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<h1 id="variables">Variables</h1>
<p>awk 는 변수를 사용할 때 일반 프로그래밍 언어와 달리 먼저 선언을 하지 않고 사용해도 됩니다.
그러면 자동으로 전역변수로 설정이 되고 프로그램이 종료될 때까지 유지가 됩니다.
awk 는 변수에 타입을 두지 않기 때문에 자유롭게 숫자나 스트링을 대입해 사용할 수 있습니다.
이와같은 특징은 간단히 쉘 스크립트를 작성할 때 편리합니다.</p>
<pre><code class="lang-bash"><span class="hljs-comment"># 동일한 변수에 숫자나 스트링을 자유롭게 대입할 수 있다.</span>
myvar = <span class="hljs-number">45</span>; myvar = <span class="hljs-string">"foo bar"</span>
</code></pre>
<p>awk 에서 사용하는 키워드나 함수명을 변수명으로 사용할 수 없습니다.</p>
<pre><code class="lang-bash">$ awk <span class="hljs-string">'BEGIN { in = 123 }'</span> <span class="hljs-comment"># in 키워드</span>
awk: cmd. line:<span class="hljs-number">1</span>: BEGIN { <span class="hljs-keyword">in</span> = <span class="hljs-number">123</span> }
awk: cmd. line:<span class="hljs-number">1</span>: ^ syntax error
<span class="hljs-comment"># 다음은 모두 오류입니다.</span>
$ awk <span class="hljs-string">'BEGIN { index = 123 }'</span> <span class="hljs-comment"># index() 함수</span>
$ awk <span class="hljs-string">'BEGIN { length = 123 }'</span> <span class="hljs-comment"># length() 함수</span>
$ awk <span class="hljs-string">'BEGIN { func = 123 }'</span> <span class="hljs-comment"># func 키워드 (function 단축형)</span>
</code></pre>
<p>존재하지 않는 변수는 산술연산에서는 <code>0</code> 과 같고 스트링 연산에서는 <code>""</code> 과 같습니다.</p>
<pre><code class="lang-bash"><span class="hljs-comment"># 현재 bb 는 존재하지 않는 변수 </span>
$ awk <span class="hljs-string">'BEGIN { aa=0; if ( aa == bb ) print "yes" }'</span>
yes
$ awk <span class="hljs-string">'BEGIN { aa=""; if ( aa == bb ) print "yes" }'</span>
yes
</code></pre>
<h4 id="하지만-scalar-변수와--array-변수는-구분합니다">하지만 scalar 변수와 array 변수는 구분합니다.</h4>
<p>awk 에서는 변수명이 처음 사용될 때 scalar 변수인지 array 변수인지가 결정됩니다.
그러므로 이후에 array 변수를 scalar 변수로 사용한다 든지 scalar 변수를 array 변수로 사용하게 되면
오류가 발생합니다.
심지어 array 변수를 delete 한 후에도 scalar 변수로 사용할 수 없습니다.</p>
<pre><code class="lang-bash"><span class="hljs-comment"># 변수 AA 가 처음 사용될 때 array 변수로 결정됨. 이후에 scalar 변수로 사용하여 오류 발생</span>
$ awk <span class="hljs-string">'BEGIN{ AA[1]="foo"; AA="bar" }'</span>
awk: cmd. line:<span class="hljs-number">1</span>: fatal: attempt to use array <span class="hljs-string">'AA'</span> <span class="hljs-keyword">in</span> a scalar context
<span class="hljs-comment"># 변수 AA 가 처음 사용될 때 scalar 변수로 결정됨. 이후에 array 변수로 사용하여 오류 발생</span>
$ awk <span class="hljs-string">'BEGIN{ AA="foo"; AA[1]="bar" }'</span>
awk: cmd. line:<span class="hljs-number">1</span>: fatal: attempt to use scalar <span class="hljs-string">'AA'</span> as an array
</code></pre>
<p><code>print</code> 문도 scalar context 이므로 array 변수를 사용할 수 없습니다.</p>
<pre><code class="lang-bash">$ awk <span class="hljs-string">'BEGIN { AA[1]="foo"; print AA }'</span>
awk: cmd. line:<span class="hljs-number">1</span>: fatal: attempt to use array `AA<span class="hljs-string">' in a scalar context
$ awk '</span>BEGIN { AA[<span class="hljs-number">1</span>]=<span class="hljs-string">"foo"</span>; <span class="hljs-built_in">print</span> AA[<span class="hljs-number">1</span>] }<span class="hljs-string">'
foo
</span></code></pre>
<h3 id="variables"><code>$</code> Variables</h3>
<p>awk 에서 사용되는 특수한 형태의 변수라고 할 수 있습니다.
레코드 값을 나타내는 <code>$0</code>, 각각의 필드 값을 나타내는 <code>$1</code>, <code>$2</code>, <code>$3</code>, ... 로 구성됩니다.
여기서 숫자 부분은 변수로 대체해 사용할 수 있습니다.</p>
<pre><code class="lang-bash">$ <span class="hljs-built_in">echo</span> <span class="hljs-number">11</span> <span class="hljs-number">22</span> <span class="hljs-number">33</span> | awk <span class="hljs-string">'{ print $1, $2, $3 }'</span>
<span class="hljs-number">11</span> <span class="hljs-number">22</span> <span class="hljs-number">33</span>
<span class="hljs-comment"># 숫자 부분을 변수로 대체해 사용</span>
$ <span class="hljs-built_in">echo</span> <span class="hljs-number">11</span> <span class="hljs-number">22</span> <span class="hljs-number">33</span> | awk <span class="hljs-string">'{ a=1; b=2; c=3; print $a, $b, $c }'</span>
<span class="hljs-number">11</span> <span class="hljs-number">22</span> <span class="hljs-number">33</span>
$ <span class="hljs-built_in">echo</span> <span class="hljs-number">11</span> <span class="hljs-number">22</span> <span class="hljs-number">33</span> | awk <span class="hljs-string">'{ v=1; print $(v++), $(v++), $v }'</span>
<span class="hljs-number">11</span> <span class="hljs-number">22</span> <span class="hljs-number">33</span>
<span class="hljs-comment"># NF 는 레코드의 필드 개수를 나타내는 builtin 변수</span>
$ <span class="hljs-built_in">echo</span> <span class="hljs-number">11</span> <span class="hljs-number">22</span> <span class="hljs-number">33</span> | awk <span class="hljs-string">'{ print $(NF-2), $(NF-1), $NF }'</span>
<span class="hljs-number">11</span> <span class="hljs-number">22</span> <span class="hljs-number">33</span>
-----------------------------------------------------------
<span class="hljs-comment"># 50점 이하 과목이 하나라도 있는 사람 출력하기</span>
$ cat file
이름 국어 산수 사회 자연 미술
AAA <span class="hljs-number">88</span> <span class="hljs-number">76</span> <span class="hljs-number">97</span> <span class="hljs-number">78</span> <span class="hljs-number">35</span>
BBB <span class="hljs-number">77</span> <span class="hljs-number">28</span> <span class="hljs-number">56</span> <span class="hljs-number">43</span> <span class="hljs-number">55</span>
CCC <span class="hljs-number">90</span> <span class="hljs-number">65</span> <span class="hljs-number">53</span> <span class="hljs-number">80</span> <span class="hljs-number">65</span>
$ awk <span class="hljs-string">'NR!=1 {
for (i=0; NF-i > 1; i++)
if ($(NF-i) < 50) { print; next }
}'</span> file
AAA <span class="hljs-number">88</span> <span class="hljs-number">76</span> <span class="hljs-number">97</span> <span class="hljs-number">78</span> <span class="hljs-number">35</span>
BBB <span class="hljs-number">77</span> <span class="hljs-number">28</span> <span class="hljs-number">56</span> <span class="hljs-number">43</span> <span class="hljs-number">55</span>
</code></pre>
<h3 id="awk-명령에-변수값-전달-방법">awk 명령에 변수값 전달 방법</h3>
<p>awk 는 코드를 작성할 때 single quotes 을 사용하므로 본문에 shell 변수를 사용하지 못하는데요.
명령의 <code>-v</code> 옵션이나 인수 전달 방식을 통해서 shell 변수값을 전달할 수 있습니다.</p>
<p>첫 번째 방법은 데이터 파일을 awk 에 인수로 전달할 때처럼 변수값을 전달하는 방법입니다.<br>이렇게 전달된 변수값은 순서에 따라서 이후의 데이터 파일에 적용됩니다.</p>
<pre><code class="lang-bash">$ cat file1
file1 record <span class="hljs-number">1</span>
file1 record <span class="hljs-number">2</span>
file1 record <span class="hljs-number">3</span>
$ cat file2
file2 record <span class="hljs-number">1</span>
file2 record <span class="hljs-number">2</span>
file2 record <span class="hljs-number">3</span>
<span class="hljs-comment"># 순서에 따라서 AA=111 값은 file1 레코드에 적용되고</span>
<span class="hljs-comment"># AA=222 값은 file2 에 적용되는 것을 볼 수 있습니다.</span>
$ awk <span class="hljs-string">'{ print AA, $0 }'</span> AA=<span class="hljs-number">111</span> file1 AA=<span class="hljs-number">222</span> file2
<span class="hljs-number">111</span> file1 record <span class="hljs-number">1</span>
<span class="hljs-number">111</span> file1 record <span class="hljs-number">2</span>
<span class="hljs-number">111</span> file1 record <span class="hljs-number">3</span>
<span class="hljs-number">222</span> file2 record <span class="hljs-number">1</span>
<span class="hljs-number">222</span> file2 record <span class="hljs-number">2</span>
<span class="hljs-number">222</span> file2 record <span class="hljs-number">3</span>
</code></pre>
<p>따라서 다음과 같이 설정하면 데이터 파일 별로 RS, FS 를 설정할 수 있습니다.</p>
<pre><code class="lang-bash"><span class="hljs-comment"># file1 에는 RS='#' FS=',' 값이 적용되고</span>
<span class="hljs-comment"># file2 에는 RS='@' FS=':' 값이 적용된다.</span>
$ awk <span class="hljs-string">'{ . . . }'</span> RS=<span class="hljs-string">'#'</span> FS=<span class="hljs-string">','</span> file1 RS=<span class="hljs-string">'@'</span> FS=<span class="hljs-string">':'</span> file2
</code></pre>
<p>BEGIN 블록은 레코드를 읽어들여 명령 사이클을 시작하기 전에 실행되므로
위와 같이 설정한 변수는 BEGIN 블록에는 적용되지 않는데요.
소스코드 전체에 적용시키려면 awk 명령의 <code>-v</code> 옵션을 사용해야 합니다.</p>
<pre><code class="lang-bash">$ awk <span class="hljs-string">'BEGIN{ print AA, BB }'</span> AA=<span class="hljs-number">11</span> BB=<span class="hljs-number">22</span>
$
$ awk -v AA=<span class="hljs-number">11</span> -v BB=<span class="hljs-number">22</span> <span class="hljs-string">'BEGIN{ print AA, BB }'</span>
<span class="hljs-number">11</span> <span class="hljs-number">22</span>
</code></pre>
<p>마지막으로 FS 변수는 자주 사용되므로 따로 <code>-F</code> 옵션을 두고 있습니다.</p>
<pre><code class="lang-bash">$ awk -F, <span class="hljs-string">'BEGIN{ print "FS is: " FS }'</span>
FS is: ,
<span class="hljs-comment"># FS 값은 ',' or ':'</span>
$ awk -F <span class="hljs-string">',|:'</span> <span class="hljs-string">'BEGIN{ print "FS is: " FS }'</span>
FS is: ,|:
</code></pre>
<h3 id="awk-에-shell-변수값을-전달할-때-주의할-점">awk 에 shell 변수값을 전달할 때 주의할 점</h3>
<p>awk 에 변수값을 전달할 때 사용하는 <code>=</code> 문자는 shell 에서 사용하는 대입 연산자가 아닙니다.
그러므로 전달하고자 하는 변수값이 공백으로 분리되어 있으면 double quotes 을 해야
오류가 생기지 않습니다.</p>
<pre><code class="lang-bash">$ MYVAR=<span class="hljs-string">"foo bar"</span> <span class="hljs-comment"># shell 변수</span>
<span class="hljs-comment"># -v AA=$MYVAR 는 -v AA=foo bar 와 같게 되므로 오류가 발생합니다.</span>
$ awk -v AA=<span class="hljs-variable">$MYVAR</span> <span class="hljs-string">'BEGIN { ... }'</span> <span class="hljs-comment"># error !</span>
<span class="hljs-comment"># double quotes 을 해야 합니다.</span>
$ awk -v AA=<span class="hljs-string">"<span class="hljs-variable">$MYVAR</span>"</span> <span class="hljs-string">'BEGIN { ... }'</span> <span class="hljs-comment"># OK</span>
$ awk -v <span class="hljs-string">"AA=<span class="hljs-variable">$MYVAR</span>"</span> <span class="hljs-string">'BEGIN { ... }'</span> <span class="hljs-comment"># OK</span>
</code></pre>
<h3 id="symtab-변수를-이용한-indirection">SYMTAB 변수를 이용한 indirection</h3>
<p>awk 는 프로그램을 실행하기 전에 먼저 전체 코드를 읽어들여 파싱 하는데요.
이때 빌드 된 심볼 테이블을 <code>SYMTAB</code> built-in 변수를 통해 접근할 수가 있습니다.
이 변수를 이용하면 일종의 indirection 을 할 수 있습니다.</p>
<pre><code class="lang-bash">$ awk <span class="hljs-string">'
BEGIN {
foo = 100
SYMTAB["foo"] = 200
print foo
bar[2] = 300
SYMTAB["bar"][2] = 400
print bar[2]
}'</span>
<span class="hljs-number">200</span>
<span class="hljs-number">400</span>
$ awk <span class="hljs-string">'
BEGIN {
answer = 10.5
multiply("answer", 4)
print "The answer is", answer
}
func multiply(var, amount) {
SYMTAB[var] *= amount
}'</span>
The answer is <span class="hljs-number">42</span>
</code></pre>
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