Ubuntu – Replace second instance of string in a line in an ASCII file using Bash

Here are a few approaches. I am using brace expansion (file{1..4}.txt) which means file1.txt file2.txt file3.txt file4.txt

1. Perl

   perl -i -pe 's/.*/ Good Morning / if $.==5' file{1..4}.txt
   

   Explanation:

   • -i: causes perl to edit the files in place, changing the original file.

     If -i is followed with a file extension suffix, then a backup is created for every file that is modified. Ex: -i.bak creates a file1.txt.bak if file1.txt is modified during the execution.

   • -p: means read the input file line by line, apply the script and print it.

   • -e: allows you to pass a script from the command line.

   • s/.*/ Good Morning /: That will replace the text in the current line (.*) with Good Morning.

   • $. is a special Perl variable that holds the current line number of the input file. So, s/foo/bar/ if $.==5, means replace foo with bar only on the 5th line.

2. sed

   sed -i '5s/.*/ Good Morning /' file{1..4}.txt
   

   Explanation:

   • -i: Like for perl, edit file in place.

   By default, sed prints each line of the input file. The 5s/pattern/replacement/ means substitute pattern with replacement on the 5th line.

3. Awk

   for f in file{1..4}.txt; do 
       awk 'NR==5{$0=" Good Morning "}1;' "$f" > foobar && mv foobar "$f"; 
   done
   

   Explanation:

   awk has no equivalent to the -i option¹ which means that we need to create a temporary file (foobar) which is then renamed to overwrite the original. The bash loop for f in file{1..4}.txt; do ... ; done simply goes through each of file{1..4}.txt, saving the current file name as $f. In awk, NR is the current line number and $0 is the content of the current line. So, the script will replace the line ($0) with " Good Morning " only on the 5th line. 1; is awk for "print the line".

   ¹_{Newer versions do as devnull showed in his answer.}

4. coreutils

   for f in file{1..4}.txt; do 
       (head -4 "$f"; echo " Good Morning "; tail -n +6 "$f") > foobar && 
       mv foobar "$f"; 
   done 
   

   Explanation:

   The loop is explained in the previous section.

   • head -4: print the first 4 lines

   • echo " Good Morning ": print " Good Morning "

   • tail -n +6: print everything from the 6th line to the end of the file

   The parentheses ( ) around those three commands allow you to capture the output of all three (so, 1st 4 lines, then " Good morning ", then the rest of the lines) and redirect them to a file.

Try this command:

 grep -v -f file2.csv file1.csv > file3.csv

According to grep manual:

  -f FILE, --file=FILE
          Obtain  patterns  from  FILE,  one  per  line.   The  empty file
          contains zero patterns, and therefore matches nothing.   (-f  is
          specified by POSIX.)

  -v, --invert-match
          Invert the sense of matching, to select non-matching lines.  (-v
          is specified by POSIX.)

As Steeldriver said in his comment is better add also -x and -F that:

  -F, --fixed-strings
          Interpret PATTERN as a  list  of  fixed  strings,  separated  by
          newlines,  any  of  which is to be matched.  (-F is specified by
          POSIX.)
  -x, --line-regexp
          Select  only  those  matches  that exactly match the whole line.
          (-x is specified by POSIX.)

So, better command is:

 grep -xvFf file2.csv file1.csv > file3.csv

This command use file2.csv line as pattern and print line of file1.csv that doesn't match (-v).