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set field delimiter
The default field delimiter is space, you can specify a delimiter explicitly with -F "delimiter":
awk -F: '{ print $NF }' /etc/passwd
# 或
awk 'BEGIN{ FS=":" } { print $NF }' /etc/passwd
In the BEGIN statement block, OFS="delimiter" can be used to set the delimiter of the output field.
flow control statement
In the while, do-while and for statements of linux awk, break and continue statements are allowed to control the flow direction, and statements such as exit are also allowed to exit. break interrupts the currently executing loop and jumps outside the loop to execute the next statement. if is process selection usage. In awk, flow control statements, grammatical structures, and c language types. With these statements, many shell programs can be handed over to awk, and the performance is very fast. The following is the usage of each statement.
conditional statement
if(表达式)
语句1
else
语句2
Statement 1 in the format can be multiple statements. For the convenience of judgment and reading, it is better to enclose multiple statements with {}. The awk branch structure allows nesting, and its format is:
if(表达式)
{
语句1}
else if(表达式)
{
语句2}
else
{
语句3}
Example:
awk 'BEGIN{
test=100;
if(test>90){
print "very good";
}
else if(test>60){
print "good";
}
else{
print "no pass";
}
}'
output:
very good
Each command statement can be followed by a ; semicolon at the end.
loop statement
while statement
while(表达式)
{
语句}
Example:
awk 'BEGIN{
test=100;
total=0;
while(i<=test){
total+=i;
i++;
}
print total;
}'
output:
5050
for loop
There are two formats for for loops:
Format 1:
for(变量 in 数组)
{
语句}
Example:
awk 'BEGIN{
for(k in ENVIRON){
print k"="ENVIRON[k];
}
}'
output:
TERM=linux
G_BROKEN_FILENAMES=1
SHLVL=1
pwd=/root/text
...
logname=root
HOME=/root
SSH_CLIENT=192.168.1.21 53087 22
Note: ENVIRON is an awk constant and is a subtypical array.
Format 2:
for(变量;条件;表达式)
{
语句}
Example:
awk 'BEGIN{
total=0;
for(i=0;i<=100;i++){
total+=i;
}
print total;
}'
output:
5050
do loop
do
{
语句} while(条件)
example:
awk 'BEGIN{
total=0;
i=0;
do {total+=i;i++;} while(i<=100)
print total;
}'
output:
5050
other statements
- break When used with a while or for statement, the break statement causes the program loop to exit.
- continue When the continue statement is used with a while or for statement, causes the program loop to move to the next iteration.
- next can cause the next input line to be read and return to the top of the script. This avoids performing additional operations on the current input line.
- The exit statement causes the main input loop to exit and transfers control to END, if END exists. If no END rule is defined, or an exit statement is applied within END, execution of the script is terminated.
Array application
Arrays are the soul of awk, and the most important thing in processing text is its array processing. Because array indices (subscripts) can be numbers and strings, arrays in awk are called associative arrays. Arrays in awk do not have to be declared ahead of time, nor do they have to declare their size. Array elements are initialized with 0 or an empty string, depending on the context.
array definition
Numbers as array indices (subscripts):
Array[1]="sun"
Array[2]="kai"
String as array index (subscript):
Array["first"]="www"
Array"[last"]="name"
Array["birth"]="1987"
In use, print Array[1] will print sun; use print Array[2] will print kai; use print["birth"] will get 1987.
read the value of the array
{
for(item in array) {
print array[item]}; } #输出的顺序是随机的
{
for(i=1;i<=len;i++) {
print array[i]}; } #Len是数组的长度
Array related functions
Get the length of the array:
awk 'BEGIN{info="it is a test";lens=split(info,tA," ");print length(tA),lens;}'
output:
4 4
length returns the string and the length of the array, split splits the string into an array, and returns the length of the array obtained by splitting.
awk 'BEGIN{info="it is a test";split(info,tA," ");print asort(tA);}'
output:
4
asort sorts an array and returns the length of the array.
Output array content (unordered, ordered output):
awk 'BEGIN{info="it is a test";split(info,tA," ");for(k in tA){print k,tA[k];}}'
output:
4 test
1 it
2 is
3 a
for...in output, because the array is an associative array, the default is unordered. So through for...in is an unordered array. If you need to get an ordered array, you need to get it by subscript.
awk 'BEGIN{info="it is a test";tlen=split(info,tA," ");for(k=1;k<=tlen;k++){print k,tA[k];}}'
output:
1 it
2 is
3 a
4 test
Note: The array subscript starts from 1, which is different from the C array.
Determine the existence of the key value and delete the key value:
Wrong judgment method:
awk 'BEGIN{tB["a"]="a1";tB["b"]="b1";if(tB["c"]!="1"){print "no found";};for(k in tB){print k,tB[k];}}'
output:
no found
a a1
b b1
c
There is a strange problem above, tB["c"] is not defined, but when looping, it is found that the key value already exists, and its value is empty. It should be noted here that the awk array is an associative array. As long as its key is referenced through the array, it will be The sequence will be created automatically.
The correct way to judge:
awk 'BEGIN{tB["a"]="a1";tB["b"]="b1";if( "c" in tB){print "ok";};for(k in tB){print k,tB[k];}}'
output:
a a1
b b1
if(key in array) uses this method to determine whether the key value is contained in the array.
Delete key value:
awk 'BEGIN{tB["a"]="a1";tB["b"]="b1";delete tB["a"];for(k in tB){print k,tB[k];}}'
output:
b b1
delete array[key] can be deleted, corresponding to the sequence value of the array key.
Use of two-dimensional and multidimensional arrays
Awk's multidimensional array is essentially a one-dimensional array. More precisely, awk does not support multidimensional arrays in storage. Awk provides access methods that logically simulate two-dimensional arrays. For example, accesses like array[2,4]=1 are allowed. Awk uses a special string SUBSEP (�34) as a split field. In the above example, the key value stored in the associative array array is actually 2�344.
Similar to the membership test of a one-dimensional array, multidimensional arrays can use syntax such as if ( (i,j) in array), but the subscript must be placed in parentheses. Similar to the iterative access of one-dimensional arrays, multidimensional arrays use for (item in array) syntax to traverse the array. Unlike one-dimensional arrays, multidimensional arrays must use the split() function to access individual subscripted components.
awk 'BEGIN{
for(i=1;i<=9;i++){
for(j=1;j<=9;j++){
tarr[i,j]=i*j; print i,"*",j,"=",tarr[i,j];
}
}
}'
output:
1 * 1 = 1
1 * 2 = 2
1 * 3 = 3
1 * 4 = 4
1 * 5 = 5
1 * 6 = 6
...
9 * 6 = 54
9 * 7 = 63
9 * 8 = 72
9 * 9 = 81
The array content can be obtained by reference to array[k,k2].
Another way:
awk 'BEGIN{
for(i=1;i<=9;i++){
for(j=1;j<=9;j++){
tarr[i,j]=i*j;
}
}
for(m in tarr){
split(m,tarr2,SUBSEP); print tarr2[1],"*",tarr2[2],"=",tarr[m];
}
}'
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