below:

Linux CommandPurpose
cutUsed to extract fields and characters from records. The option -c option is used to cut by a character or a range of characters. The -f option indicates the field number or field range to display (this may require using the -d option to indicate the field separator (delimiter).

Examples:
cut -f2 filename - extract 2nd field from all records in file, using tab as delimiter (default)
cut -d' ' -f2,5 filename - extract 2nd and 5th field, using space as delimiter
cut -d' ' -f1-3,5 filename - extract 1st through 3rd and 5th fields, using space as delimiter
cut -c3-5 filename - extract 3rd to 5th characters
trUsed to translate characters to different characters.

Examples:
tr a A < filename - translate all characters "a" to "A"
tr "[a-z]" "[A-Z]" < filename - translate lowercase "a" through "z" to uppercase
tr "a-z" "A-Z" < filename - translate lowercase "a" through "z" to uppercase, different syntax (non-System V)
tr ':' ' ' < filename - translate all colons to spaces
tr ' ' '\n' < filename - translate all spaces to newline characters
tr 'abc' 'A' < filename - translate 'a', 'b', and 'c' to 'A', the last character in the "to" string repeats
wcDisplays various counts of the contents of a file.

Examples:
wc -l filename - displays number of lines in file
wc -c filename - displays number of characters in file
wc -w filename - displays number of words in fil


Piping (Using Pipes)

 
A pipeline command sends a command's standard output directly to standard input of other command(s) without having to create temporary files.

Pipeline Command: Having commands send their standard output directly to standard input of other commands without having to use temporary files.

A few simple commands can be combined to form a more powerful command line.


Pipes are represented by | Many commands can be "piped" together, filter commands are especially useful Each filter processes the initial input based on it's design Filters must be chained in a specific order, depending on what you wish to accomplish Example piping use: ls -al | more

Discuss pipes with the tee filter

Multiple Commands Using Semicolon ; / Grouping Commands ( )

x

INVESTIGATION 1: BASICS OF REDIRECTION


In this section, you will learn how to ...



Perform the Following Steps:

  1. x

In the next investigation, you will ...

INVESTIGATION 2: REDIRECTION USING PIPES

In this section, you will learn how to ...


Perform the Following Steps:

  1. x

In the next investigation, you will ...

INVESTIGATION 3: MULTIPLE COMMANDS / COMMAND GROUPING

In this section, you will learn how to ...


Perform the Following Steps:

  1. x

LINUX PRACTICE QUESTIONS

The purpose of this section is to obtain extra practice to help with quizzes, your midterm, and your final exam.

Here is a link to the MS Word Document of ALL of the questions displayed below but with extra room to answer on the document to simulate a quiz:

https://ict.senecacollege.ca/~murray.saul/uli101/uli101_week5_practice.docx

Your instructor may take-up these questions during class. It is up to the student to attend classes in order to obtain the answers to the following questions. Your instructor will NOT provide these answers in any other form (eg. e-mail, etc).


Review Questions:

  1. x
  2. x
  3. x
  4. x
  5. x
  6. x
  7. x
  8. x
  9. Create a table listing each Linux command, useful options that were mentioned in this tutorial for the following Linux commands: cut , tr , wc , and tee.