Awk array usage

Two files. Find the line equal to file a from the first column of file B and Output
[[Email protected] ~] # Cat
1
3
6
7
[[Email protected] ~] # Cat B
1 2 3
2 3 4
3 4 5
4 5 6
5 6 7
6 7 8
7 8 9

Shell script
[[Email protected] ~] # Cat jieguo
#! /Bin/bash
Y = 'sed-n' P' A | WC-l' # number of a rows
X = 'sed-n' P' B | WC-l' # Number of B rows
For (I = 1; I <= $ Y; I ++ ))
Do
Value_a = 'sed-N $ {I} p a' # output by row, assigned to
For (j = 1; j <= $ X; j ++ ))
Do
Value_ B = 'awk' {print $1} 'B | sed-n "/$ J/P"' # first output column, assigned to B
If ["$ value_a" = "$ value_ B"] # judge whether value_a and value_ B are equal
Then
Value_c = 'sed-N $ {J} p B'
Echo $ value_c>/root/value_c # equal output of rows in B to value_c
Fi
Done
Done

Awk command line
Awk 'nr = FNR {k [$1] = $1} Nr> FNR {If (K [$1] = $1) {print $1, $2, $3} 'a B or
Awk 'nr = FNR {k [$1] = $1} Nr> FNR {If (K [$1] = $1) {print $0} 'a B
Nr: Total number of lines for awk to process files
FNR: number of lines that awk processes the current file
Array K [$1]: pay the value of file a to array K.

Condition Statement: Nr = FNR: execute the operation in {} on file.
NR> FNR: perform operations in {} on file B
K [$1] = $1 is the first domain in file.
If (K [$1] ==1 1) {print $1, $2, $3} is the 1, 2, and 3 fields in file B.
$0 is the current complete record.

Differences:

Script: nesting for is required to traverse the entire file. That is to say, to compare whether the first line of A and B is the same, each number in a needs to traverse the first line of data in B cyclically. X Y is required.

Awk: array K [$1] can be understood as converting the value of file a from a column to a line. In this way, when awk makes the if judgment, it is to find the target in a row of data, the for loop statement is not required to traverse the entire file like a column. It is to compare the data from the first line of B with the K array. If the data is the same, it is output. You only need to compare X times.

Running result:
[[Email protected] ~] #./Jieguo
1 2 3
3 4 5
6 7 8
7 8 9
[[Email protected] ~] # Awk 'nr = FNR {k [$1] = $1} Nr> FNR {If (K [$1] = $1) {print $0} 'a B
1 2 3
3 4 5
6 7 8
7 8 9

If the number of lines in a file is tens of thousands or hundreds of thousands, the script takes a lot of time. Awk second output result...

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