使用 UNIX/Linux 操作（排除行）a.csv 文件的特定列

Question

I want to access and manipulate the fourth column of a csv file.In particular I want to exclude the lines that don't meet a specific requirement ( exclude the lines that do not have 3 character country code).

My data set:

Luxembourg,LUX,2017,9294689.12
Aruba,ABW,2017,927865.82
Nepal,NPL,2017,9028196.37
Bangladesh,BGD,2017,88057460.51
Costa Rica,CRI,2017,8695008.05
Chile,CHL,2017,84603249.72
Cook Islands,COK,2017,82045.41
World,OWIDWRL,1755,9361520
India,INDIA,1763,0
Asia and Pacific (other),,2017,5071156099
World,OWID_WRL,1752,9354192
Middle East,,1751,0
International transport,,1751,0
India,IND,1751,0
Europe (other),,1751,0
China,CHN,1751,0
Asia and Pacific (other),,1751,0
Americas (other),,1751,0
Africa,,1751,0

Thanks in advance.

I have already sorted my data file by year, but I don't know how to access the 4th column and use awk or sed.

Expected Data set:

Luxembourg,LUX,2017,9294689.12
Aruba,ABW,2017,927865.82
Nepal,NPL,2017,9028196.37
Bangladesh,BGD,2017,88057460.51
Costa Rica,CRI,2017,8695008.05
Chile,CHL,2017,84603249.72
Cook Islands,COK,2017,82045.41

Answer 1

If I got your question correctly, could you please try following. Where code looks if 2nd field of any line is NOT having exact 3 characters in it then DO NOT print that line.

awk 'BEGIN{FS=","} $2~/^[a-zA-Z]{3}$/' Input_file

In case you have OLD awk where range {3} doesn't work the try.

awk 'BEGIN{FS=","} $2~/^[a-zA-Z][a-zA-Z][a-zA-Z]$/' Input_file

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Explanation: Adding explanation for above code here.

awk '                  ##Starting awk program here.
BEGIN{                 ##Starting BEGIN section from here. Which will be executed before Input_file is being read
  FS=","               ##Setting field separator as comma here.
}                      ##Closing BEGIN section here.
$2~/^[a-zA-Z]{3}$/     ##Checking condition if 2nd field is starting with alphabets 3 occurrence of it and ending with it too.
                       ##Since awk works on method of condition then action; so if condition is TRUE then perform certain action.
                       ##In this case no action given so  by default print of line will happen.
' Input_file           ##Mentioning Input_file name here.

Answer 2

The below would output only lines with a 3 letter value in the second field:

awk --re-interval -F, 'tolower($2) ~ /^[a-z]{3}$/' country.txt

Checking the length is also possible, but this ensures only 3 letters are provided.

--re-internval allows you to use itnernval expressions in RE's as braces are reserved characters in awk.

-F, tells awk the input delimiter is comma.

print is the default action in awk, so tolower($2) ~ /^[az]{3}$/ is a shorthand way of saying tolower($2) ~ /^[az]{3}$/ {print}

The tolower($2) is lowercasing the value of the second field, and ~ is the regex comparison operator, which we use to check for the beginning of the string ^ , then [az] repeated {3} times and the end of the string $ .