Linux System Administration Recipes A Problem-Solution Approach

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CHAPTER 9 ■ WORKING WITH TEXT IN FILESThis snippet will print only the lines that match your search string (set in line 05) to the output file.(Alternatively, use awk to do this; see recipe 9-3.) To append to OUTPUT rather than overwriting, replaceline 07 with the following:open (OUTPUT, ">>outputfile");You can put a + in front of the > or < to indicate that you want both read and write access to the file.+< is almost always preferred for read-write updates, because the +> mode would clobber the file first, sothere won’t be anything there to read from.9-4a. Perl, Files, and Command-Line OptionsA couple of command-line options make Perl particularly useful for editing files, enabling you to dosome quite powerful things without having to leave the command line.Specifically, the -p option sets Perl up as a stream editor, rather like sed (see recipe 9-2) and awk (seerecipe 9-3). It prints out each line that it processes as it processes it. -n does the same thing except itsuppresses the printing (so if you want output, you have to put in a print statement). This one-liner doesexactly the same thing as the previous script:perl -ne 'print if ! /192.168/' < /var/log/auth.log > testout.txtIt checks each line of /var/log/auth.log, and if it doesn’t find part of the line that matches 192.168,it prints it to standard out, which is then redirected to testout.txt.Unlike a traditional editor, because Perl is reading the file in only a single line at a time, it can dealwith enormous files quite happily and without any memory impact. So, if you have a couple of monthsof Apache log files to deal with, you can search through them and edit them with a Perl one-liner likethis, where you might have to wait for a noticeable time to get vim to open up the file.To some extent, this does much the same as sed and awk. Perl syntax may be more familiar to you,and Perl is a little more powerful. However, its power also means that if you’re doing somethingstraightforwardly text-based, sed or awk may require less coding. To some extent, it depends on whichyou get along with best!You can edit a file with Perl as well. For example, say someone wants a long document on a webpage to have all the references to widgets changed to wodgets. Use this one-liner:perl -pe 's/widget/wodget/g' < biglist.html > newbiglist.htmlThe g at the end of the search command means that the search and replace is to be done globally.Otherwise, it’ll just be done on the first instance of every line. We use -p here because there’s no explicitprint statement in the code. We want each line to be printed with the change made (any lines that arenot changed will also be printed, which again is what we want here).■ Note You can edit a file in place with the -i option, overwriting the previous version. However, if somethinggoes wrong, you’ve lost your file, and you’ll have to restore from backup or from your version control systemrepository. If you use -i.bak, it’ll save a backup copy as filename.bak before making any changes.194Download at WoweBook.Com
CHAPTER 9 ■ WORKING WITH TEXT IN FILESBut if you’ve had to change something in one file on the web server, you’ll probably have to do it inmore than one file. Happily, this also works with shell wildcards. So, to make the previous change toevery file in a particular directory, use this:perl -i.bak -pe 's/widget/wodget/g' *.htmlOr to do this recursively through all the directories (this might be useful if you change the name ofyour main CSS file, for example), use this:find . -type f | xargs perl -p -i.old -e 's/old.css/new.css/g'The -type f argument to find limits this to files, so you won’t get errors from Perl trying to do thissubstitution in directories. You can also use Perl to split columned data, in much the same way as youcan use awk. This is the -a option, which adds an implicit split (by default on whitespace) inside thewhile() loop that -p provides. As with awk, you can change the character to split on with -F. So, to getthe user and home directory information out of /etc/passwd, use this:# perl -an -F: -e 'print $F[0], "\n", $F[5], "\n";' /etc/passwd9-5. When It’s Not ASCII: Dealing with UTF-8Before talking about how <strong>Linux</strong> handles various sorts of text, let’s establish what text encodings areavailable:• ASCII: This is the most basic encoding available. It uses 7 bits to encode the 128characters that it uses, which means that the eighth bit of every byte is left empty.It’s limited, but you can rely on ASCII characters being viewable and available verynearly everywhere. (The exception is certain sorts of mainframe, but if you’reusing these you’ll know about it!)• ISO-8859-1: Given that ASCII has only 128 characters (including 33 mostlyobsolete nonprinting characters), all the accented characters and some othernonletter standard characters (such as the U.K. pound sign) are missing from it.Thus, people started creating extensions to it. The first extension, aka Latin 1, wasISO-8859-1, which broadly speaking deals with Western Europe. There are lotsmore similarly numbered encodings, such as ISO-8859-15, and so on.• UTF-8: The extensions system is cumbersome and requires multiple encodings(which may be incompatible) to be stored on your system. UTF-8 aims to fix thisby encoding all available characters, using between one and four bytes. Westernstylecharacters all fit within the first two bytes. Everything else in the BasicMultilingual Plane, which covers almost all characters in common use in anylanguage, fits within three bytes. The fourth byte is used for characters in the othercharacter planes (various less-common characters from Asian scripts, historicallanguages, and noncharacter notations such as musical notation). UTF-8 is veryspace-efficient for Western-style languages and less so for other languages.195Download at WoweBook.Com
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Contents■About the Author .......
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■ CONTENTS3-7. Writing a Nagios P
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■ CONTENTS■Chapter 8: Using the
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■ CONTENTSAbout the Author■Juli
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■ CONTENTSAcknowledgmentsMany tha
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■ CONTENTSIntroductionThis book i
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■ INTRODUCTIONDownloading the Cod
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C H A P T E R 1■ ■ ■Saving Yo
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CHAPTER 1 ■ SAVING YOURSELF EFFOR
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C H A P T E R 2■ ■ ■Centraliz
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CHAPTER 2 ■ CENTRALIZING YOUR NET
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C H A P T E R 3■ ■ ■Monitorin
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CHAPTER 3 ■ MONITORING AND UPDATI
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C H A P T E R 4■ ■ ■Taking Ba
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CHAPTER 4 ■ TAKING BACKUPS AND MA
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C H A P T E R 5■ ■ ■Working w
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CHAPTER 5 ■ WORKING WITH FILESYST
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C H A P T E R 6■ ■ ■Securing
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CHAPTER 6 ■ SECURING YOUR SYSTEMS
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Page 191 and 192: C H A P T E R 8■ ■ ■Making Be
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Page 225 and 226: CHAPTER 10 ■ THINGS GO IN, THINGS
Page 233 and 234: C H A P T E R 11■ ■ ■Tracking
Page 235 and 236: CHAPTER 11 ■ TRACKING DOWN BUGSTh
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Page 251 and 252: C H A P T E R 12■ ■ ■Managing
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CHAPTER 12 ■ MANAGING TIME AND PE
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APPENDIX■ ■ ■Perl TipsSee rec
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APPENDIX ■ PERL TIPS• Date::Par
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APPENDIX ■ PERL TIPSPerl Syntax N
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■ ■ ■Index■Symbols$CDPATH,
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■ INDEXenscript option (printing)
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■ INDEX■NNagiosadding hosts to,
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■ INDEXsyslogcentralized logging
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