Master awk: From Simple Column Extraction to Powerful Data Aggregation

awk

works similarly to sed, but while sed excels at line selection, awk excels at column selection. It processes input line by line, applying commands to each line.

Basic Syntax

awk [options] 'BEGIN{commands} pattern{commands} END{commands}' filename

Printing Specific Columns

$ echo 'I love you' | awk '{print $3 $2 $1}'
# youloveI

By default, fields are split on whitespace. To change the delimiter, use -F:

$ echo '192.168.1.1' | awk -F "." '{print $2}'
# 168

Conditional Filtering (pattern)

The pattern part selects which lines to process. For example, to print rows where the second column is at least 90:

$ awk '$2>=90{print $0}' score.txt
kitty 90 95 87

If Statements

$ awk '{if($2>=90) print $0}' score.txt
kitty 90 95 87
$ awk '{if($2>=90) print $1,"优秀"; else print $1,"良好"}' score.txt
tom 良好
kitty 优秀
jack 良好

BEGIN Block – Defining Header

$ awk 'BEGIN{print "姓名 语文 数学 英语"}{printf "%-8s%-5d%-5d%-5d
",$1,$2,$3,$4}' score.txt
姓名 语文 数学 英语
tom   60  60  60
kitty 90  95  87
jack  72  84  99

The printf format %-8s left‑aligns the field to width 8, similar to C.

END Block – Adding Footer

$ echo ok | awk '{print $1} END{print "end"}'
ok
end

Data Aggregation Example

$ awk 'BEGIN{print "姓名 语文 数学 英语 总成绩"; sum1=0;sum2=0;sum3=0;sumall=0} {printf "%5s%5d%5d%5d%5d
",$1,$2,$3,$4,$2+$3+$4; sum1+=$2; sum2+=$3; sum3+=$4; sumall+=$2+$3+$4} END{printf "%5s%5d%5d%5d%5d
","总成绩",sum1,sum2,sum3,sumall}' score.txt
姓名 语文 数学 英语 总成绩
  tom   60   60   60  180
 kitty   90   95   87  272
  jack   72   84   99  255
总成绩   222   239   246  707

Useful Built‑in Variables

NF – number of fields in the current line (e.g., $NF is the last field)

NR – current record number (line number)

FILENAME – name of the current input file

OFMT – output format for numbers (default %.6g)

Common Built‑in Functions

Mathematical functions such as sqrt() and random number generation with rand() and srand():

$ echo 1 2 | awk '{print $1+sqrt($2)}'
2.41421
$ echo 1 | awk 'BEGIN{srand()}{print rand()}'
0.929885