Finding a pattern, then executing a line change only after the pattern-CodePudding

I have a file of the like:

H 1 2 3 4
H 1 2 3 4
C 1 2 3 4
 $END

 $EFRAG
COORD=CART
FRAGNAME=H2ODFT
O 1 2 3 4
H 1 2 3 4
H 1 2 3 4
FRAGNAME=H2ODFT
O 1 2 3 4
H 1 2 3 4
H 1 2 3 4

I want to remove the column "1" from the lines only after the $EFRAG line. and add a label to the O H H as well. My expected output is:

H 1 2 3 4
H 1 2 3 4
C 1 2 3 4
 $END

 $EFRAG
COORD=CART
FRAGNAME=H2ODFT
Oa  2 3 4
Hb  2 3 4
Hc  2 3 4
FRAGNAME=H2ODFT
Oa  2 3 4
Hb  2 3 4
Hc  2 3 4

I'm new to coding in bash, and I'm not quite sure where to start.

I was thinking of piping a grep command to a sed command, but I'm not sure how that syntax would look. Am also trying to learn awk, but that syntax is even more confusing to me. Currently trying to read a book on it's capabilities.

Any thoughts or ideas would be greatly appreciated!

CodePudding user response：

Use the following awk processing:

awk '$0~/\$EFRAG/ {
    start = 1;  # marker denoting the needed block
    split("a b c", suf);  # auxiliary array of suffixes
}
start {
    if (/^FRAGNAME/) idx = 1;  # encountering subblock
    if (/^[OH]/) {  # if starting with O or H char
        $2 = "";
        $1 = $1""suf[idx  ];
    }
}1' test.txt

H 1 2 3 4
H 1 2 3 4
C 1 2 3 4
 $END

 $EFRAG
COORD=CART
FRAGNAME=H2ODFT
Oa  2 3 4
Hb  2 3 4
Hc  2 3 4
FRAGNAME=H2ODFT
Oa  2 3 4
Hb  2 3 4
Hc  2 3 4