Shell – Replace nth Line from the Matched Pattern

You probably want the -wflag - from man grep

   -w, --word-regexp
          Select  only  those  lines  containing  matches  that form whole
          words.  The test is that the matching substring must  either  be
          at  the  beginning  of  the  line,  or  preceded  by  a non-word
          constituent character.  Similarly, it must be either at the  end
          of  the  line  or  followed by a non-word constituent character.
          Word-constituent  characters  are  letters,  digits,   and   the
          underscore.

i.e.

grep -wFf patfile file
denovo1 xxx yyyy oggugu ddddd
denovo22 hhhh yyyy kkkk iiii

To enforce matching only in the first column, you would need to modify the entries in the pattern file to add a line anchor: you could also make use of the \b word anchor instead of the command-line -w switch e.g. in patfile:

^denovo1\b
^denovo3\b
^denovo22\b

then

grep -f patfile file
denovo1 xxx yyyy oggugu ddddd
denovo22 hhhh yyyy kkkk iiii

Note that you must drop the -F switch if the file contains regular expressions instead of simple fixed strings.

If your files are not too large to fit in memory, you could use perl to slurp the file:

perl -0777pe 's/.*?PAT[^\n]*\n?//s' file

Just change PAT to whatever pattern you're after. For example, given these two input files and the pattern 5:

$ cat file
1
2
3
4
5
11
12
13
14
15
$ cat file1 
foo
bar
$ perl -0777pe 's/.*?5[^\n]*\n?//s' file
11
12
13
14
15
$ perl -0777pe 's/.*?10[^\n]*\n?//s' file1
foo
bar

Explanation

• -pe : read the input file line by line, apply the script given by -e to each line and print.
• -0777 : slurp the entire file into memory.
• s/.*?PAT[^\n]*\n?//s : remove everything until the 1st occurrence of PAT and until the end of the line.

For larger files, I don't see any way to avoid reading the file twice. Something like:

awk -vpat=5 '{
              if(NR==FNR){
                if($0~pat && !a){a++; next} 
                if(a){print}
              }
              else{ 
                if(!a){print}
                else{exit} 
              }
             }' file1 file1

Explanation

• awk -vpat=5 : run awk and set the variable pat to 5.
• if(NR==FNR){} : if this is the 1st file.
• if($0~pat && !a){a++; next} : if this line matches the value of pat and a is not defined, increment a by one and skip to the next line.
• if(a){print} : if a is defined (if this file matches the pattern), print the line.
• else{ } : if this is not the 1st file (so it's the second pass).
• if(!a){print} if a is not defined, we want the whole file, so print every line.
• else{exit} : if a is defined, we've already printed in the 1st pass so there's no need to reprocess the file.