Shell Programming and Scripting

file1.txt
abc12345
def12345
ghi54321
...

file2.txt
abc1,text1,texta
abc,text2,textb
def123,text3,textc
gh,text4,textd
...

file3.txt
abc12345,abc1,text1,texta 
abc12345,abc,text2,textb
def12345,def123,text3,textc
ghi154321,gh,text4,textd
...

#!/bin/sh
# read from f1, print in this order
while read f1line
do
  # read from f2, find matches => print
  while IFS="," read f2col1 f2othercols
  do
    case $f1line in
    ("$f2col1"*)
      echo "$f1line,$f2col1,$f2othercols"
    esac
  done < file2.txt
done < file1.txt

#!/bin/sh
awk -F"," '
{
  if (NR==FNR) {
  # read from f2 into associative array col1[]
    col1[$1]=($2 FS $3)
  } else {
  # read from f1, find matches => print
    for (c in col1)
      if (c == substr($0,1,length(c)))
        print $0 FS c FS col1[c]
  }  
}
' file2.txt file1.txt

#!/bin/sh
awk -F"," '
{
  if (NR==FNR) {
  # read from f2 into associative array col1[]
    col1[$1]=($2 FS $3)
  } else {
  # read from f1, find matches => print
  for (i=length; i>=1; i--)
    if ((c=substr($0,1,i)) in col1)
      print $0 FS c FS col1[c]
  }  
}
' file2.txt file1.txt

awk -F, 'FNR==NR {T[$0]; next} {for (t in T) if (t ~ $1) print t, $0}' OFS=, file[12]
abc12345,abc1,text1,texta
abc12345,abc,text2,textb
def12345,def123,text3,textc
ghi54321,gh,text4,textd