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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 DEBIAN

column

COLUMN(1)						    BSD General Commands Manual 						 COLUMN(1)

NAME

     column -- columnate lists

SYNOPSIS

     column [-entx] [-c columns] [-s sep] [file ...]

DESCRIPTION

     The column utility formats its input into multiple columns.  Rows are filled before columns.  Input is taken from file operands, or, by
     default, from the standard input.	Empty lines are ignored unless the -e option is used.

     The options are as follows:

     -c      Output is formatted for a display columns wide.

     -s      Specify a set of characters to be used to delimit columns for the -t option.

     -t      Determine the number of columns the input contains and create a table.  Columns are delimited with whitespace, by default, or with
	     the characters supplied using the -s option.  Useful for pretty-printing displays.

     -x      Fill columns before filling rows.

     -n      By default, the column command will merge multiple adjacent delimiters into a single delimiter when using the -t option; this option
	     disables that behavior. This option is a Debian GNU/Linux extension.

     -e      Do not ignore empty lines.

ENVIRONMENT

     The COLUMNS, LANG, LC_ALL and LC_CTYPE environment variables affect the execution of column as described in environ(7).

EXIT STATUS

     The column utility exits 0 on success, and >0 if an error occurs.

EXAMPLES

	   (printf "PERM LINKS OWNER GROUP SIZE MONTH DAY " ; 
	   printf "HH:MM/YEAR NAME
" ; 
	   ls -l | sed 1d) | column -t

SEE ALSO

     colrm(1), ls(1), paste(1), sort(1)

HISTORY

     The column command appeared in 4.3BSD-Reno.

BUGS

     Input lines are limited to LINE_MAX (2048) bytes in length.

BSD
								   July 29, 2004							       BSD
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