Why awk if conditional matching is wrong - shell

Question

In my project, I have two files.
The content userid is :
6534
4524
4522
6635
The content userpwinfo.txt is:
nsgg315_RJ:x:4520:100::/home-gg/users/nsgg315_RJ:/bin/bash
nsgg316_ZJY:x:4521:100::/home-gg/users/nsgg316_ZJY:/bin/bash
nsgg317_CPA:x:4522:100::/home-gg/users/nsgg317_CPA:/bin/bash
nsgg318_ZRL:x:4523:100::/home-gg/users/nsgg318_ZRL:/bin/bash
nsgg319_YYM:x:4524:100::/home-gg/users/nsgg319_YYM:/bin/bash
Now I want to print the username which id is in userid. I writed a bash shell like:
for i in $(cat userid)
do
#username=`awk -F: '{if($3=="$i") print $1}' /root/userpwinfo.txt`
#username=`awk -F: '$3=="$i" {print $1}' /root/userpwinfo.txt`
#username=`awk -F: '{if($3~/$i/) print $1}' /root/userpwinfo.txt`
username=`awk -F: '{if($3==$i) print $1}' /root/userpwinfo.txt`
echo $username
done
But unlucky, it shows nothing. The correct result should be:
nsgg319_YYM
nsgg317_CPA
I have tried in command line:
awk -F: '{if($3==4524) print $1}' /root/userpwinfo.txt
It is OK
Maybe if($3==$i) is wrong in shell, Who can help me?

Answer 1

Your $i is the shell variable, but it's inside the quotation mark ' so awk will try to interpret it instead of the shell.
Try this:
username=`awk -F: '{if($3=='$i') print $1}' /root/userpwinfo.txt`
Note that the $i is between ' marks, meaning it's outside of the block that will be interpreted by awk, meaning it should be interpreted by the shell.
Also note that if you have an empty line in the input file, your awk command would be if($3==) which is invalid and will yield an error.
I'd like to comment also that awk is meant to have a filter and an execution block. You shouldn't need to write an if inside a block, unless you want something unusual. Meaning, your command would be more appropriately written as:
username=`awk -F: '($3=='$i'){print $1}' /root/userpwinfo.txt`
Note that even this is not a very good solution, but you already have much to think about with only these changes. When you're more familiar with awk or getting more professional, come back and check the comments. ;)

Answer 2

If username is what you needed using the 2 files, you could try
$ cat userpwinfo.txt
nsgg315_RJ:x:4520:100::/home-gg/users/nsgg315_RJ:/bin/bash
nsgg316_ZJY:x:4521:100::/home-gg/users/nsgg316_ZJY:/bin/bash
nsgg317_CPA:x:4522:100::/home-gg/users/nsgg317_CPA:/bin/bash
nsgg318_ZRL:x:4523:100::/home-gg/users/nsgg318_ZRL:/bin/bash
nsgg319_YYM:x:4524:100::/home-gg/users/nsgg319_YYM:/bin/bash
$ cat userid.txt
6534
4524
4522
6635
$ awk -F":" ' { if( NR==FNR ) { a[$3]=$1; next } ; if(a[$1]) print a[$1] }' userpwinfo.txt userid.txt
nsgg319_YYM
nsgg317_CPA