Shell Programming and Scripting

awk 'FNR==NR{A[$1]=$1;next} ($1 in A){print $0 FS A[$1]} !($1 in A){print $0}' file1 FS=, file2

abc,123,abc
mno,456
def,989,def
pqr,787,pqr
ghj,678

awk 'NR==FNR {T[$1]=FS $1; next} {print $0 T[$1]}' FS="," file1 file2
abc,123,abc
mno,456
def,989,def
pqr,787,pqr
ghj,678

awk 'NR==FNR {T[$1]=FS $1; next}

{print $0 T[$1]}

awk 'NR==FNR          ####### This condition will be TRUE when first file named file is being read, because FNR attribute will be RESET when new file will be read and NR will be keep on increasing it's count till last file read successfully.
{T[$1]=FS $1; next}   ####### Creating an array named T whose index is $1 and value is FS(Field separator  which is , here mentioned later in code), after doing this next means leave all coming statements, NOT to execute.
{print $0 T[$1]}'     ####### This will be executed when 2nd file is being read because when NR==FNR will NOT be TRUE then control will come here and execute it, to print the complete line by $0 and print array T's value whose index is $1.
FS="," file1 file2    ####### mentioning Field separator here which is comma and mentioning Input_file which are file1 and file2.