"Forget to learn again" shell Basics - 29. Awk built-in variables

1、AWK Built-in variables

AWK The built-in variables are shown in the following table ：

awk Built-in variables	effect
$0	Represents the present AWK Read the entire row of data . We know AWK Read data line by line ,$0 It represents the entire row of data currently read into the row .
$n	It stands for the No. of the current reading line n A field .
NF	Fields owned by the current row （ Column ） total .
NR	At present AWK Rows processed , Is the row number of the total data .
FS	User defined separator .AWK The default delimiter for is any space （tab Key or space ）, If you want to use other delimiters （ Such as “：”）, Need FS Variable definitions .
ARGC	Number of command line parameters .
ARGV	Array of command line arguments .
FNR	The current number of records in the current file （ The input file starts with 1）.
OFMT	The output format of the value （ The default is %.6g）.
OFS	Separator for output field （ Default is space ）.
ORS	Output record separator （ The default is line break ）.
RS	Enter record separator （ The default is line break ）.

2、 Exercise instructions

（1）$n Variable practice

Use the following text ：

ID      Name    Python  Linux   MySQL   Java
1       Tangs   88      87      86      85.55
2       Sunwk   99      98      97      96,66
3       Zhubj   77      76      75      74.44
4       Shahs   66      65      64      63.33

For example, we can extract the number in the text 2 Column data , Execute the following command ：

[[email protected] tmp]# awk '{printf $2 "\n"}' student.txt
Name
Tangs
Sunwk
Zhubj
Shahs

（2）FS Variable practice

AWK The default delimiter for is any space （tab Key or space ）, If you want to use other delimiters , Need FS Variable definitions .

cut The order defaults to tab Key as separator .

We used cut Command extracted /etc/passwd User name of ordinary user in the file , Now let's use AWK To extract the user name of the user who can log in normally .

Execute the following command ：

[[email protected] tmp]# cat /etc/passwd | grep "/bin/bash" | awk '{FS=":"} {printf $1 "\n"}'
root:x:0:0:root:/root:/bin/bash
user1
user2

explain ：FS Variable specifies that the delimiter is a separate action , Printout is another action .

Looking at the above results, we will find ,user1 and user2 The user's information is correctly extracted , And the first line root User information , Is to print out the whole line of data , I didn't press : The colon is used as a separator to correctly extract .

Because AWK First read a row of data into AWK in , And then use the following action , Then process the read data .

That is to say, I have put the first line of root User information , Read in to awk in ,$0、$1、$2 And other variables have been assigned , Only then did I see that you specified in the action behind : Colon as separator , At this time, it is too late to process the first row of data , Only use AWK The default treatment , Use spaces as separators to handle , There are no spaces on this line , All the data in this row will be printed out .

It's time to process the second row of data ,AWK Already know to use : Colon as separator , At this time, the data can be processed correctly .

What you need now BEGIN To deal with this problem , Specify the separator {FS=":"} Put in BEGIN Medium will do .

[[email protected] tmp]# cat /etc/passwd | grep "/bin/bash" | awk 'BEGIN{FS=":"} {printf $1 "\n"}'
root
user1
user2

In this way, we can correctly extract the data we need .

So it's using AWK When , If you need to specify the separator manually , The action specifying the delimiter is written in BEGIN in .

summary ： If there is an obvious separator , Recommended priority cut command , Because of the simple .

But if you need some judgment , For example, I need to according to the user ID, View the user name of a user .

Use this time AWK A lot of convenience ,cut Commands cannot be handled directly , You need to write a script to filter .

Like printing uid=500 User name of the user for , The order is as follows ：

[[email protected] tmp]# cat /etc/passwd | grep "/bin/bash" | awk 'BEGIN{FS=":"} $3=="500" {printf $1 "\n"}'
user1

# $3=="500" Or you could write it as $3==500 perhaps $3=/500/ Fine 

（3）NF Variables and NR Variable practice

We still use /etc/passwd Take the contents of the file as an example , Print out the user name of the user who can log in , user ID, Line number , Number of fields （ That's the number of columns ）.

Execute the following command ：

#  Tips ： It's written in , The output format is in double quotation marks , Variables are outside double quotation marks .
[[email protected] tmp]# cat /etc/passwd | grep "/bin/bash" | awk 'BEGIN{FS=":"} {printf $1 "\t" $3 "\t  Line number ：" NR "\t  Number of fields ：" NF "\n"}'
root    0     Line number ：1     Number of fields ：7
user1   500   Line number ：2     Number of fields ：7
user2   501   Line number ：3     Number of fields ：7

Pay attention to the , Finally, it was introduced into AWK Only three lines of data are processed in , as follows ：

[[email protected] tmp]# cat /etc/passwd | grep "/bin/bash"
root:x:0:0:root:/root:/bin/bash
user1:x:500:500::/home/user1:/bin/bash
user2:x:501:501::/home/user2:/bin/bash

3、 summary ：

We will learn before 5 individual AWK Built in variables are OK , The back AWK Built in variables are generally unavailable , In other words, you can use Shell To deal with the , Use less AWK Handle , If you really need it in the future AWK Deeper programming , I will study it alone AWK.