AWK Programming Language Notes Chapter 1: Basic Syntax

1 AWK command

pattern {
    
     action }

patternIndicates pattern matching and actionindicates operation. Both are optional and are distinguished by curly braces.

{action}You can append multiple files later, or you can use -f:

awk 'program' input files
awk -f progfile optional list of input files

Build data:

$ touch emp.data
$ vi emp.data
$ cat emp.data
Beth 4.00 0
Dan 3.75 0
Kathy 4.00 10
Mark 5.00 20
Mary 5.50 22
Susie 4.25 18

2 Extract fields

awk $extracts fields by 1starting with the field, 0then the entire line.

{
    
     print $1, $3 }
{
    
     print $O }
{
    
     print }

3 Calculate the number of fields

NF: number of fields, $NFindicating the last field.

$ awk ' {print NF, $1, $NF}' emp.data
3 Beth 0
3 Dan 0
...

4 Count the number of lines read

NR：counts the number of lines read so far

$ awk ' {print NR, $0}' emp.data
1 Beth 4.00 0
2 Dan 3.75 0
...

5 Add text to the print field

$ awk '{print "line number:",NR, $0}' emp.data
line number: 1 Beth 4.00 0
line number: 2 Dan 3.75 0
...

6 `%`Different formats by printing

$ awk '{printf("total pay for %s is $%.2f\n", $1, $2 * $3)}' emp.data
total pay for Beth is $0.00
total pay for Dan is $0.00
...

# -8表示8个字符并向左对齐
$ awk '{ printf("%-8s $%6.2f\n", $1, $2 * $3) }' emp.data
Beth     $  0.00
Dan      $  0.00
...

7 operators

$ awk '$3>0 {print $1, $2*$3}' emp.data
Kathy 40
Mark 100
...

$ awk '$2 >=5 || $3 >= 20 {print $0}' emp.data
Mark 5.00 20
Mary 5.50 22

$ awk '$1=="Susie" {print $0}' emp.data
Susie 4.25 18

$ awk '!($2<5 && $3<20) {print $0}' emp.data
Mark 5.00 20
Mary 5.50 22

8 BEGIN and END

The special pattern BEGINmatches before the first line of the first input file is read and ENDafter the last line of the last file is processed.

# print ""， 打印空白行
$ awk 'BEGIN { print "NAME RATE HOURS";print ""}{print}' emp.data
NAME RATE HOURS

Beth 4.00 0
Dan 3.75 0
...

$ awk '{pay=pay+$2*$3} 
END {print NR, "employees"                                      
print "total pay is ", pay
print "average pay is", pay/NR}' emp.data
6 employees
total pay is  337.5
average pay is 56.25

9 character processing

# 像处理数字一样处理文本
$ awk '$2>maxrate {maxrate=$2;maxemp=$1}
END {print "highest hourly rate:",maxrate,"for",maxemp}' emp.data
highest hourly rate: 5.50 for Mary

# 字符串连接
$ awk '{names=names $1 " "} END {print names}' emp.data
Beth Dan Kathy Mark Mary Susie

# 打印最后一行
$ awk '{last =$0} END {print last}' emp.data
Susie 4.25 18

10 built-in functions

$ awk '{print $1, length($1)}' emp.data
Beth 4
Dan 3
...

$ awk '{nc=nc+length($0)+1
 nw=nw+NF}
 END {print NR , "lines,",nw,"words,",nc,"characters"}' emp.data
6 lines, 18 words, 77 characters

11 Control flow

$ awk '$2>6 {n=n+1;pay=pay+$1*$3}
END {if (n>0)
    print n, "employees, total pay is",pay
   else
    print "no employees are paid more than $6/hour"
  }' emp.data
no employees are paid more than $6/hour

$ awk '{i=1;while(i <=2){print $1;i=i+1}}' emp.data

$ awk '{for (i=1;i<=3;i++) printf $2}' emp.data

12 Processing Arrays

line[NR]=$0, assign each row to linethe array.

~$ awk '{line[NR]=$0} END {i=NR;while (i>0){print line[i];i--}}' emp.data
Susie 4.25 18
Mary 5.50 22
...

13 One-liners

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