1. AWK tool introduction

AWK is a language for manipulating text files and is a powerful text analysis tool
Complex text manipulation can be achieved in non-interactive mode
Compared with sed, which usually works on a whole line, awk is more inclined to divide a line into several fields, because awk is quite suitable for small text data

1.1 Basic format of AWK command

awk [选项] ‘模式条件{
    
    操作}’ 文件1 文件2...
awk -f|-v 脚本文件 文件1 文件.....

1.2 Working principle of AWK

As mentioned earlier, the sed command is often used to process a whole line, while awk tends to divide a line into multiple "fields" and then process them. By default, the field separator is a space or a tab key. The execution result of awk can print and display the field data through the print function.
In the process of using the awk command, you can use the logical operator "&&" to represent "and", "" to represent "or", "!" to represent "not"; you can also perform simple mathematical operations, such as +, -, * , /, %, ^ represent addition, subtraction, multiplication, division, remainder and power respectively.
Awk is followed by two single quotation marks and curly braces { } to set the processing operation you want to perform on the data. Awk can process subsequent files, and can also read the standard output from the previous command.

1.3 Common built-in variables

FS: column separator, specify the field separator of each line of text, the default is space or tab, the same as "-F"
NF: the number of fields in the currently processed row
NR: the row number (ordinal number) of the currently processed row
$0: the entire line content of the currently processed line
$n: the nth field (column n) of the currently processed row
FILENAME: the name of the file being processed
RS: line separator, when awk reads data from a file, it will cut the data into many records according to the definition of RS, and awk only reads one record at a time for processing, the default value is '\n'

2. Example

2.1 Print text content

Awk can automatically compress multiple spaces into one space
Printing strings requires double quotes

Example 1: Print disk usage

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Example 2: print a string

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Example 3: Print a string to determine how many lines a file has

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2.2 Extract fields according to $n

Example 1: Extract IP address
$n, which means take the third column

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Example 2: extract mac address

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2.3 Specify the delimiter according to option -F

Example 1: Print all usernames in /etc/passwd

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Example 2: Print multi-column content.
Commas can represent spaces when printing. If you use ":" or "+", you need to print special symbols with double quotes as strings

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