rha030-workbook07-student-3.0-0

Workbook 7. Standard I/O and Pipes

Red Hat, Inc.

Workbook 7. Standard I/O and Pipesby Red Hat, Inc.Copyright © 2003-2005 Red Hat, Inc.

Revision HistoryRevision rha030-2.0-2003_11_12-en 2003-11-12First RevisionRevision rha030-3.0-0-en-2005-08-17T07:23:17-0400 2005-08-17First Revision

Red Hat, Red Hat Network, the Red Hat "Shadow Man" logo, RPM, the RPM logo, PowerTools, and all Red Hat-based trademarks and logos aretrademarks or registered trademarks of Red Hat, Inc. in the United States and other countries.Linux is a registered trademark of Linus Torvalds.Motif and UNIX are registered trademarks of The Open Group.Windows is a registered trademark of Microsoft Corporation.Intel and Pentium are a registered trademarks of Intel Corporation. Itanium and Celeron are trademarks of Intel Corporation.SSH and Secure Shell are trademarks of SSH Communications Security, Inc.All other trademarks and copyrights referred to are the property of their respective owners.

Published 2005-08-17

Table of Contents1. Standard In and Standard Out ............................................................................................................ 4

Discussion .......................................................................................................................................... 4Three types of programs........................................................................................................... 4Standard in (stdin) and Standard out (stdout)........................................................................... 5Redirecting stdout..................................................................................................................... 5Redirecting stdin....................................................................................................................... 6Under the Hood: Open Files and File Descriptors ...................................................................7

Examples............................................................................................................................................ 9Example 1. Getting Out of sort’s ............................................................................................. 9Example 2. Automated FTP Transfers ................................................................................... 10Example 3. Automating Graph Generation with gnuplot ......................................................13

Online Exercises............................................................................................................................... 15Online Exercise 1. Using Standard In and Standard Out .......................................................15

Questions.......................................................................................................................................... 162. Standard Error .................................................................................................................................... 20

Discussion ........................................................................................................................................ 20Standard Error (stderr)............................................................................................................ 20Redirecting stderr ................................................................................................................... 21Combining stdout and stderr: Old School .............................................................................. 22Combining stdout and stderr: New School............................................................................. 23Summary................................................................................................................................. 24

Examples.......................................................................................................................................... 24Example 1. Using /dev/null to filter out stderr ..................................................................24

Online Exercises............................................................................................................................... 25Specification ........................................................................................................................... 25Deliverables ............................................................................................................................ 26

Questions.......................................................................................................................................... 263. Pipes ...................................................................................................................................................... 30

Discussion ........................................................................................................................................ 30Pipes ....................................................................................................................................... 30Filtering output using grep..................................................................................................... 31Pipes and stderr....................................................................................................................... 32Commands as filters ............................................................................................................... 32

Examples.......................................................................................................................................... 33Example 1. Listing Processes by Name ................................................................................. 33Example 2. Searching Command History Efficiently ............................................................33Example 3. Unix philosophy: Simple Tools that Work Well Together...................................34

Online Exercises............................................................................................................................... 37Specification ........................................................................................................................... 37Deliverables ............................................................................................................................ 37Clean Up................................................................................................................................. 37

Questions.......................................................................................................................................... 37

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Chapter 1. Standard In and Standard Out

Key Concepts• Terminal based programs tend to read information from one source, and write information to one

destination.

• The source programs read from is referred to as Standard In (stdin), and is usually connected to aterminal’s keyboard.

• The destination programs write to is referred to as Standard Out (stdout), and is usually connected to aterminal’s display.

• When using the bash shell, stdout can be redirected using > or >>, and stdin can be redirected using <.

DiscussionMany Linux commands read input from the keyboard and display output to the terminal. In thisWorkbook, you’ll learn how you can redirect where input is read from and where output goes. Theoutput of one command can be used as the input for another command, allowing simple commands to beused together to perform more complicated tasks.

Three types of programsIn Linux (and Unix), programs can generally be grouped into the following three designs.

Graphical Programs

Graphical programs are designed to run in the X graphical environment. They expect the user to beusing a mouse, and use common graphical components, such as popup menus and buttons, for userinput. The mozilla web browser is an example of a graphical program.

Screen Programs

Screen based programs expect to use a text console. They make use of the entire display, and handletext placement and screen redraws in sophisticated ways. They do not require a mouse, and areappropriate for terminals and virtual consoles. The vi and nano text editors, and links web browser,are examples of screen based programs.

Terminal Programs

Terminal programs collect input and display output in a stream, seldom if ever redrawing the screen,as if writing directly to a printer that does not allow the cursor to move back up the page. Because oftheir simplicity, terminal based programs are often called simply commands. ls, grep, and useraddare examples of terminal based programs.

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This chapter focuses on the latter type of program. Do not let the simplicity of the way these commandsreceive input and output fool you. You will find that many of these commands are very sophisticated, andallow you to use the command line interface in powerful ways.

Standard in (stdin) and Standard out (stdout)Terminal based programs generally read information as as stream from a single source, such as aterminal’s keyboard. Likewise, they generally write information as a steam to a single destination, suchas a display. In Linux (and Unix), the input stream is referred to as Standard In (usually abbreviatedstdin), and the output stream is referred to as Standard Out (usually abbreviated stdout).

Usually, stdin and stdout are connected to the terminal that runs the command. Sometimes, in order toautomate commonly repeated commands, or in order to record the output of a command for laterinclusion in a report or email, people find it convenient to redirect stdin from or stdout into files.

Redirecting stdout

Writing Output to a FileWhen a terminal based program generates output, it generally writes that output to its stdout stream,without knowing what is connected to the receiving end of that stream. Usually, the stdout stream isconnected to the terminal that started the process, so the output is written to the terminal’s display. Thebash shell uses > to redirect a process’s stdout stream to a file.

For example, suppose the machine elvis is using becomes very sluggish and non-responsive. In order todiagnose the problem, elvis would like to examine the currently running processes. Because the machineis so sluggish, however, he wants to collect the information now, but analyze it later. He can redirect theoutput of the ps aux command into the file sluggish.txt, and come back to examine the file when themachine is more responsive.

[elvis@station elvis]$ ps aux > sluggish.txt[elvis@station elvis]$

Notice that no output is displayed to the terminal. The ps command writes to stdout, as it always does,but stdout is redirected by the bash shell to the file sluggish.txt. The user elvis can examine the filelater, at a more convenient time.

[elvis@station elvis]$ head sluggish.txtUSER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMANDroot 1 0.0 0.0 1380 76 ? S Jun02 0:04 init [root 2 0.0 0.0 0 0 ? SW Jun02 0:00 [keventd]root 3 0.0 0.0 0 0 ? SW Jun02 0:00 [kapmd]root 4 0.0 0.0 0 0 ? SWN Jun02 0:00 [ksoftirqd_CPU0]root 9 0.0 0.0 0 0 ? SW Jun02 0:00 [bdflush]root 5 0.0 0.0 0 0 ? SW Jun02 0:00 [kswapd]root 6 0.0 0.0 0 0 ? SW Jun02 0:00 [kscand/DMA]root 7 0.0 0.0 0 0 ? SW Jun02 0:37 [kscand/Normal]root 8 0.0 0.0 0 0 ? SW Jun02 0:00 [kscand/HighMem]

rha030-3.0-0-en-2005-08-17T07:23:17-0400

Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any otheruse is a violation of U.S. and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwiseduplicated whether in electronic or print format without prior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, orotherwise improperly distributed please email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

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Appending Output to a FileIf the file sluggish.txt already existed, its original contents would be lost. This is often referred to asclobbering a file. To append a command’s output to a file, rather than clobbering it, bash uses >>.

Suppose that elvis wanted to record a timestamp of when the sluggish behavior was happening, as wellas a list of currently running processes. He could first create (or clobber) the file with the output of thedate command, using >, and then append to it the output of the ps aux command using >>.

[elvis@station elvis]$ date > sluggish.txt[elvis@station elvis]$ ps aux >> sluggish.txt[elvis@station elvis]$ head sluggish.txtTue Jun 3 16:57:23 EDT 2003USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMANDroot 1 0.0 0.0 1380 76 ? S Jun02 0:04 init [root 2 0.0 0.0 0 0 ? SW Jun02 0:00 [keventd]root 3 0.0 0.0 0 0 ? SW Jun02 0:00 [kapmd]root 4 0.0 0.0 0 0 ? SWN Jun02 0:00 [ksoftirqd_CPU0]root 9 0.0 0.0 0 0 ? SW Jun02 0:00 [bdflush]root 5 0.0 0.0 0 0 ? SW Jun02 0:00 [kswapd]root 6 0.0 0.0 0 0 ? SW Jun02 0:00 [kscand/DMA]root 7 0.0 0.0 0 0 ? SW Jun02 0:37 [kscand/Normal]

Redirecting stdinJust as bash uses > to coax commands into delivering their output somewhere other than the display,bash uses < to cause them to read input from somewhere other than the keyboard. The user elvis is stilltrying to figure out why his machine was acting sluggish. He talked to his local system administrator,who thought that looking at the list of currently running processes sounded like a good idea, and askedelvis to mail him a copy.

Using the terminal based mail command, elvis first writes an email message to the administrator"manually", from the keyboard. The mail command expects a recipient as an argument, and the subjectline can be specified with the -s command line switch. The email body is then entered from the keyboard.The end of the message text is signaled by a lone period on a line.

[elvis@station elvis]$ mail -s "Computer is sluggish" [email protected]

Hey sysadmin...

I’m sending a list of processes that were running when the computer was runningin a separate email.

Thanks! --elvis.Cc:

For his follow-up message, elvis can easily mail the output of the ps command he recorded in the filesluggish.txt. He just redirects the mail command’s stdin stream to be read from the file.

[elvis@station elvis]$ mail -s "ps output" [email protected] < sluggish.txt

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The system administrator will receive an email from elvis, with "ps output" as it’s subject line, and thecontents of the file sluggish.txt as its body.

In the first case, the mail process’s stdin was connected to the terminal, and the message body wasprovided by the keyboard. In the second case, bash arranged for the mail process’s stdin to be connectedto the file sluggish.txt, and the message body was provided by its contents. The mail commanddoesn’t change its basic behavior: It reads the body of the email message from stdin. 1

Under the Hood: Open Files and File Descriptors

Open Files and File DescriptorsTo fully appreciate how processes manage Standard In, Standard Out, and files, we must introduce theconcept of a file descriptor. In order to read information from or write information to a file, a processmust open the file. Linux (and Unix) processes keep track of the files they currently have open byassigning each an integer. The integer is called a file descriptor.

The Linux kernel provides an easy way to examine the open files and file descriptors of a currentlyrunning process, using the /proc file system. Every process has an associated subdirectory under/proc, named after its PID (process ID). The process’s subdirectory in turn has a subdirectory called fd(for f ile descritptor). Within the /proc/pid/fd subdirectory, a symbolic links exists for every file theprocess has open. The name of the symbolic link is the open file’s integer file descriptor, and thesymbolic link resolves to the open file itself.

In the following, elvis cats the file /etc/termcap, and then almost immediately suspends the programwith a CTRL-Z.

[elvis@station elvis]$ cat /etc/termcap

[1]+ Stopped cat /etc/termcap

Using the ps command to look up the process’s PID, elvis next examines the process’s /proc/pid/fddirectory.

[elvis@station elvis]$ psPID TTY TIME CMD

1368 pts/1 00:00:00 bash1910 pts/1 00:00:00 cat Ê

1911 pts/1 00:00:00 ps[elvis@station elvis]$ ls -l /proc/1910/fd Ë

total 0lrwx------ 1 elvis elvis 64 Sep 13 06:42 0 -> /dev/tty1lrwx------ 1 elvis elvis 64 Sep 13 06:42 1 -> /dev/tty1lrwx------ 1 elvis elvis 64 Sep 13 06:42 2 -> /dev/tty1lr-x------ 1 elvis elvis 64 Sep 13 06:42 3 -> /etc/termcap

Ê elvis observes that the PID of the cat process is 1910.

Ë elvis now looks in the subdirectory which corresponds to the observed PID.

rha030-3.0-0-en-2005-08-17T07:23:17-0400Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any otheruse is a violation of U.S. and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwiseduplicated whether in electronic or print format without prior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, orotherwise improperly distributed please email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

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Not surprisingly, the cat process has the file /etc/termcap open (it must be able to read the file todisplay its contents). Perhaps a little surprising, it is not the only, or even the first, file that the process hasopen. The cat command has three open files before it, or, more exactly, the same file open three times:/dev/tty1.

As a Linux (and Unix) convention, every process inherits three open files upon startup. The first, filedescriptor 0, is Standard In. The second, file descriptor 1, is Standard Out, and the third, file descriptor 2,is Standard Error (to be discussed in the next Lesson). What open files did the cat command inherit fromthe bash shell that started it? The device node /dev/tty1 for all three.

Table 1-1. Standard In, Standard Out, and Standard Error File Descriptors

Stream Descriptor AbbreviationStandard In 0 stdinStandard Out 1 stdoutStandard Error 2 stderr

Recall that /dev/tty1 is the device node which connects to the console serial driver within the kernel.Whatever elvis types can be read from this file, and whatever is written to this file is displayed on elvis’sterminal. What happens if the cat process reads from stdin? It reads input from elvis’s keyboard. Whathappens if it writes to stdout? Whatever is written is displayed on elvis’s terminal.

RedirectionIn the next example, elvis cat’s the /etc/termcap file, but this time redirects stdout to the file/tmp/foo. Again, elvis suspends the command in mid-stride with the CTRL-Z control sequence.

[elvis@station elvis]$ cat /etc/termcap > /tmp/foo

[1]+ Stopped cat /etc/termcap >/tmp/foo

Using the same technique as above, elvis examines the files opened by the cat command, and the filedescriptors associated with them.

[elvis@station elvis]$ psPID TTY TIME CMD

1368 pts/1 00:00:00 bash1976 pts/1 00:00:00 cat1977 pts/1 00:00:00 ps

[elvis@station elvis]$ ls -l /proc/1976/fdtotal 0lrwx------ 1 elvis elvis 64 Sep 13 07:05 0 -> /dev/pts/1l-wx------ 1 elvis elvis 64 Sep 13 07:05 1 -> /tmp/foo Ê

lrwx------ 1 elvis elvis 64 Sep 13 07:05 2 -> /dev/pts/1lr-x------ 1 elvis elvis 64 Sep 13 07:05 3 -> /etc/termcap

Ê Notice that file descriptor 1 (in other words, Standard Out) is no not connected to the terminal, butinstead to the file /tmp/foo.

rha030-3.0-0-en-2005-08-17T07:23:17-0400

Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any other use isa violation of U.S. and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwise duplicated whetherin electronic or print format without prior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, or otherwise improperly distributedplease email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

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What happens when elvis redirects both Standard Out and Standard In?

[elvis@station elvis]$ cat < /etc/termcap > /tmp/foo

[1]+ Stopped cat </etc/termcap >/tmp/foo[elvis@station elvis]$ ps

PID TTY TIME CMD1368 pts/1 00:00:00 bash1980 pts/1 00:00:00 cat1988 pts/1 00:00:00 ps

[elvis@station elvis]$ ls -l /proc/1980/fdtotal 0lr-x------ 1 elvis elvis 64 Sep 13 07:07 0 -> /etc/termcap Ê

l-wx------ 1 elvis elvis 64 Sep 13 07:07 1 -> /tmp/foolrwx------ 1 elvis elvis 64 Sep 13 07:07 2 -> /dev/pts/1

Ê File descriptor 0 (Standard In) is not connected to the terminal, but instead to the file/etc/termcap.

When the cat command is called without arguments (i.e., without any filenames of files to display), itdisplays Standard In instead. Rather than opening a specified file (using file descriptor 3, as above), thecat command reads from stdin instead.

What is the effective difference between the following three commands?

[elvis@station elvis]$ cat < /etc/termcap > /tmp/foo[elvis@station elvis]$ cat /etc/termcap > /tmp/foo[elvis@station elvis]$ cp /etc/termcap /tmp/foo

There is none. In order to appreciate the real benefit of designing commands to read from Standard In inlieu of named files, we must wait until pipes are introduced in a subsequent Lesson.

ExamplesThe following examples include a quick example of how new users can often get confused by commandsthat read from Standard In, and a couple of more "real world" examples that use the ftp and gnuplotprograms. The ftp and gnuplot programs are both complicated programs, and these examples merelyintroduce enough of their functionality to emphasize one of the main themes of this Workbook: if theprogram is driven from a command line interface, it can usually be automated with a simple text scriptand redirection.

Example 1. Getting Out of sort’sIn the following, blondie uses the sort command to sort the animals found in the text file zoo.

[blondie@station blondie]$ cat zooelephantseal

rha030-3.0-0-en-2005-08-17T07:23:17-0400Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any other use isa violation of U.S. and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwise duplicated whetherin electronic or print format without prior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, or otherwise improperly distributedplease email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

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apegiraffefish[blondie@station blondie]$ sort zooapeelephantfishgiraffeseal

As the name of the command suggests, the sort command (in its simplest form) reads a file, and writesthe contents sorted line by line alphabetically. Like the cat command, the sort command, when notprovided any arguments (i.e., filenames of files to sort), the sort command will look to stdin for its input.

[blondie@station blondie]$ sort < zooapeelephantfishgiraffeseal

While this behavior seems (and is) perfectly reasonable, it often confuses new users who innocently typea command’s name, "just to see what it does". In the following, assume that blondie does not yet knowabout Standard In. Exploring, she invokes the sort command. Not understanding that the sort commandis waiting to read Standard In, i.e., her keyboard, she tries to somehow exit the command she’s started.Finally, a friend whispers to her, "CTRL-D".

[blondie@station blondie]$ sortlsquitman sortexitget me out of thisCTRL-Dexitget me out of thislsman sortquit[blondie@station blondie]$

Upon typing CTRL-D, the conventional "End of File" control sequence (recall Workbook 1), the sortcommand prints a sorted list of everything it read on Standard In.

Example 2. Automated FTP TransfersThe user blondie routinely grabs a README file from the ftp server for the linux kernel project,ftp.kernel.org. The kernel.org ftp server allows anonymous users, that is, users who sign in using theusername "anonymous". When prompted for a password, anonymous ftp users do not need to supplyone, but conventionally give their email address in lieu of a password.


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[blondie@station student]$ ftp ftp.kernel.orgConnected to ftp.kernel.org (204.152.189.116).220 ProFTPD [ftp.kernel.org]Name (ftp.kernel.org:blondie): anonymous331 Anonymous login ok, send your complete email address as your password.Password: (blondie types in her email address)230 Anonymous access granted, restrictions apply.Remote system type is UNIX.Using binary mode to transfer files.ftp> ls227 Entering Passive Mode (204,152,189,116,237,224).150 Opening ASCII mode data connection for file listdrwxr-s--- 2 korg mirrors 4096 May 21 2001 for_mirrors_onlydrwx------ 2 root root 16384 Mar 18 00:27 lost+founddrwxrwsr-x 8 korg korg 4096 Mar 24 17:46 pub226 Transfer complete.ftp> cd pub250 CWD command successful.ftp> ls227 Entering Passive Mode (204,152,189,116,237,229).g150 Opening ASCII mode data connection for file listdrwxrws--- 2 korg korg 4096 Mar 18 04:05 RCS-r--r--r-- 1 korg korg 1963 Oct 4 2001 README-r--r--r-- 1 korg korg 578 Mar 18 04:04 README_ABOUT_BZ2_FILESdrwxrwsr-x 4 korg korg 4096 Oct 26 2000 dist...226 Transfer complete.ftp> get READMElocal: README remote: README227 Entering Passive Mode (204,152,189,116,237,237).150 Opening BINARY mode data connection for README (1963 bytes).226 Transfer complete.1963 bytes received in 0.000564 secs (3.4e+03 Kbytes/sec)ftp> quit221 Goodbye.

When the ftp command pauses with the ftp> prompt, blondie types commands to navigate the ftpserver’s directories. If blondie downloaded this file often, she might be tempted to write a simple text file,getreadme.ftp, that would reproduce the commands she typed from the keyboard. She could then runthe same command, ftp ftp.kernel.org. This time, however, she would use < to cause bash to redirectstdin from the file getreadme.ftp. When the ftp command reads input from its stdin stream, theinformation is provided by the file instead of the keyboard.

First, blondie uses a simple text editor to create the file getreadme.ftp containing all of the commandsthat she typed interactively on the keyboard (including the password she supplied to the anonymous ftpserver, [email protected]).

[blondie@station blondie]$ cat [email protected] publs


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get READMEquit

Notice how the contents of the file match exactly what she typed when using the ftp command above.Next, she reruns the ftp ftp.kernel.org, but redirects stdin from the newly created file.

[blondie@station blondie]$ ftp ftp.kernel.org < getreadme.ftpPassword:Name (ftp.kernel.org:blondie):?Invalid commanddrwxr-s--- 2 korg mirrors 4096 May 21 2001 for_mirrors_onlydrwx------ 2 root root 16384 Mar 18 00:27 lost+founddrwxrwsr-x 8 korg korg 4096 Mar 24 17:46 pubdrwxrws--- 2 korg korg 4096 Mar 18 04:05 RCS-r--r--r-- 1 korg korg 1963 Oct 4 2001 README-r--r--r-- 1 korg korg 578 Mar 18 04:04 README_ABOUT_BZ2_FILESdrwxrwsr-x 4 korg korg 4096 Oct 26 2000 dist-r--r--r-- 1 korg korg 1507 Oct 11 2001 index.htmldrwxrwsr-x 8 korg korg 4096 Jan 21 2002 linuxdrwxrwsr-x 3 korg korg 4096 Mar 24 17:46 scmdrwxrwsr-x 3 korg korg 4096 Oct 11 2001 sitedrwxrwsr-x 11 korg korg 4096 Jan 1 2002 software[blondie@station blondie]$ ls -l README-rw-rw-r-- 1 blondie blondie 1963 Jun 3 17:37 README

After the command has run, blondie has a new README file in her directory, which was downloaded bythe ftp command. blondie did encounter a couple of hitches, however.

• First, the command paused, and she needed to hit RETURN once to get it to complete. For securityreasons, many commands, when reading passwords, do not read the passwords from stdin, but fromthe terminal directly. (Commands do not have to rely on stdin as their sole source of input, but mostchoose to do so.) When ftp attempted to read the password from the terminal, the program suspendeduntil blondie hit the RETURN key.

• Secondly, there is an odd line specifying ?Invalid Input. Because the password was read from theterminal directly, it was not consumed from getreadme.ftp file. When the ftp command went toread its next line of input, it read [email protected], which it reasonably didn’t recognize as acommand.

• Lastly, directory listings were dumped to the terminal when the command was run. When the ftpcommand ran the ls commands from getreadme.txt, it wrote the output to stdout, which is stillconnected to the terminal. Since blondie knows where the file is located, and has embedded thatinformation into the script, she does not need to see these listings every time she runs the command.

To address these issues, she first takes advantage of a ~/.netrc file. The ftp command is designed tolook for such a file in the user’s home directory, and if it exists, use it to provide the user’s username andpassword. After examining the netrc(5) man page, blondie uses a simple text editor to create thefollowing ~/.netrc file.

[blondie@station blondie]$ cat .netrcdefault login anonymous password [email protected]

Because the ~/.netrc file will now provide her username and password, she removes them from hergetreadme.ftp script. Secondly, she removes the unnecessary ls commands from the script as well.


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[blondie@station blondie]$ cat getreadme.ftpcd pubget READMEquit

Armed with her ~/.netrc file (to provide her username and password) and her modifiedgetreadme.txt (to provide the commands for the ftp program), she reruns the ftp command, and theoperation runs smoothly.

[blondie@station blondie]$ head .netrc getreadme.ftp==> .netrc <==default login anonymous password user@site

==> getreadme.ftp <==cd pubget READMEquit[blondie@station blondie]$ lsgetreadme.ftp[blondie@station blondie]$ ftp ftp.kernel.org < getreadme.ftp[blondie@station blondie]$ lsgetreadme.ftp README

Example 3. Automating Graph Generation with gnuplotThe user madonna would like to be able to easily generate plots of her machine’s CPU activity. She isfamiliar with the vmstat command, which samples and tables many parameters which concern systemperformance. The command can take two numeric arguments, the first which specifies the samplingperiod in seconds, the last the number of samples to collect.

She is interested in the last three columns, which are the the percentage of time the CPU is spending inthe user ("us"), system ("sy"), and idle ("id") state. She collects 60 seconds worth of data from hermachine, sampling every second.

[madonna@station madonna]$ vmstat 1 60 > stats.txt[madonna@station madonna]$ head stats.txtprocs memory swap io system cpur b swpd free buff cache si so bi bo in cs us sy id wa2 6 0 17348 65604 277768 0 0 15 16 126 221 1 0 97 11 5 0 15736 66008 277788 0 0 376 6269 314 725 5 2 0 931 6 0 11496 67224 277392 0 0 1216 8 422 1533 15 16 0 690 6 0 10492 67944 277676 0 0 940 28 338 1193 7 8 0 850 6 0 10168 68324 277644 0 0 576 0 261 992 6 1 0 933 3 0 8848 69424 277864 0 0 1252 64 429 1386 10 16 0 743 3 0 8056 70188 277892 0 0 1068 1148 422 1215 8 16 0 761 6 0 12248 71084 277636 0 0 940 28 341 1275 9 4 0 87

A little frustrated that the two lines of text headers will interfere with the plotting of the data, madonnaopens the file stats.txt in a text editor and easily deletes them.


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To plot the data, she uses gnuplot, a sophisticated plotting package which uses commands read from aterminal interface to generate plots of mathematical functions and numeric data. After some browsingthrough the online help available within gnuplot, she develops the following commands to plot her dataas a PNG graphics file called cpu.png.

[madonna@station madonna]$ gnuplot

G N U P L O TVersion 3.7 patchlevel 3

...

Terminal type set to ’x11’gnuplot> set term pngTerminal type set to ’png’Options are ’ small color’gnuplot> set output ’cpu.png’gnuplot> plot ’stats.txt’ using 0:13 title "user" with lines, ’stats.txt’ using0:14 title "system" with lines, ’stats.txt’ using 0:15 title "idle" with linesgnuplot> quit

After quiting gnuplot, she returns to the bash shell, where she uses the eog image viewer to view herplot.

[madonna@station madonna]$ eog cpu.png

Figure 1-1. madonna’s plot of CPU activity

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Because madonna thinks she will want to generate a similar plot often, and doesn’t go through the agonyof typing in gnuplot’s plot command every time, she generates a script which can be used to automategnuplot. Using a text editor, she creates the file cpu_plot.gnuplot, which contains all of the gnuplotcommands she entered from the keyboard, paying close attention to put one command per line.

[madonna@station madonna]$ cat cpu_plot.gnuplotset term pngset output ’cpu.png’plot ’stats.txt’ using 0:13 title "user" with lines, ’stats.txt’ using 0:14 title "system" with lines, ’stats.txt’ using 0:15 title "idle" with lines

Now she can easily plot newly collected data by redirecting her script as gnuplot’s stdin.

[madonna@station madonna]$ gnuplot < cpu_plot.gnuplot

Online Exercises

Online Exercise 1. Using Standard In and Standard Out

Lab ExerciseObjective: Use bash shell redirection to effectively control Standard In andStandard Out.

Estimated Time: 20 mins.

Specification

1. The hostname command reports your station’s currently assigned hostname. Run the command(without arguments), and redirect the output to the file ~/stdoutlab.txt.

2. The uptime command reports how much time has passed since your machine was booted, and othersystem utilization information. Run the uptime command (without arguments), using redirection toappend the output to the file ~/stdoutlab.txt.

3. The uname -a command lists information about your current kernel version. Run the command,using redirection to append the output to the file ~/stdoutlab.txt.

If you have completed the previous three steps successfully, you should be able to reproduce outputsimilar to the following. (Do not be concerned if your actual information differs from that shownbelow).[student@station student]$ cat stdoutlab.txtstation07:09:31 up 11:30, 5 users, load average: 0.19, 0.06, 0.01

Linux station 2.4.20-20.9 #1 Mon Aug 18 11:45:58 EDT 2003 i686 i686 i386 GNU/Linux

rha030-3.0-0-en-2005-08-17T07:23:17-0400Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any otheruse is a violation of U.S. and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwiseduplicated whether in electronic or print format without prior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used,copied, or otherwise improperly distributed please email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

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4. Generate a simple text file, ~/script.gnuplot, that will act as script to drive gnuplot. Whengnuplot reads your script from stdin, it should generate a plot of a simple mathematical expression,such as the sine of x (sin(x)), or x squared (x**2). The plot should be generated as a PNGgraphics file called "gnuplot.png".

Once completed, your script should be able to be used as in the following example.[student@station student]$ lsscript.gnuplot[student@station student]$ gnuplot < script.gnuplot[student@station student]$ lsgnuplot.png script.gnuplot[student@station student]$ file gnuplot.pnggnuplot.png: PNG image data, 640 x 480, 8-bit colormap, non-interlaced[student@station student]$ eog gnuplot.png

Deliverables

1. A file called ~/stdoutlab.txt, which contains the output of the hostname command, followed by the outputof the uptime command, followed by the output of the uname -a command.

2. A script ~/script.gnuplot, which when used as stdin for the gnuplot command, generates a PNG graphicsfile titled gnuplot.png containing a plot of a simple mathematical function.

Suggested Strategy for Automating gnuplotUsing Example 3 as your guide, experiment with gnuplot interactively, until you are able to generate asimple plot. If you are using a text terminal instead of the X graphics environment, you can generate textplots by setting your gnuplot output terminal as "dumb":

gnuplot> set term dumb

Once you can produce graphs, set your terminal type to png (for PNG graphics), and your output file to"gnuplot.png", using the following two commands ...

gnuplot> set term pnggnuplot> set output "gnuplot.png"

... and generate your plot again. Once you have figured out the sequence of commands to generate a plotas a PNG file, record the commands as your gnuplot script.

rha030-3.0-0-en-2005-08-17T07:23:17-0400

Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any other use is a violation of U.S.and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwise duplicated whether in electronic or print format withoutprior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, or otherwise improperly distributed please email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

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QuestionsUse the following transcript to answer the next 3 questions.

[prince@station prince]$ whoprince tty1 Sep 22 20:02[prince@station prince]$ cat < /etc/services >> ~/services

1. Which file is the cat command using for Standard In?

( ) a. /dev/tty1( ) b. /etc/services( ) c. /dev/null( ) d. ~/services( ) e. None of the above

2. Which file is the cat command using for Standard Out?

( ) a. ~/services( ) b. /dev/tty1( ) c. /etc/services( ) d. /dev/null( ) e. None of the above

3. Which file is the bash shell using for Standard Out?

( ) a. /dev/tty1( ) b. ~/services( ) c. /dev/null( ) d. /etc/services( ) e. None of the above

4. Which of the following command lines would append lines to the (already existing) file df.out?

( ) a. df > df.out

( ) b. df >+ df.out

( ) c. df.out << df

( ) d. df.out +< df

( ) e. None of the above

rha030-3.0-0-en-2005-08-17T07:23:17-0400

Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any other use is a violation of U.S.and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwise duplicated whether in electronic or print format withoutprior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, or otherwise improperly distributed please email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

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Use the following transcript to answer the next 3 questions.

[prince@station prince]$ psPID TTY TIME CMD

1409 pts/0 00:00:00 bash1542 pts/0 00:00:00 vmstat1543 pts/0 00:00:00 ps

[prince@station prince]$ ls -l /proc/1542/fdtotal 6lrwx------ 1 prince prince 64 Sep 22 20:42 0 -> /dev/pts/0l-wx------ 1 prince prince 64 Sep 22 20:42 1 -> /tmp/vmstat.outlrwx------ 1 prince prince 64 Sep 22 20:42 2 -> /dev/pts/0lr-x------ 1 prince prince 64 Sep 22 20:42 3 -> /proc/uptimelr-x------ 1 prince prince 64 Sep 22 20:42 4 -> /proc/statlr-x------ 1 prince prince 64 Sep 22 20:42 5 -> /proc/meminfo

5. Which of the following files is the vmstat command using for Standard Out?

( ) a. /proc/stat( ) b. /dev/tty1( ) c. /dev/pts/0( ) d. /tmp/vmstat.out( ) e. There is not enough information provided.

6. Which of the following occurred when the vmstat command was started?

( ) a. The bash shell created a new file.

( ) b. The bash shell clobbered an already existing file.

( ) c. The bash shell appended to an already existing file.

( ) d. The vmstat command created a new file.

( ) e. There is not enough information provided.

7. Which of the following files is the vmstat command using for Standard In?

( ) a. /proc/stat( ) b. /tmp/vmstat.out( ) c. /dev/pts/0( ) d. /dev/tty1( ) e. There is not enough information provided.


[prince@station prince]$ cal > cal.out[prince@station prince]$ ls -l /dev/stdin

rha030-3.0-0-en-2005-08-17T07:23:17-0400Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any other use is a violation of U.S.and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwise duplicated whether in electronic or print format withoutprior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, or otherwise improperly distributed please email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

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lrwxrwxrwx 1 root root 17 Apr 1 11:13 /dev/stdin -> ../proc/self/fd/0[prince@station prince]$ cat /dev/stdin < cal.out

September 2003Su Mo Tu We Th Fr Sa

1 2 3 4 5 67 8 9 10 11 12 13

14 15 16 17 18 19 2021 22 23 24 25 26 2728 29 30

8. Which of the following would best describe the file /proc/self?

( ) a. The file is a symbolic link which resolves to the current login shell.

( ) b. The file is a symbolic link which resolves to the root directory.

( ) c. The file is a symbolic link which resolves to /dev/stdout.

( ) d. The file is a symbolic link which resolves to /tmp.

( ) e. The file is a symbolic link which resolves to the /proc/pid directory corresponding to the current process.

9. For the cat process, to what file would the symbolic link /proc/self/fd/0 resolve?

( ) a. /dev/stdin( ) b. cal.out( ) c. /dev/tty1( ) d. /tmp( ) e. /proc/self

10. For the cat process, to what file would the symbolic link /proc/self/fd/1 most likely resolve?

( ) a. /proc/self( ) b. /tmp( ) c. cal.out( ) d. /dev/stdin( ) e. /dev/tty1

Notes1. This is an oversimplification. Some commands do respond differently if stdin or stdout is a terminal

instead of a file. The mail command, for instance, prompted elvis with a Cc: prompt when stdin wasa terminal, but not when stdin was the file sluggish.txt. In general, however, commands shouldbe thought of reading from a file or a terminal interchangeably, with only occasional, minor changesin behavior.


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Chapter 2. Standard Error

Key Concepts• Unix programs commonly report error conditions to a destination called Standard Error (stderr).

• Usually, stderr is connected to a terminal’s display, and error messages are found intermixed withstandard output.

• When using the bash shell, the stderr stream can be redirected to a file using 2>.

• When using bash, the stderr stream can be combined with stdout stream using 2>&1 or >&

Discussion

Standard Error (stderr)We have discussed the standard input and output streams, stdin and stdout, and how to use > and < in thebash command line to redirect them. We are now ready to confuse matters a little by introducing asecond output stream, commonly used for reporting error conditions, called Standard Error (oftenabbreviated stderr).

In the following sequence, elvis is using the head -1 command to generate a list of the first lines of allthe files in the /etc/rc.d directory.

[elvis@station elvis]$ ls -F /etc/rc.d/init.d/ rc0.d/ rc2.d/ rc4.d/ rc6.d/ rc.sysinit*rc* rc1.d/ rc3.d/ rc5.d/ rc.local* rc.sysinit.rpmsave*[elvis@station elvis]$ head -1 /etc/rc.d/*==> /etc/rc.d/init.d <==head: /etc/rc.d/init.d: Is a directory

==> /etc/rc.d/rc <==#! /bin/bash

==> /etc/rc.d/rc0.d <==head: /etc/rc.d/rc0.d: Is a directory




==> /etc/rc.d/rc4.d <==

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head: /etc/rc.d/rc4.d: Is a directory



==> /etc/rc.d/rc.local <==#!/bin/sh

==> /etc/rc.d/rc.sysinit <==#!/bin/bash

==> /etc/rc.d/rc.sysinit.rpmsave <==#!/bin/bash

The head command, when fed multiple file names as arguments, conveniently decorates the name of thefile, followed by the first specified number of lines (in this case, one). When the head commandencounters a directory, however, it merely complains. Next, elvis runs the same command, redirectingstdout to the file rcsummary.out.

[elvis@station elvis]$ head -1 /etc/rc.d/* > rcsummary.outhead: /etc/rc.d/init.d: Is a directoryhead: /etc/rc.d/rc0.d: Is a directoryhead: /etc/rc.d/rc1.d: Is a directoryhead: /etc/rc.d/rc2.d: Is a directoryhead: /etc/rc.d/rc3.d: Is a directoryhead: /etc/rc.d/rc4.d: Is a directoryhead: /etc/rc.d/rc5.d: Is a directoryhead: /etc/rc.d/rc6.d: Is a directory

Most of the output is obediently redirected to the file rcsummary.out, but the directory complaints arestill displayed. Although not obvious at the outset, the head command is really sending output to twoindependent streams. Normal output is written to Standard Out, but error message are written to aseparate stream called Standard Error (often abbreviated stderr). Usually, both streams are connected tothe terminal, and so the two are difficult to distinguish. By redirecting stdout, however, the informationwritten to stderr is obvious.

Redirecting stderrJust as bash uses > to redirect stdout, bash uses 2> to redirect stderr. For example, elvis repeats the headcommand from above, but instead of redirecting stdout to rcsummary.out, he redirects stderr to the filercsummary.err.

[elvis@station elvis]$ head -1 /etc/rc.d/* 2> rcsummary.err==> /etc/rc.d/init.d <==



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==> /etc/rc.d/rc0.d <==

==> /etc/rc.d/rc1.d <==

==> /etc/rc.d/rc2.d <==

==> /etc/rc.d/rc3.d <==

==> /etc/rc.d/rc4.d <==

==> /etc/rc.d/rc5.d <==

==> /etc/rc.d/rc6.d <==




The output is the complement to the previous example. We now see the normal output displayed to thescreen, but no error messages. Where did the error messages go? It shouldn’t be hard to guess.

[elvis@station elvis]$ cat rcsummary.errhead: /etc/rc.d/init.d: Is a directoryhead: /etc/rc.d/rc0.d: Is a directoryhead: /etc/rc.d/rc1.d: Is a directoryhead: /etc/rc.d/rc2.d: Is a directoryhead: /etc/rc.d/rc3.d: Is a directoryhead: /etc/rc.d/rc4.d: Is a directoryhead: /etc/rc.d/rc5.d: Is a directoryhead: /etc/rc.d/rc6.d: Is a directory

In the following example, both > and 2> are used to redirect stdout and stderr independently.

[elvis@station elvis]$ head -1 /etc/rc.d/* > rcsummary.out 2> rcsummary.err[elvis@station elvis]$

In this case, the standard output can be found in the file rcsummary.out, error messages can be foundin rcsummary.err, and nothing is left over to be displayed to the screen.

Combining stdout and stderr: Old SchoolOften, someone would like to redirect the combined stdout and stderr streams to a single file. As a firstattempt, elvis tries the following command.

[elvis@station elvis]$ head -1 /etc/rc.d/* > rcsummary.both 2> rcsummary.both

rha030-3.0-0-en-2005-08-17T07:23:17-0400


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Upon examining the file rcsummary.both, however, elvis doesn’t find what he expects.

[elvis@station elvis]$ cat rcsummary.bothhead: /etc/rc.d/init.d: I==> /etc/rc.dhead: /etc/rc.d/rc0.d: Is a directoryhead: /etc/rc.d/rc1.d: Is a direc==> head: /etc/rc.d/rc2.==> /etc/rc.d/rc3head: /==> /etc/rc.d/rc4.d <==

==> /etc/rc.d/rc5.d <==

==> /etc/rc.d/rc6.d <==




The bash shell opened the file rcsummary.both twice, but treated each open file independently. Whenstdout and stderr both wrote to the file, they clobbered each other’s information. What is needed insteadis some way to tell bash to effectively combine stderr and stdout into a single stream, and then redirectthat stream to a single file. As you would expect, there is such a way.

[elvis@station elvis]$ head -1 /etc/rc.d/* > rcsummary.both 2>&1

Although awkward, the last token 2>&1 should be thought of as saying "take stderr, and send itwherever stdout is currently going". Now rcsummary.both contains the expected output.

[elvis@station elvis]$ cat rcsummary.both==> /etc/rc.d/init.d <==head: /etc/rc.d/init.d: Is a directory



==> /etc/rc.d/rc1.d <==head: /etc/rc.d/rc1.d: Is a directory...

Much of this output was truncated, and replaced with "...".

rha030-3.0-0-en-2005-08-17T07:23:17-0400


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Combining stdout and stderr: New SchoolUsing 2>&1 to combine stdout and stderr was introduced in the original Unix shell, the Bourne shell (sh).Because bash is designed to be backwards compatible with sh, it supports the syntax as well. Thesyntax, however, is inconvenient. Besides being difficult to write, the order of the redirections isimportant. Using ">out.txt 2>&1" and "2>&1 >out.txt" does not have the same effect!

In order to simplify things, bash uses >& to combine both stdin and stdout, as in the following example.

[elvis@station elvis]$ head -1 /etc/rc.d/* >& rcsummary.both

SummaryThe following table summarizes the syntax used by the bash shell for redirecting stdin, stdout, and stderrlearned in this and the previous lesson.

Table 2-1. Redirecting stdin, stdout, and stderr in bash

syntax effectcmd < file Redirect stdin from filecmd > file Redirect stdout into file, overwriting (clobbering) file if it exists.cmd >> file Redirect stdout into file, appending to file if it exists.cmd 2> file Redirect stderr into file, overwriting (clobbering) file if it exists.cmd 2>> file Redirect stderr into file, appending to file if it exists.cmd > file 2>&1 Combine stdout and stderr, and redirect both into file. (Portable syntax)cmd >& file Combine stdout and stderr, and redirect both into file. (Convenient syntax)

Examples

Example 1. Using /dev/null to filter out stderrThe user elvis is has recently learned that, besides the /home/elvis and /tmp directories he’s familiarwith, he may also own files in the /var directory. These files are usually spooling files for received butnot yet viewed email, print jobs waiting to be sent to the printer, etc.

Curious, he uses the find command to find all files within the /var directory that he owns.

[elvis@station elvis]$ find /var -user elvisfind: /var/lib/slocate: Permission deniedfind: /var/lib/nfs/statd: Permission deniedfind: /var/lib/xdm/authdir: Permission denied...find: /var/spool/lpd/three-west: Permission deniedfind: /var/spool/lpd/one-east-color: Permission denied


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find: /var/spool/lpd/server1: Permission denied/var/spool/mail/elvisfind: /var/spool/at: Permission denied...find: /var/tux: Permission deniedfind: /var/tomcat4/webapps: Permission denied

(Much of the output of this command has been truncated, and replaced with "...").

Although the find command appropriately reported the /var/spool/mail/elvis file, the output isdifficult to find among all of the "Permission denied" error messages being reported from varioussubdirectories of /var. In order to help separate the wheat from the chaff, elvis redirects stderr to somefile in the /tmp directory.

[elvis@station elvis]$ find /var -user elvis 2> /tmp/foo/var/spool/mail/elvis

While this works, elvis is left with a file called /tmp/foo that he really didn’t want. In situations likethis, when a user wants to discard a stream of information, experienced Unix users usually redirectoutput to a pseudo device called /dev/null.

[elvis@station elvis]$ find /var -user elvis 2> /dev/null/var/spool/mail/elvis

As the following long listing shows, /dev/null is a character device node, like those used forconventional device drivers.

[elvis@station elvis]$ ls -l /dev/nullcrw-rw-rw- 1 root root 1, 3 Jan 30 05:24 /dev/null

When a user writes to /dev/null, the information is merely discarded by the kernel. When a user readsfrom /dev/null, they encounter an immediate end of file. Notice that /dev/null is one of the fewfiles in Red Hat Enterprise Linux that has world writable permissions by default.

Online Exercises

Lab ExerciseObjective: Effectively manage Standard Out and Standard Error Streams


Specification

1. Use the following command line to display the contents of all files within the /etc/X11 directory.[elvis@station elvis]$ cat /etc/X11/*cat: /etc/X11/applnk: Is a directory

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cat: /etc/X11/desktop-menus: Is a directorycat: /etc/X11/fs: Is a directorycat: /etc/X11/gdm: Is a directorycat: /etc/X11/lbxproxy: Is a directory#!/bin/sh

PATH=/sbin:/usr/sbin:/bin:/usr/bin:/usr/X11R6/bin...

2. Repeat the command line, but redirect stdout to a file called ~/stderrlab.out and stderr to a filecalled ~/stderrlab.err.

3. Repeat the command again, but combine stdout and stderr into a single stream, and redirect thestream to the file ~/stderrlab.both.

Deliverables

1. A file called ~/stderrlab.out, which contains the stdout stream from the command cat /etc/X11/*.

2. A file called ~/stderrlab.err, which contains the stderr stream from the command cat /etc/X11/*.

3. A file called ~/stderrlab.both, which contains the combined stdout and stderr streams from the commandcat /etc/X11/*.

QuestionsUse the following transcript to answer the next question.

[madonna@station madonna]$ cat chmod.errchmod: changing permissions of ‘/tmp/orbit-elvis’: Operation not permittedchmod: changing permissions of ‘/tmp/orbit-elvis-a8e8d915’: Operation not permittedchmod: changing permissions of ‘/tmp/orbit-hogan’: Operation not permittedchmod: changing permissions of ‘/tmp/orbit-root’: Operation not permittedchmod: changing permissions of ‘/tmp/orbit-student’: Operation not permitted

1. Which of the following command lines most likely created the file chmod.err?

( ) a. chmod a+r /tmp/* > chmod.err

( ) b. chmod a+r /tmp/* 2> chmod.err

( ) c. chmod a+r /tmp/* >> chmod.err

( ) d. chmod a+r /tmp/* e> chmod.err


rha030-3.0-0-en-2005-08-17T07:23:17-0400Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any other use is a violationof U.S. and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwise duplicated whether in electronic or printformat without prior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, or otherwise improperly distributed please [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

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2. Which of the following command lines would combine stdout and stderr, and redirect the combined stream to thefile /tmp/find.out?

( ) a. find /etc > /tmp/find.out 2>&1

( ) b. find /etc >> /tmp/find.out

( ) c. find /etc >& /tmp/find.out

( ) d. find /etc >>& /tmp/find.out

( ) e. Both A and C

3. Which of the following command lines would combine stdout and stderr, and redirect the combined stream to thefile /tmp/find.out, appending to the file if it already existed?

( ) a. find /etc > /tmp/find.out 2>&1

( ) b. find /etc >> /tmp/find.out

( ) c. find /etc >> /tmp/find.out 2>&1

( ) d. find /etc >>& /tmp/find.out

( ) e. Both A and C

Use the following transcript to answer the next two questions.

[madonna@station madonna]$ cat /etc/t* > /tmp/cat.out 2> /tmp/cat.err

[1]+ Stopped cat /etc/t* >/tmp/cat.out 2>/tmp/cat.err[madonna@station madonna]$ ps

PID TTY TIME CMD2419 pts/1 00:00:00 bash3126 pts/1 00:00:00 cat3127 pts/1 00:00:00 ps

[madonna@station madonna]$ ls -l /proc/3126/fdtotal 4lrwx------ 1 madonna madonna 64 Sep 23 04:15 0 -> /dev/pts/1l-wx------ 1 madonna madonna 64 Sep 23 04:15 1 -> /tmp/cat.outl-wx------ 1 madonna madonna 64 Sep 23 04:15 2 -> /tmp/cat.errlr-x------ 1 madonna madonna 64 Sep 23 04:15 3 -> /etc/termcap[madonna@station madonna]$ fg[madonna@station madonna]$ ???????cat: /etc/tomcat4: Is a directory

4. Which file descriptor does Linux use as Standard Error?

( ) a. 0

( ) b. 1

( ) c. 2


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( ) d. 3


5. Which of the following command lines could replace the question marks as the last command line in thetranscript?

( ) a. cat /dev/pts/1

( ) b. cat /tmp/cat.out

( ) c. cat /tmp/cat.err

( ) d. cat /etc/termcap

( ) e. cat /dev/null

6. To which of the following files would you expect the symbolic link /dev/stderr to resolve?

( ) a. ../proc/self/fd/2( ) b. ../proc/self/fd/1( ) c. ../proc/self/fd/0( ) d. /dev/null( ) e. None of the above

Use the following transcript to answer the next two questions.

[madonna@station madonna]$ cat groups.sh#!/bin/bash

if id $1then

echo "The user $1 belongs to the following groups: $(id -Gn $1) "else

echo "The user $1 does not exist"fi[madonna@station madonna]$ ./groups.sh elvisuid=501(elvis) gid=501(elvis) groups=501(elvis),201(wrestle),202(physics),203(emperors),205(music)The user elvis belongs to the following groups: elvis wrestle physics emperors music[madonna@station madonna]$ ./groups.sh barneyid: barney: No such userThe user barney does not exist


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7. Which of the following replacements for line number 3 of the file groups.sh would cause the script to display asingle line beginning "The user", in all cases?

( ) a. if id -q $1

( ) b. if id $1 >/dev/null 2>&1

( ) c. if id $1 2> /dev/null

( ) d. if id $1 > /dev/null


8. Which of the following replacements for line number 7 of the file groups.sh would cause the script to complainon Standard Error if the user does not exist (instead of Standard Out)?

( ) a. echo "The user $1 does not exist" > /dev/stderr

( ) b. echo -e "The user $1 does not exist"

( ) c. echo "The user $1 does not exist" > /proc/self/fd/2

( ) d. echo -E "The user $1 does not exist"

( ) e. A and C

9. Assuming cmd is some simple command and its arguments (but no shell metacharacters), which of the followingwould redirect Standard Error (only) to the file /tmp/errors, appending to the file if it already existed?

( ) a. cmd 2> /tmp/errors

( ) b. cmd >> /tmp/errors

( ) c. cmd >+ /tmp/errors

( ) d. cmd 2>+ /tmp/errors

( ) e. cmd 2>> /tmp/errors

10. Assuming cmd is some simple command and its arguments (but no shell metacharacters), which of the followingwould be guaranteed to execute cmd, but generate no visible output?

( ) a. cmd 2> /dev/null

( ) b. cmd > /dev/null

( ) c. (cmd)( ) d. cmd > /dev/null 2>&1

( ) e. silent (cmd)

rha030-3.0-0-en-2005-08-17T07:23:17-0400


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Chapter 3. Pipes

Key Concepts• The stdout stream from one process can be connected to the stdin stream of another process, using

what Unix calls a "pipe".

• Many commands in Unix are designed to operate as a filter, reading input from stdin and sendingoutput to stdout.

• bash uses "|" to create a pipe between two commands.

Discussion

PipesIn the previous Lessons, we have seen that a process’s output can be redirected to somewhere other thanthe terminal display, or that a process can be asked to read input from some location other than theterminal keyboard. One of the most common, and most powerful, forms of redirection is a combinationof the two, where the output (Standard Out) of one command is "piped" directly into the input (StandardIn) of another command, forming what Linux (and Unix) refers to as a pipe.

When two commands are joined by a pipe, the stdout stream of the first process is tied directly to thestdin sequence of the second process, so that multiple processes can be combined in a sequence. In orderto create a pipe using bash, the two commands are joined with a vertical bar |. (On most keyboards, thischaracter is found on the same key as the backslash, above the RETURN key.) All processes that arejoined in a pipe are referred to as a process group.

As an example, consider prince, who is trying to find the largest files underneath the /etc directory. Hebegins by composing a find command that will list all file with a size greater than 100Kbytes.

[prince@station prince]$ find /etc -size +100k 2>/dev/null/etc/termcap/etc/gconf/gconf.xml.defaults/schemas/desktop/gnome/interface/%gconf.xml/etc/gconf/gconf.xml.defaults/schemas/apps/mailcheck_applet/prefs/%gconf.xml/etc/gconf/gconf.xml.defaults/schemas/apps/tasklist_applet/prefs/%gconf.xml...

Observing that the find command seems to list the files in no particular order, prince decides he wouldlike the files to be listed alphabetically. He could redirect the output to a file, and then sort the file.Instead, he takes advantage of the fact that the sort command, when invoked without arguments, looks toStandard In for the data to sort. He pipes the output of his find command into sort.

[prince@station prince]$ find /etc -size +100k 2>/dev/null | sort/etc/aep/aeptarg.bin/etc/gconf/gconf.xml.defaults/schemas/apps/gedit-2/preferences/editor/save/%gconf.xml/etc/gconf/gconf.xml.defaults/schemas/apps/gnomemeeting/general/%gconf.xml

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.../etc/makedev.d/cciss/etc/makedev.d/dac960/etc/squid/squid.conf/etc/squid/squid.conf.default/etc/termcap

The files are now listed in alphabetical order.

Filtering output using grepThe traditional Unix grep command is commonly used in pipes to reduce data to only the "interesting"parts. The grep command will be discussed in detail in a later Workbook. Here, we introduce grep in itssimplest form.

The grep command is used to search for and extract lines which contain a specified string of text. Forexample, in the following, prince prints all lines that contain the text "root" from the /etc/passwd file.

[prince@station prince]$ grep root /etc/passwdroot:x:0:0:root:/root:/bin/bashoperator:x:11:0:operator:/root:/sbin/nologin

The first argument to the grep command is the string of text to be searched for, and any remainingarguments are files to be searched for the text. If the grep command is called with only one argument (astring to be searched for, but no files to search), it looks to Standard In as its source of data on which tooperate.

In the following, prince has so many files in his home directory that he is having trouble keeping track ofthem. He’s trying to find a directory called templates that he created a few months ago. He uses thelocate command to help him find it.

[prince@station prince]$ locate templates/etc/openldap/ldaptemplates.conf/usr/share/doc/libxslt-1.0.27/html/libxslt-templates.html/usr/share/doc/libxslt-1.0.27/templates.gif/usr/share/doc/docbook-style-xsl-1.58.1/docsrc/templates.xml/usr/share/man/man5/ldaptemplates.conf.5.gz/usr/share/man/man3/ldap_free_templates.3.gz/usr/share/man/man3/ldap_init_templates_buf.3.gz/usr/share/man/man3/ldap_init_templates.3.gz...

Unfortunately for prince, there are many files which contain the text templates in their name on thesystem, and prince becomes overwhelmed with lines and lines of output. In order to reduce theinformation to more relevant files, prince next takes stdout from the locate command, and creates a pipeto stdin of the grep command, "grepping" for the word "prince".

[prince@station 010_section_discussion]$ locate templates | grep prince/home/prince/.kde/share/apps/quanta/templates/home/prince/proj/templates

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Because the grep command is not given a file to search, it looks to stdin, where it finds the stdout streamof the locate command. Filtering the stream, grep only duplicates to its stdout lines that matched thespecified text, "prince". The rest were discarded. The user prince easily finds his directory under~/proj, as well as another directory created by the application quanta.

Pipes and stderrIn the next example, prince is curious to see where he shows up in the system’s configuration files, and"greps" for his name in the /etc directory.

[prince@station prince]$ grep prince /etc/*grep: /etc/aliases.db: Permission deniedgrep: /etc/at.deny: Permission deniedgrep: /etc/default: Permission denied/etc/group:music:x:205:elvis,blondie,prince,madonna,student/etc/group:prince:x:502:grep: /etc/group-: Permission deniedgrep: /etc/group.lock: Permission denied...grep: /etc/lvmtab: Permission denied/etc/passwd:prince:x:502:502::/home/prince:/bin/bashgrep: /etc/passwd-: Permission deniedgrep: /etc/passwd.lock: Permission denied...grep: /etc/sudoers: Permission denied/etc/termcap:# From: John Doe <[email protected]>grep: /etc/vsftpd.conf.rpmsave: Permission denied

Again, prince is overwhelmed by the amount of output from this command. He tries the same trick,"grepping" it down for all lines that contain the word "passwd".

[prince@station prince]$ grep prince /etc/* | grep passwdgrep: /etc/aliases.db: Permission deniedgrep: /etc/at.deny: Permission deniedgrep: /etc/default: Permission deniedgrep: /etc/group-: Permission deniedgrep: /etc/group.lock: Permission denied...grep: /etc/lvmtab: Permission denied/etc/passwd:prince:x:502:502::/home/prince:/bin/bashgrep: /etc/passwd-: Permission deniedgrep: /etc/passwd.lock: Permission denied...

While stdout from the first grep command was appropriately filtered, stderr is unaffected, and still getsdisplayed to the screen. How would prince go about suppressing stderr as well?

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Commands as filtersThe concept of a pipe extends naturally, so that multiple commands can be used together, each readinginformation from stdin, somehow modifying or filtering the information, and passing the result to stdout.In a subsequent Workbook, you will find that there are many standard Linux (and Unix) commands thatare designed for this purpose, including some that you are already familiar with: grep, head, tail, cut,sort, sed, and awk, to name a few.

Examples

Example 1. Listing Processes by NameOften, one would like to list information about processes which are running a specific command. Whileps aux tables a lot of information about currently running processes, the number of processes running onthe machine can make the output overwhelming. The grep command can help simplify the output.

In the following, prince would like to list information about the processes which are implementing hisweb server, the httpd command. He lists all processes, but then reduces the output to only those lineswhich contain the text httpd.

[prince@station prince]$ ps aux | grep httpdroot 889 0.0 0.0 18248 100 ? S Sep22 0:00 /usr/sbin/httpdapache 907 0.0 0.5 18436 1320 ? S Sep22 0:00 /usr/sbin/httpdapache 913 0.0 0.7 18436 1952 ? S Sep22 0:00 /usr/sbin/httpdapache 914 0.0 0.5 18436 1332 ? S Sep22 0:00 /usr/sbin/httpdapache 1979 0.0 0.5 18360 1524 ? S Sep22 0:00 /usr/sbin/httpdapache 1980 0.0 0.8 18388 2140 ? S Sep22 0:00 /usr/sbin/httpdapache 1981 0.0 0.5 18360 1524 ? S Sep22 0:00 /usr/sbin/httpdprince 4905 0.0 0.2 3572 640 pts/1 S 06:19 0:00 grep httpd

Example 2. Searching Command History EfficientlyThe user prince recently spent some time constructing a find command line which listed all large filesunderneath the /etc directory, including size. Rather than repeating his efforts, he would like to see ifthe command is still in his history. Because his history contains hundreds of lines, he uses the grepcommand to help reduce the output.

[prince@station prince]$ history | grep find102 find /var -user elvis175 find -exec file {} \;434 find /etc -name *.conf | head675 find /etc -size +100k680 find /etc -size +100k -exec ls -s {} \; 2>/dev/null682 find -size +100k /etc683 find /etc -size +100k690 history | grep find

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He now locates the command line he wanted, and uses history substitution to easily repeat the command.

[prince@station prince]$ !680find /etc -size +100k -exec ls -s {} \; 2>/dev/null728 /etc/termcap132 /etc/gconf/gconf.xml.defaults/schemas/desktop/gnome/interface/%gconf.xml304 /etc/gconf/schemas/gedit.schemas

...

Example 3. Unix philosophy: Simple Tools that Work WellTogetherLinux, like Unix, is fundamentally based on the philosophy that complex systems should be created outof simple, specialized components that inter-operate easily. Following this philosophy, many standardLinux programs are designed to work as filters, reading data from a standard source, manipulating thedata, and delivering the result to a standard destination.

This philosophy is important enough to be illustrated using a long, detailed example. The example willuse commands you’re probably not yet familiar with. Don’t worry about the details of how to use thecommands, but instead focus on how the commands work together, each command performing a smallpart, to produce the desired result.

Suppose a system administrator is examining DHCP lease messages in a well known log file,/var/log/messages. If you are not familiar with DHCP, it is a protocol by which IP addresses can beassigned to machines based on the hardware (MAC) address that is built into a machine’s Ethernet card.In the following lines extracted from /var/log/messages, focus on the line containing the wordDHCPOFFER. Note that the IP address 192.168.0.11 is being offered to the Ethernet card with ahardware address of 00:08:74:37:c5:c3.

...May 27 12:18:21 server1 dhcpd: DHCPACK on 192.168.0.110 to 00:09:6b:d0:ce:8f via eth0May 27 12:18:27 server1 login(pam_unix)[1981]: session closed for user rootMay 27 12:19:15 server1 named[24350]: listening on IPv4 interface eth1, 192.168.22.20#53May 27 12:19:21 server1 vsftpd: warning: can’t get client address: Bad file descriptorMay 27 12:19:21 server1 last message repeated 3 timesMay 27 12:20:27 server1 dhcpd: DHCPDISCOVER from 00:08:74:37:c5:c3 via eth0May 27 12:20:27 server1 dhcpd: DHCPOFFER on 192.168.0.11 to 00:08:74:37:c5:c3 via eth0May 27 12:20:27 server1 dhcpd: DHCPREQUEST for 192.168.0.11 (192.168.0.254) from 00:08:74:37:c5:c3 via eth0...

Without worrying about the details of the DHCP protocol, suppose the administrator wanted to extract alist of IP addresses and the hardware addresses they were offered to from the log file. An experiencedadministrator might take the following approach.

Realizing that the /var/log/message file is a very large file, in this case over 1000 lines in length, theadministrator first uses the grep command to reduce the information to only the relevant lines.

[root@station log]# grep DHCPOFFER messagesMay 27 11:46:22 server1 dhcpd: DHCPOFFER on 192.168.0.1 to 00:08:74:d9:41:9e via eth0May 27 11:46:22 server1 dhcpd: DHCPOFFER on 192.168.0.1 to 00:08:74:d9:41:9e via eth0May 27 11:46:30 server1 dhcpd: DHCPOFFER on 192.168.0.1 to 00:08:74:d9:41:9e via eth0


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May 27 11:46:30 server1 dhcpd: DHCPOFFER on 192.168.0.1 to 00:08:74:d9:41:9e via eth0May 27 11:48:40 server1 dhcpd: DHCPOFFER on 192.168.0.2 to 00:08:74:d9:41:32 via eth0...

This is a start, but the administrator is still dealing with too much information (the above screenshot onlylists the first 5 of 90 lines produced by this command). In order to extract just the relevant information,namely the IP address and hardware address, the administrator takes the output of the grep command,and pipes it to a command called sed, which strips the first few words from each line.

[root@station log]# grep DHCPOFFER messages | sed "s/^.*on //"192.168.0.1 to 00:08:74:d9:41:9e via eth0192.168.0.1 to 00:08:74:d9:41:9e via eth0192.168.0.1 to 00:08:74:d9:41:9e via eth0192.168.0.1 to 00:08:74:d9:41:9e via eth0192.168.0.2 to 00:08:74:d9:41:32 via eth0192.168.0.2 to 00:08:74:d9:41:32 via eth0192.168.0.2 to 00:08:74:d9:41:32 via eth0192.168.0.2 to 00:08:74:d9:41:32 via eth0192.168.0.3 to 00:08:74:d9:40:a4 via eth0...

If you are not familiar with the sed command (and you probably aren’t), don’t worry about the details.Just note that sed’s argument had the effect of removing the leading text from each line, up to the word"on". There’s still too much extra text, however, so the administrator takes the output of this grep-sedcombination, and pipes it to a command called awk.

[root@station log]$ grep DHCPOFFER messages | sed "s/^.*on //" | awk ’{print $1,$3}’...192.168.0.14 00:08:74:34:fe:bc192.168.0.5 00:08:74:34:fd:36192.168.0.15 00:08:74:37:c8:eb192.168.0.15 00:08:74:37:c8:eb192.168.0.6 00:08:74:d9:41:a3192.168.0.16 00:08:74:d9:41:ac192.168.0.7 00:08:74:d9:41:53192.168.0.16 00:08:74:d9:41:ac192.168.0.17 00:08:74:35:00:e3...

Again, not worrying about the details of the awk command, note that the result was to extract just thefirst and third column from the previous output. In order to sort the data, and remove duplicate lines, theadministrator now takes the output from the chain, and pipes it in turn through the commands sort anduniq.

[root@station log]$ grep DHCPOFFER messages | sed "s/^.*on //" | awk ’{print $1,$3}’ | sort | uniq192.168.0.10 00:08:74:d9:40:95192.168.0.1 00:08:74:d9:41:9e192.168.0.110 00:09:6b:d0:ce:8f192.168.0.11 00:08:74:37:c5:c3192.168.0.12 00:08:74:d9:41:dd192.168.0.13 00:08:74:35:00:d0


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192.168.0.14 00:08:74:34:fe:bc192.168.0.15 00:08:74:37:c8:eb192.168.0.16 00:08:74:d9:41:ac192.168.0.17 00:08:74:35:00:e3192.168.0.2 00:08:74:d9:41:32192.168.0.3 00:08:74:d9:40:a4192.168.0.4 00:08:74:d9:3f:7f192.168.0.5 00:08:74:34:fd:36192.168.0.6 00:08:74:d9:41:a3192.168.0.7 00:08:74:d9:41:53192.168.0.8 00:08:74:d9:41:7b192.168.0.9 00:08:74:35:00:1f

This is almost the list that the administrator wants, but the sort command didn’t work quite right. Theinformation is sorted, but it’s sorted alphabetically, not by IP address. The administrator modifies thesort command with a couple of command line switches, specifying to sort numerically, keying on thefourth field, where fields are separated by a period:

[root@station log]$ grep DHCPOFFER messages | sed "s/^.*on //" | awk ’{print $1,$3}’ | sort -n -k4 -t. | uniq192.168.0.1 00:08:74:d9:41:9e192.168.0.2 00:08:74:d9:41:32192.168.0.3 00:08:74:d9:40:a4192.168.0.4 00:08:74:d9:3f:7f192.168.0.5 00:08:74:34:fd:36192.168.0.6 00:08:74:d9:41:a3192.168.0.7 00:08:74:d9:41:53192.168.0.8 00:08:74:d9:41:7b192.168.0.9 00:08:74:35:00:1f192.168.0.10 00:08:74:d9:40:95192.168.0.11 00:08:74:37:c5:c3192.168.0.12 00:08:74:d9:41:dd192.168.0.13 00:08:74:35:00:d0192.168.0.14 00:08:74:34:fe:bc192.168.0.15 00:08:74:37:c8:eb192.168.0.16 00:08:74:d9:41:ac192.168.0.17 00:08:74:35:00:e3192.168.0.110 00:09:6b:d0:ce:8f

This is the list that the administrator wanted, in the order that she wanted. She redirects all of this outputto a file in her home directory.

[root@station log]$ grep DHCPOFFER messages | sed "s/^.*on //" | awk ’{print $1,$3}’ | sort -n -k4 -t. | uniq > ~/ip_dhcp.txt

In this example, an (admittedly experienced) administrator browsing a log file was able to spend a fewminutes and develop a chain of commands that filtered the original information down to exactly theinformation that she wanted. She did this with a handful of tools that are in most Unix administrator’smental toolkit: grep, sed, awk, sort, and uniq.

If the administrator were using an operating system that wasn’t designed around the philosophy "smalltools that work together", she would have needed to rely on some programmer to develop theip_mac_extractor utility, and possibly rely on that programmer to create a graphical user interface for


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the utility as well. Instead, because she was able to use the flexibility of the command line, she was ableto handle the information herself.

Online Exercises

Lab ExerciseObjective: Use pipes to effectively filter information.


Specification

1. You would like to create a sorted list of all TCP services found in the file /etc/services. Pipe theoutput of the command grep tcp /etc/services into the command sort. Redirect the output of thispipe into the file ~/pipelab.txt.

2. Using the less pager, you would like to browse the output of the ls -R / command, viewing only filesthat contain the letter s. Compose a command line using two pipes to chain the commands ls -R /,grep s, and less. Leave the less pager in the foreground while you grade your exercise.

Deliverables

1. A file called ~/pipelab.txt, which contains the output of the command grep tcp /etc/services pipedthrough the command sort.

2. An active less pager, which is browsing the output of the command ls -R /, piped through the command grep s.

Clean UpAfter your Exercise has been graded, you may quit the less pager.


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Questions

1. Which of the following command lines would reduce the output of the locate conf command to only files whosename or path contains the text python?

( ) a. locate conf >> grep python

( ) b. locate conf | grep python

( ) c. locate conf < grep python

( ) d. locate conf | grep < python


2. Which of the following command lines would produce a sorted list of all processes which contain the text sshd?

( ) a. ps aux | sort | grep sshd

( ) b. ps aux | grep sshd | sort

( ) c. grep sshd | ps aux | sort

( ) d. sort | ps aux | grep sshd

( ) e. A or B


[elvis@station elvis]$ ls -R / 2> /dev/null | grep etc | less

[1]+ Stopped ls --color=tty -R / 2>/dev/null | grep etc | less[elvis@station elvis]$ ps

PID TTY TIME CMD1603 pts/2 00:00:01 bash2391 pts/2 00:00:00 ls2392 pts/2 00:00:00 grep2393 pts/2 00:00:00 less2394 pts/2 00:00:00 ps

[elvis@station elvis]$ ls -l /proc/2391/fdtotal 0lrwx------ 1 elvis elvis 64 Sep 23 09:49 0 -> /dev/pts/2l-wx------ 1 elvis elvis 64 Sep 23 09:49 1 -> pipe:[20966]l-wx------ 1 elvis elvis 64 Sep 23 09:49 2 -> /dev/null

3. Which file (or pipe) is tied to stderr of the ls process?

( ) a. A pipe to the grep process

( ) b. /dev/null( ) c. /dev/pts/2( ) d. A pipe to the less process


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4. Which file (or pipe) is tied to stderr of the grep process?

( ) a. /dev/null( ) b. /dev/pts/2( ) c. A pipe to the ls process

( ) d. A pipe to the less process


5. Which file (or pipe) is tied to stdin of the grep process?

( ) a. A pipe to the less process

( ) b. A pipe to the ls process

( ) c. /dev/pts/2( ) d. /dev/null( ) e. None of the above

6. Which file (or pipe) is tied to stdin of the less process?

( ) a. A pipe to the ls process

( ) b. /dev/pts/2( ) c. /dev/null( ) d. A pipe to the grep process


7. To what expression would the symbolic link /proc/2392/fd/0 resolve?

( ) a. /dev/pts/2( ) b. pipe:[20966]( ) c. /dev/null( ) d. /tmp( ) e. /dev/tty2

rha030-3.0-0-en-2005-08-17T07:23:17-0400Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any other use is aviolation of U.S. and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwise duplicated whether inelectronic or print format without prior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, or otherwise improperly distributedplease email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

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8. When bash reports the stopped process group, the ls alias has been expanded to ls --color=tty. The --color=ttyoption tells the ls command to generate color control sequences only if its stdout is tied to a terminal. Which of thefollowing is true?

( ) a. The ls command generates color control sequences, which can be observed by the less pager.

( ) b. The ls command does not generate color control sequences.( ) c. The ls command generates color control sequences, but they are filtered out by the grep command.

( ) d. The ls command never generates color control sequences (the --color=tty is only included for purposes ofbackwards compatibility).

( ) e. None of the above apply.

9. Which of the following command lines would allow the less pager to browse grep’s error messages, as well asoutput?

( ) a. grep root /etc/* 2>&1 | less

( ) b. grep root /etc/* | less 2>&1

( ) c. grep root /etc/* >>| less

( ) d. grep root /etc/* 2| less


The -j command line switch causes the ps command to generate "job control oriented" output. Use the followingtranscript to answer the next question.

[elvis@station elvis]$ ls -R / | grep etc | less

[1]+ Stopped ls --color=tty -R / | grep etc | less[elvis@station elvis]$ ps -j

PID PGID SID TTY TIME CMD1215 1215 1215 pts/0 00:00:00 bash3242 3242 1215 pts/0 00:00:00 ls3243 3242 1215 pts/0 00:00:00 grep3244 3242 1215 pts/0 00:00:00 less3246 3246 1215 pts/0 00:00:00 ps

10. Which of the following would be the most reasonable expansion of the second column header, PGID?

( ) a. Process Group ID

( ) b. Parent Generation ID

( ) c. Process Gender ID

( ) d. Powder Gondola ID


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