Replace lines matching a pattern with lines from another file in order

awkfilesperlsedtext processing

I want to replace the lines matching a pattern from one file from the lines in order from another file, for example, given:

file1.txt:

aaaaaa
bbbbbb
!! 1234
!! 4567
ccccc
ddddd
!! 1111

we like to replace the lines starting with !! with the lines of this file:

file2.txt:

first line
second line
third line

so the result should be:

aaaaaa
bbbbbb
first line
second line
ccccc
ddddd
third line

Best Answer

Easy can be done with awk

awk '
    /^!!/{                    #for line stared with `!!`
        getline <"file2.txt"  #read 1 line from outer file into $0 
    }
    1                         #alias for `print $0`
    ' file1.txt

Other version

awk '
    NR == FNR{         #for lines in first file
        S[NR] = $0     #put line in array `S` with row number as index 
        next           #starts script from the beginning
    }
    /^!!/{             #for line stared with `!!`
        $0=S[++count]  #replace line by corresponded array element
    }
    1                  #alias for `print $0`
    ' file2.txt file1.txt
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