How to use sed replace matching string of line contents at fixed position

lwsunny · 07-26-2013, 06:41 AM

hi all, i need to loop each lines of a file, and look up if there is a matching word at specific position, eg. at position 6= ABC;
then replace the words at position 10 with DEF;
thanks for any help!!
e.g.
input_file.txt
123ABC789JKL123
123ABC789MNL123
123FGH789PQR123

output_file as:
output.txt
123ABC789DEF123
123ABC789DEF123
123FGH789PQR123

Ser Olmy · 07-26-2013, 07:16 AM

Basically, you search for the following pattern (pseudo-code):

<start of line>(<X random characters>)ABC(<y random characters>)<3 random characters>(<rest of line>)

The parts in parentheses must be stored for backreferencing. Now replace with:

(backref 1)ABC(backref 2)DEF(backref 3)

The following sed command will replace characters 10-12 with "DEF" in any string where characters 4-6 are "ABC":

Code:

sed 's/^\(.\{3\}\)ABC\(.\{3\}\).\{3\}\(.*\)/\1ABC\2DEF\3/'

lwsunny · 07-26-2013, 07:23 AM

thanks for quick reply!

i am new to linux script, how can i loop each line of the input file, then output the replace one to another file...

Ser Olmy · 07-26-2013, 07:35 AM

That's a very common question, and it has been answered here on LQ quite a few times.

Most would recommend using a while loop and input redirection:

Code:

while read x ; do
  [... process $x here and write the results to another file ...]
done < input_file

A quick search of the LQ forums should turn up a few examples.

anon237 · 07-26-2013, 08:03 AM

@lwsunny: These bash related links might come in handy:

danielbmartin · 07-31-2013, 08:50 PM

Quote:

Originally Posted by lwsunny

i am new to linux script, how can i loop each line of the input file, then output the replace one to another file...

No need for an explicit loop. You may convert an entire input file with a single line of code.

Using sed, as coded by Ser Olmy:

Code:

sed 's/^\(.\{3\}\)ABC\(.\{3\}\).\{3\}\(.*\)/\1ABC\2DEF\3/' $InFile >$OutFile

Using awk, as coded by yours truly:

Code:

awk 'BEGIN{FS=OFS=""} {if (substr($0,4,3)=="ABC") {$10="DEF";$11=$12=""}}1' $InFile >$OutFile

Daniel B. Martin