CST8207: GNU/Linux Operating Systems I

Lab Worksheet Five

I/O Redirection and Pipes

This is Lab Worksheet 5 - not an Assignment

This Lab Worksheet contains some practical examples that will prepare you to complete your Assignments.
You do not have to hand in this Lab Worksheet. Make sure you complete the separate Assignments on time. Quizzes and tests may refer to work done in this Lab Worksheet; save your answers.

Before you get started - REMEMBER TO READ ALL THE WORDS

You must have an account on the Course Linux Server to do this lab. Log in to the server and use the shell. Review the Class Notes related to this worksheet as you work through it. Leave your work on the Linux server. Do not delete any work from the Linux server until the term is over and your marks are complete!

Commands introduced and used in this Lab

bash – Shell command pipelining: |
bash – Shell I/O redirection: > < >>
date – show the current time and date
head – select lines at the start (head) of the input (default is to select first 10 lines)
nl – read lines and prefix them with line numbers (see also: cat -n )
tail – select lines at the end (tail) of the input (default is to select last 10 lines)
tr – translate characters from standard input, e.g. lower-case to upper-case, etc.
wc – count lines, words, and characters

Log in to the Course Linux Server to do all commands in this lab. Set your bash PS1 shell prompt to show your login name, computer name, and the basename of your current directory, just as you did in the previous Lab. Leave your finished work on the server; do not delete it when you are finished the worksheet.

Using Shell I/O Redirection

Redirection happens first, before the command runs. That means shell I/O redirection can appear anywhere in the command line, even at the very beginning: $ >out date
The redirection syntax is removed from the command line before calling the command to run, so it is not counted as an argument on the command line. The command knows nothing about it.
Redirection files are erased to empty even if the command is not found or has no output. This truncation happens before the command runs.
You can only redirect what you can see. If you don't see any output on your screen, adding redirection won't create any.
Redirection goes into only one output file.

Lab 5.1: Redirection happens first, before the command runs

[user@localhost ]$ cd ; rm -rf lab5.1 ; mkdir lab5.1 ; cd lab5.1

[user@localhost lab5.1]$ date > foo (erase foo and then put the date in it)

[user@localhost lab5.1]$ cat foo foo foo (show the file contents three times)

[user@localhost lab5.1]$ cat foo foo foo >foo (send foo into foo three times)

[user@localhost lab5.1]$ cat foo (what is in foo now?)

Explain in detail what you see inside the file foo now and exactly how the contents got that way:

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[user@localhost lab5.1]$ date > foo (erase foo and then put the date in it)

[user@localhost lab5.1]$ cat foo (show the file contents on the screen)

[user@localhost lab5.1]$ cp foo bar >foo (copy the file with some added redirection)

[user@localhost lab5.1]$ cat bar (what is in bar now?)

Explain in detail what you see inside the file bar now and exactly how the contents got that way:

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Lab 5.2: You can only redirect output that you can see

[user@localhost ]$ cd ; rm -rf lab5.2 ; mkdir lab5.2 ; cd lab5.2

[user@localhost lab5.2]$ date >foo (erase foo and then put the date in it)

[user@localhost lab5.2]$ cat foo (show the contents of the file)

[user@localhost lab5.2]$ cp foo bar (copy command has no output to see)

[user@localhost lab5.2]$ cp foo bar >out (redirect output of copy command)

[user@localhost lab5.2]$ cat out (what is in out now?)

Explain in detail what you see inside the file out now and exactly how the contents got that way:

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Lab 5.3: Redirection goes only to one place

[user@localhost ]$ cd ; rm -rf lab5.3 ; mkdir lab5.3 ; cd lab5.3

[user@localhost lab5.3]$ date >a (erase a and then put the date in it)

[user@localhost lab5.3]$ cat a (show the contents of the file on the screen)

[user@localhost lab5.3]$ date >a >b >c (erase three files; put date only in last one)

[user@localhost lab5.3]$ cat a (show content of file a now)

[user@localhost lab5.3]$ cat b (show content of file b now)

[user@localhost lab5.3]$ cat c (show content of file c now)

Explain in detail what you see inside the three files and exactly how the contents got that way:

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Lab 5.4: Only standard output is redirected, by default

[user@localhost ]$ cd ; rm -rf lab5.4 ; mkdir lab5.4 ; cd lab5.4

[user@localhost lab5.4]$ date >foo (erase foo and then put the date in it)

[user@localhost lab5.4]$ cat foo nofile (will generate error message for missing file)

[user@localhost lab5.4]$ cat foo nofile >out (redirect stdout only)

[user@localhost lab5.4]$ cat out

[user@localhost lab5.4]$ cat foo nofile >both 2>&1 (redirect stderr with stdout)

[user@localhost lab5.4]$ cat both

Explain in detail what you see inside the files out and both and exactly how the contents got that way:

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Lab 5.5a: Creating a text file from keyboard input using keyboard EOF ^D

Most Unix/Linux commands that read data from file names as arguments will read your keyboard (“standard input”) if you don't supply any file names to read. Examples: cat, wc, sum, sort, fgrep

[user@localhost ]$ cd ; rm -rf lab5.5 ; mkdir lab5.5 ; cd lab5.5

[user@localhost lab5.5]$ cat > myfile (no file names are given to the cat command)

The cursor waits at the beginning of the next line for input from the keyboard. No prompt appears!

Type: Hello world!

Press [Enter]

Press [Ctrl]+d (hold Ctrl down and then press the d key to close the keyboard and send EOF)

[user@localhost lab5.5]$ cat myfile (This is Output One.)

Note the output of the above command. This is Output One.

[user@localhost lab5.5]$ cat > myfile (no file names are given to the cat command)

Type your name. (No prompt appears! You are typing directly into the cat program.)

Press [Enter]

Press [Ctrl]+d (hold Ctrl down and then press the d key to close the keyboard and send EOF)

[user@localhost lab5.5]$ cat myfile (This is Output Two.)

Explain exactly why does Output Two show none of the text from Output One?

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Lab 5.5b: Appending to or Overwriting an existing file using > and >>

[user@localhost lab5.5]$ date > bar (note only one > on this line!)

[user@localhost lab5.5]$ date >> bar

[user@localhost lab5.5]$ date > bar (note only one > on this line!)

[user@localhost lab5.5]$ date >> bar

How many lines are in output file bar now (guess without executing commands first)? ____________

Lab 5.5c: Concatenating multiple files using cat

In this section you will be using cat without any file name argument, which means it will be reading your keyboard. Type the given text and use the EOF character to complete the input. The output from cat will be redirected into the given output file. (Nothing will appear on your screen – look in the file.)

[user@localhost lab5.5]$ cat > f1

Enter the following keyboard text (and close the file as before using EOF): Hello everybody

[user@localhost lab5.5]$ cat > f2

Enter the following keyboard text (and close the file as before using EOF): My name is My Name

[user@localhost lab5.5]$ cat > f3

Enter the following keyboard text (and close the file as before using EOF): Good-bye

[user@localhost lab5.5]$ cat f1 f2 f3 (see all three files – three lines – on your screen)

[user@localhost lab5.5]$ cat f1 f2 f3 > f4

Describe the contents of file f4 after the last command, above. What happened?

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Lab 5.5d: Throwing away output using /dev/null

[user@localhost lab5.5]$ date >foo (erase foo and then put the date in it)

[user@localhost lab5.5]$ cat foo nofile (will generate error message for missing file)

[user@localhost lab5.5]$ cat foo nofile 2>/dev/null (discard error message)

[user@localhost lab5.5]$ find /etc >out (generates some error messages)

[user@localhost lab5.5]$ find /etc >out 2>/dev/null (discard the error messages)

[user@localhost lab5.5]$ find /etc >/dev/null 2>out (keep only the error messages)

What is the purpose of the special output file pathname /dev/null ?

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Lab 5.6a: Using shell pipelines | to connect commands

[user@localhost ]$ cd ; rm -rf lab5.6 ; mkdir lab5.6 ; cd lab5.6

[user@localhost lab5.6]$ ls -l /bin (too much output to see on one screen)

[user@localhost lab5.6]$ ls -l /bin | less

Use the [spacebar] to jump to the next screen of information in less. You can use q to exit the command.

[user @localhost lab5.6]$ fgrep 'the' /etc/fstab

[user @localhost lab5.6]$ fgrep 'none' /etc/fstab

[user @localhost lab5.6]$ fgrep 'the' /etc/fstab | fgrep 'none'

The first two commands show output, so why is there no output from the last command pipeline, above?

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[user@localhost lab5.6]$ nl /etc/passwd

[user@localhost lab5.6]$ nl /etc/passwd | head -n 5

[user@localhost lab5.6]$ nl /etc/passwd | head -n 5 | tail -n 1

Describe concisely what the last pipeline above selects from any input file (read some man pages first):

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Lab 5.6b: Pipes and Redirecting standard input (stdin)

[user@localhost lab5.6]$ date >foo

[user@localhost lab5.6]$ cat foo

[user@localhost lab5.6]$ echo hi | cat

[user@localhost lab5.6]$ echo hi | cat foo

[user@localhost lab5.6]$ echo hi | cat <foo

[user@localhost lab5.6]$ echo hi | ls

[user@localhost lab5.6]$ echo hi | pwd

Explain in detail why hi does not show on your screen in the last four commands, above?

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[user@localhost lab5.6]$ date >foo | wc

Explain why the output of the above command pipeline is: 0 0 0

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Lab 5.6c: The tr translate command

[user@localhost lab5.6]$ echo 'hello world'

[user@localhost lab5.6]$ echo 'hello world' | tr '[:lower:]' '[:upper:]'

Explain the function of the tr translate command, as shown above?

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Does the tr translate command accept pathnames as command arguments?

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Review: Basic Commands

Describe the purpose of the command: head

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Describe the purpose of the command: tail

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Describe the purpose of the command: fgrep and how it differs from grep

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Describe the purpose and output of the wc command:

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Use a head and tail pipeline to show only lines 10-20 from the file /etc/passwd:

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Use a head and tail and sort pipeline to show only lines 10-20 from the file /etc/passwd, sorted in descending (not ascending) order (read the sort man page):

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Give a command pipeline (two commands) that would show the current month calendar on your screen all in UPPER CASE LETTERS, e.g SEPTEMBER instead of September:

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Do you know the Four Rules for Output Redirection from the class notes?

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Do you know the Three Rules for Pipes from the class notes?

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What is wrong with this command: $ sort file >file

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Shawn Unger, Todd Kelley, Ian Allen

worksheet05.odt

Version 16 01/27/2017, 01:36