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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 ULTRIX

diff3

diff3(1)						      General Commands Manual							  diff3(1)

Name
       diff3 - 3-way differential file comparison

Syntax
       diff3 [-ex3] file1 file2 file3

Description
       The command compares three versions of a file, and publishes the ranges of text that disagree, flagged with the following codes:

	  ====	      all three files differ

	  ====1       file1 is different

	  ====2       file2 is different

	  ====3       file3 is different

       The type of change needed to convert a given range of a given file to some other is indicated in one of these ways:

	  f : n1 a    Text is to be appended after line number n1 in file f, where f = 1, 2, or 3.

	  f : n1 , n2 c
		      Text is to be changed in the range line n1 to line n2.  If n1 = n2, the range may be abbreviated to n1.

       The original contents of the range follows immediately after a c indication.  When the contents of two files are identical, the contents of
       the lower-numbered file is suppressed.

Options
       -3   Produces an editor script containing the changes between file1 and file2 that are to be incorporated into file3.

       -e	   Produces an editor script containing the changes between file2 and file3 that are to be incorporated into file1.

       -x	   Produces an editor script containing the changes among all three files.

Examples
       Under the -e option, publishes a script for the editor that incorporates into file1 all changes between file2 and  file3  -  that  is,  the
       changes	that would normally be flagged ==== and ====3.	Option -x (-3) produces a script to incorporate only changes flagged ==== (====3).
       The following command applies the resulting script to `file1':
       (cat script; echo '1,$p') | ed - file1

Restrictions
       Text lines that consist of a single `.'	defeat -e.

Files
       /tmp/d3?????
       /usr/lib/diff3

See Also
       cmp(1), comm(1), diff(1), dffmk(1), join(1), sccsdiff(1), uniq(1)

																	  diff3(1)
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