Update rows by referring another file

Question

I have two comma separated files as mentioned below,

cat source.txt
A,1000
B,3000
C,2500
D,5000
E,4000
F,3000

cat test.txt
A,1000
C
D,5000
B
E

How can I update file test.txt by referring file source.txt, such that the expected output is as below,

cat test.txt
A,1000
C,2500
D,5000
B,3000
E,4000

Answer 1

Try:

$ awk -F, -v OFS=, 'FNR==NR{a[$1]=$2; next} NF==1{$2=a[$1]} 1' source.txt test.txt 
A,1000
C,2500
D,5000
B,3000
E,4000

How it works:

1. -F, -v OFS=,

   This sets the input and output field separator to a comma.

2. FNR==NR{a[$1]=$2; next}

   When reading the first file, source.txt, save the value of the second field, $2, in associative array a under the key of the first field, $1. Then skip the rest of the commands and jump to start over on the next line.

3. NF==1{$2=a[$1]}

   If the current line has only one field, assign the second field to the value in associative array a.

4. 1

   This is short-hand for print the line.

Making permanent changes to the file

With any version of awk, we can make a permanent update to file test.txt with (recommended):

awk -F, -v OFS=, 'FNR==NR{a[$1]=$2; next} NF==1{$2=a[$1]} 1' source.txt test.txt >temp && mv temp test.txt

Alternatively, if one has a recent version of GNU awk (sometimes called gawk):

awk -i inplace -F, -v OFS=, 'FNR==NR{a[$1]=$2; print; next} NF==1{$2=a[$1]} 1' source.txt test.txt

Answer 2

I have done by below method and it worked fine

command:

for i in `awk -F "," '{print $1}' file2`; do k=`awk -F "," -v i="$i" '$1 == i {print $2}' file1`; echo $i;echo $k;done| sed "N;s/\n/,/g"

output

A,1000
C,2500
D,5000
B,3000
E,4000