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Full Discussion: Match text to lines in a file, iterate backwards until text or text substring matches, print to file
Top Forums Shell Programming and Scripting Match text to lines in a file, iterate backwards until text or text substring matches, print to file Post 303038582 by MadeInGermany on Friday 6th of September 2019 01:36:12 PM
Old 09-06-2019
A shell script that works as I described:
Code:
#!/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

The same idea in awk (file2 is read into an array variable first):
Code:
#!/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

In awk your propsed way can be implemented with no big overhead:
Code:
#!/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

 

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LEARN ABOUT BSD

join

JOIN(1) 						      General Commands Manual							   JOIN(1)

NAME

       join - relational database operator

SYNOPSIS

       join [ options ] file1 file2

DESCRIPTION

       Join  forms,  on the standard output, a join of the two relations specified by the lines of file1 and file2.  If file1 is `-', the standard
       input is used.

       File1 and file2 must be sorted in increasing ASCII collating sequence on the fields on which they are to be joined, normally the  first	in
       each line.

       There  is  one line in the output for each pair of lines in file1 and file2 that have identical join fields.  The output line normally con-
       sists of the common field, then the rest of the line from file1, then the rest of the line from file2.

       Fields are normally separated by blank, tab or newline.	In this case, multiple separators count as one, and leading  separators  are  dis-
       carded.

       These options are recognized:

       -an    In addition to the normal output, produce a line for each unpairable line in file n, where n is 1 or 2.

       -e s   Replace empty output fields by string s.

       -jn m  Join on the mth field of file n.	If n is missing, use the mth field in each file.

       -o list
	      Each output line comprises the fields specified in list, each element of which has the form n.m, where n is a file number and m is a
	      field number.

       -tc    Use character c as a separator (tab character).  Every appearance of c in a line is significant.

SEE ALSO

       sort(1), comm(1), awk(1)

BUGS

       With default field separation, the collating sequence is that of sort -b; with -t, the sequence is that of a plain sort.

       The conventions of join, sort, comm, uniq, look and awk(1) are wildly incongruous.

7th Edition							  April 29, 1985							   JOIN(1)
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