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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><HTML><HEAD><TITLE>Awk</TITLE><METANAME="GENERATOR"CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+"><LINKREL="HOME"TITLE="Advanced Bash-Scripting Guide"HREF="index.html"><LINKREL="UP"TITLE="A Sed and Awk Micro-Primer"HREF="sedawk.html"><LINKREL="PREVIOUS"TITLE="A Sed and Awk Micro-Primer"HREF="sedawk.html"><LINKREL="NEXT"TITLE="Exit Codes With Special Meanings"HREF="exitcodes.html"><METAHTTP-EQUIV="Content-Style-Type"CONTENT="text/css"><LINKREL="stylesheet"HREF="common/kde-common.css"TYPE="text/css"><METAHTTP-EQUIV="Content-Type"CONTENT="text/html; charset=iso-8859-1"><METAHTTP-EQUIV="Content-Language"CONTENT="en"><LINKREL="stylesheet"HREF="common/kde-localised.css"TYPE="text/css"TITLE="KDE-English"><LINKREL="stylesheet"HREF="common/kde-default.css"TYPE="text/css"TITLE="KDE-Default"></HEAD><BODYCLASS="SECT1"BGCOLOR="#FFFFFF"TEXT="#000000"LINK="#AA0000"VLINK="#AA0055"ALINK="#AA0000"STYLE="font-family: sans-serif;"><DIVCLASS="NAVHEADER"><TABLESUMMARY="Header navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><THCOLSPAN="3"ALIGN="center">Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH></TR><TR><TDWIDTH="10%"ALIGN="left"VALIGN="bottom"><AHREF="sedawk.html"ACCESSKEY="P">Prev</A></TD><TDWIDTH="80%"ALIGN="center"VALIGN="bottom">Appendix C. A Sed and Awk Micro-Primer</TD><TDWIDTH="10%"ALIGN="right"VALIGN="bottom"><AHREF="exitcodes.html"ACCESSKEY="N">Next</A></TD></TR></TABLE><HRALIGN="LEFT"WIDTH="100%"></DIV><DIVCLASS="SECT1"><H1CLASS="SECT1"><ANAME="AWK"></A>C.2. Awk</H1><P><ANAME="AWKREF"></A></P><P><ICLASS="FIRSTTERM">Awk</I> is a full-featured text processing language with a syntax reminiscent of <ICLASS="FIRSTTERM">C</I>. While it possesses an extensive set of operators and capabilities, we will cover only a few of these here - the ones most useful in shell scripts.</P><P>Awk breaks each line of input passed to it into <ICLASS="FIRSTTERM">fields</I>. By default, a field is a string of consecutive characters delimited by <AHREF="special-chars.html#WHITESPACEREF">whitespace</A>, though there are options for changing this. Awk parses and operates on each separate field. This makes it ideal for handling structured text files -- especially tables -- data organized into consistent chunks, such as rows and columns.</P><P><AHREF="variables.html#SNGLQUO">Strong quoting</A> and <AHREF="special-chars.html#CODEBLOCKREF">curly brackets</A> enclose blocks of awk code within a shell script.</P><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 # $1 is field #1, $2 is field #2, etc. 2 3 echo one two | awk '{print $1}' 4 # one 5 6 echo one two | awk '{print $2}' 7 # two 8 9 # But what is field #0 ($0)? 10 echo one two | awk '{print $0}' 11 # one two 12 # All the fields! 13 14 15 awk '{print $3}' $filename 16 # Prints field #3 of file $filename to stdout. 17 18 awk '{print $1 $5 $6}' $filename 19 # Prints fields #1, #5, and #6 of file $filename. 20 21 awk '{print $0}' $filename 22 # Prints the entire file! 23 # Same effect as: cat $filename . . . or . . . sed '' $filename</PRE></TD></TR></TABLE></P><P>We have just seen the awk <ICLASS="FIRSTTERM">print</I> command in action. The only other feature of awk we need to deal with here is variables. Awk handles variables similarly to shell scripts, though a bit more flexibly.</P><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 { total += ${column_number} }</PRE></TD></TR></TABLE> This adds the value of <TTCLASS="PARAMETER"><I>column_number</I></TT> to the running total of <TTCLASS="PARAMETER"><I>total</I></TT>>. Finally, to print <SPANCLASS="QUOTE">"total"</SPAN>, there is an <BCLASS="COMMAND">END</B> command block, executed after the script has processed all its input. <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 END { print total }</PRE></TD></TR></TABLE></P><P>Corresponding to the <BCLASS="COMMAND">END</B>, there is a <BCLASS="COMMAND">BEGIN</B>, for a code block to be performed before awk starts processing its input.</P><P>The following example illustrates how <BCLASS="COMMAND">awk</B> can add text-parsing tools to a shell script.</P><DIVCLASS="EXAMPLE"><HR><ANAME="LETTERCOUNT2"></A><P><B>Example C-1. Counting Letter Occurrences</B></P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #! /bin/sh 2 # letter-count2.sh: Counting letter occurrences in a text file. 3 # 4 # Script by nyal [nyal@voila.fr]. 5 # Used in ABS Guide with permission. 6 # Recommented and reformatted by ABS Guide author. 7 # Version 1.1: Modified to work with gawk 3.1.3. 8 # (Will still work with earlier versions.) 9 10 11 INIT_TAB_AWK="" 12 # Parameter to initialize awk script. 13 count_case=0 14 FILE_PARSE=$1 15 16 E_PARAMERR=65 17 18 usage() 19 { 20 echo "Usage: letter-count.sh file letters" 2>&1 21 # For example: ./letter-count2.sh filename.txt a b c 22 exit $E_PARAMERR # Too few arguments passed to script. 23 } 24 25 if [ ! -f "$1" ] ; then 26 echo "$1: No such file." 2>&1 27 usage # Print usage message and exit. 28 fi 29 30 if [ -z "$2" ] ; then 31 echo "$2: No letters specified." 2>&1 32 usage 33 fi 34 35 shift # Letters specified. 36 for letter in `echo $@` # For each one . . . 37 do 38 INIT_TAB_AWK="$INIT_TAB_AWK tab_search[${count_case}] = \ 39 \"$letter\"; final_tab[${count_case}] = 0; " 40 # Pass as parameter to awk script below. 41 count_case=`expr $count_case + 1` 42 done 43 44 # DEBUG: 45 # echo $INIT_TAB_AWK; 46 47 cat $FILE_PARSE | 48 # Pipe the target file to the following awk script. 49 50 # --------------------------------------------------------------------- 51 # Earlier version of script: 52 # awk -v tab_search=0 -v final_tab=0 -v tab=0 -v \ 53 # nb_letter=0 -v chara=0 -v chara2=0 \ 54 55 awk \ 56 "BEGIN { $INIT_TAB_AWK } \ 57 { split(\$0, tab, \"\"); \ 58 for (chara in tab) \ 59 { for (chara2 in tab_search) \ 60 { if (tab_search[chara2] == tab[chara]) { final_tab[chara2]++ } } } } \ 61 END { for (chara in final_tab) \ 62 { print tab_search[chara] \" => \" final_tab[chara] } }" 63 # --------------------------------------------------------------------- 64 # Nothing all that complicated, just . . . 65 #+ for-loops, if-tests, and a couple of specialized functions. 66 67 exit $? 68 69 # Compare this script to letter-count.sh.</PRE></TD></TR></TABLE><HR></DIV><P>For simpler examples of awk within shell scripts, see: <OLTYPE="1"><LI><P><AHREF="internal.html#EX44">Example 14-13</A></P></LI><LI><P><AHREF="redircb.html#REDIR4">Example 19-8</A></P></LI><LI><P><AHREF="filearchiv.html#STRIPC">Example 15-31</A></P></LI><LI><P><AHREF="wrapper.html#COLTOTALER">Example 33-5</A></P></LI><LI><P><AHREF="ivr.html#COLTOTALER2">Example 9-25</A></P></LI><LI><P><AHREF="internal.html#COLTOTALER3">Example 14-20</A></P></LI><LI><P><AHREF="procref1.html#PIDID">Example 27-2</A></P></LI><LI><P><AHREF="procref1.html#CONSTAT">Example 27-3</A></P></LI><LI><P><AHREF="loops.html#FILEINFO">Example 10-3</A></P></LI><LI><P><AHREF="extmisc.html#BLOTOUT">Example 15-58</A></P></LI><LI><P><AHREF="randomvar.html#SEEDINGRANDOM">Example 9-31</A></P></LI><LI><P><AHREF="moreadv.html#IDELETE">Example 15-4</A></P></LI><LI><P><AHREF="string-manipulation.html#SUBSTRINGEX">Example 9-15</A></P></LI><LI><P><AHREF="assortedtips.html#SUMPRODUCT">Example 33-16</A></P></LI><LI><P><AHREF="loops.html#USERLIST">Example 10-8</A></P></LI><LI><P><AHREF="wrapper.html#PRASC">Example 33-4</A></P></LI><LI><P><AHREF="mathc.html#HYPOT">Example 15-51</A></P></LI></OL> </P><P>That's all the awk we'll cover here, folks, but there's lots more to learn. See the appropriate references in the <AHREF="biblio.html"><I>Bibliography</I></A>.</P></DIV><DIVCLASS="NAVFOOTER"><HRALIGN="LEFT"WIDTH="100%"><TABLESUMMARY="Footer navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top"><AHREF="sedawk.html"ACCESSKEY="P">Prev</A></TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="index.html"ACCESSKEY="H">Home</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top"><AHREF="exitcodes.html"ACCESSKEY="N">Next</A></TD></TR><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top">A Sed and Awk Micro-Primer</TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="sedawk.html"ACCESSKEY="U">Up</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top">Exit Codes With Special Meanings</TD></TR></TABLE></DIV></BODY></HTML>⌨️ 快捷键说明
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