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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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DIFF(1) 						      General Commands Manual							   DIFF(1)

NAME
diff - print differences between two files SYNOPSIS
diff [-c | -e | -C n] [-br]file1 file2 OPTIONS
-C n Produce output that contains n lines of context -b Ignore white space when comparing -c Produce output that contains three lines of context -e Produce an ed-script to convert file1 into file2 -r Apply diff recursively to files and directories of EXAMPLES
diff file1 file2 # Print differences between 2 files diff -C 0 file1 file2 # Same as above diff -C 3 file1 file2 # Output three lines of context with every diff -c file1 file2 # Same diff /etc /dev # Compares recursively the directories /etc and /dev diff passwd /etc # Compares ./passwd to /etc/passwd DESCRIPTION
the same name, when file1 and file2 are both directories" difference encountered" Diff compares two files and generates a list of lines telling how the two files differ. Lines may not be longer than 128 characters. If the two arguments on the command line are both directories, diff recursively steps through all subdirectories comparing files of the same name. If a file name is found only in one directory, a diagnostic message is written to stdout. A file that is of either block special, character special or FIFO special type, cannot be compared to any other file. On the other hand, if there is one directory and one file given on the command line, diff tries to compare the file with the same name as file in the directory directory. SEE ALSO
cdiff(1), cmp(1), comm(1), patch(1). DIFF(1)
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