Awk

Every input line is tested against each of the patterns in turn. For each pattern that matches, the corresponding action (which may involve multiple steps) is performed. Then the next line is read and the matching starts over. This continues until all the input has been read.

Awk 支持的模式如表 1 所示。

实例：若某列符合指定模式（第 1 列为 abc 时），则输出整行。

$ cat file1
abc 123
abcd 456
abc 789
$ awk '$1 ~ /^abc$/ {print $0}' file1
abc 123
abc 789

默认地，awk 以空格和 tab 来对每一行进行切分。 $1, $2, $3 等分别表示一行的第 1、2、3 等列，而 $0 则表示整行。

An action is a sequence of statements of the following kinds:

break
continue
delete array-element
do statement while (expression)
exit [expression]
expression
if (expression) statement [else statement]
input-output statement
for (expression; expression; expression) statement
for (variable in array) statement
next
return [expression]
while (expression) statement
{ statements }

Comments may be inserted at the end of any line. A comment starts with the character # and finishes at the end of the line.

You can check whether a particular array index "indexname" exists by using the following if condition syntax. This will return true, if the index "indexname" exists in the array.

if ( "indexname" in arrayname ) ...

注意：不要用下面的语句来测试是否在数组 arrayname 中存在元素 arrayname["indexname"]，因为它有副作用：不存在时会创建元素 arrayname["indexname"]。

if ( arrayname["indexname"] != "" ) ...

If you want to access all the array elements, you can use a special instance of the for loop to go through all the indexes of an array:

for (var in arrayname)
    print arrayname[var]

var can be any variable, which holds the index of array.

An array element may be deleted with delete arrayname[subscript]

For example, this loop removes all the elements from the array arrayname:

for (i in arrayname)
    delete arrayname[i]

还有一种更简单的办法删除数组里所有元素，但是古老的 awk 实现可能不支持。
All the elements of an array may be deleted with a single statement by leaving off the subscript in the delete statement, as follows:

delete arrayname

For many years, using delete without a subscript was a common extension. In September 2012, it was accepted for inclusion into the POSIX standard. See the Austin Group website.

还有第三种删除数组里所有元素的方法（可移植性好，但不直观）：
The following statement provides a portable but nonobvious way to clear out an array:

split("", arrayname)

GNU awk contains two array sorting functions:

asort(source [, dest [, how ] ])
asorti(source [, dest [, how ] ])

Both functions return the number of elements in the array source. For asort(), gawk sorts the values of source and replaces the indices of the sorted values of source with sequential integers starting with one. If the optional array dest is specified, then source is duplicated into dest. dest is then sorted, leaving the indices of source unchanged.

For example, if the contents of array a are as follows:

a["last"] = "de"
a["first"] = "sac"
a["middle"] = "cul"

Calling 'asort(a)' would yield:

a[1] = "cul"
a[2] = "de"
a[3] = "sac"

The asorti() function works similarly to asort(); however, the indices are sorted, instead of the values. Thus, in the previous example, starting with the same initial set of indices and values in a, calling 'asorti(a)' would yield:

a[1] = "first"
a[2] = "last"
a[3] = "middle"

参考：
https://www.gnu.org/software/gawk/manual/gawk.html#String-Functions
https://www.gnu.org/software/gawk/manual/gawk.html#Array-Sorting-Functions

A multidimensional array is an array in which an element is identified by a sequence of indices instead of a single index.

$ cat array-multi3.awk
BEGIN {
  item["1,1"]=10;
  item["1,2"]=20;
  item[2,1]=30;
  item[2,2]=40;
  for (x in item)
    print "Index",x,"contains",item[x];
}

$ awk -f array-multi3.awk
Index 1,1 contains 10
Index 1,2 contains 20
Index 2X1 contains 30  # X is non-printable character "\034"
Index 2X2 contains 40  # X is non-printable character "\034"

In above example.
Indexes "1,1" and "1,2" are enclosed in quotes. So, this is treated as a one dimensional array index, no subscript separator is used by awk. So, the index gets printed as is.
Indexes 2,1 and 2,2 are not enclosed in quotes. So, this is treated as a multi-dimensional array index, and awk uses a subscript separator. So, the index is "2\0341" and "2\0342", which is printed with the non-printable character "\034" between the subscripts.

Reference: Sed and Awk 101 Hacks

ARGC and ARGV include the name of the invoking program (usually awk) but not the program arguments or options.

For example, with the command line

awk -f progfile a v=1 b

ARGC has the value 4, ARGV[0] contains awk, ARGV[1] contains a, ARGV[2] contains v=1, and ARGV[3] contains b.

Another example, with the command line

awk -F'\t' 'S3 > 100' countries

ARGC is 2, ARGV[0] is awk, ARGV[1] is countries.

表 3 是 awk 中支持的字符串处理函数。

说明，awk 中开始位置是 1（而不是 0），如：

$ echo "abc" | awk '{print index($0, "a")}'
1
$ echo "abc" | awk '{print index($0, "b")}'
2

使用内置函数 getline 可以显式地控制 awk 的读取过程。getline 成功时返回 1，文件结束时返回 0，出错时返回-1。

使用关键字 function 可以自定义函数。如：

# Returns minimum number
function min(num1, num2){
    return num1 < num2 ? num1 : num2
}

BEGIN {
    print(min(10, 20))
}

首先，通过一个简单例子回顾下 regexp constant (i.e., a string of characters between slashes)的用法：

$ cat file1
123
abcd
xyz
$ awk '$0 ~ /^[[:digit:]]+$/ { print }' file1         # 这里， /^abc$/ 是regexp constant（用两个斜杠包围）
123

有时，我们想在正则匹配符~右边使用变量，这就是 Dynamic Regexp（也称为 Computed Regexp）。如，上面例子也可以写为 Dynamic Regexp 的形式：

$ awk 'BEGIN { digits_regexp = "^[[:digit:]]+$" } $0 ~ digits_regexp { print }' file1   # Dynamic Regexp实例
123
$ awk -v regexp='^[[:digit:]]+$' '$0 ~ regexp { print }' file1                          # Dynamic Regexp实例（同上）
123

需要注意的是， Dynamic Regexp 不能用两个斜杠包围（两个斜杠包围是 regexp constant 的记法）。 如，下面是错误的用法：

$ awk -v regexp='/^[[:digit:]]+$/' '$0 ~ regexp { print }' file1             # 错误用法！正确用法为regexp='^[[:digit:]]+$'
awk: syntax error in regular expression /^[[:digit:]]+$/ at [[:digit:]]+$/
 input record number 1, file file1
 source line number 1

参考：Using Dynamic Regexps