Insert a new line only before a first matching pattern

Question

My input file contains data as below

acb/xyz/row<t>
acb/xyz/row<t>
abc/xyz/row<b>
abc/xyz/row<b>
abc/xyz/row<0>
abc/xyz/row<0>
abc/xyz/row<1>
abc/xyz/row<1>
abc/xyz/row<2>
abc/xyz/row<2>
abc/xyz/row<3>
abc/xyz/row<3>
abc/xyz/row<4>
abc/xyz/row<4>

So I want the output to be as shown below:

#Sector Top
acb/xyz/row<t>
acb/xyz/row<t>
#Sector Bottom
abc/xyz/row<b>
abc/xyz/row<b>
#Sector 0
abc/xyz/row<0>
abc/xyz/row<0>
#Sector 1
abc/xyz/row<1>
abc/xyz/row<1>
#Sector 2
abc/xyz/row<2>
abc/xyz/row<2>
#Sector 3
abc/xyz/row<3>
abc/xyz/row<3>
#Sector 4
abc/xyz/row<4>
abc/xyz/row<4>

• Each sector shown above contains multiple lines, here I have shown only 2 lines per sector.
• There are top, bottom and from 0 to 30 sectors, here I have shown up to 4 sectors.

I tried for one of the sectors by using sed command as:

sed '/row<1>/i #Sector 1' myfile

which gives me output like:

#Sector 1
abc/xyz/row<1>
#Sector 1
abc/xyz/row<1>

I don't need a new line before every match, only needed before first match for all the sectors.

Answer 1

Using any awk:

$ cat tst.awk
BEGIN {
    FS = "[<>]"
    map["t"] = "Top"
    map["b"] = "Bottom"
}
{
    sector = $(NF-1)
    if ( sector != prev ) {
        print "#Sector", (sector in map ? map[sector] : sector)
        prev = sector
    }
    print
}

$ awk -f tst.awk file
#Sector Top
acb/xyz/row<t>
acb/xyz/row<t>
#Sector Bottom
abc/xyz/row<b>
abc/xyz/row<b>
#Sector 0
abc/xyz/row<0>
abc/xyz/row<0>
#Sector 1
abc/xyz/row<1>
abc/xyz/row<1>
#Sector 2
abc/xyz/row<2>
abc/xyz/row<2>
#Sector 3
abc/xyz/row<3>
abc/xyz/row<3>
#Sector 4
abc/xyz/row<4>
abc/xyz/row<4>

Answer 2

You can do this using the N;P;D cycle of sed and only insert the #Sector line if the row number did change:

sed -E 'N;P;/^(.*)\n\1/D;s/.*(\n.*)(row<)([0-9bt])*>/#Sector \3\1\2\3>/;P;D'

Now we need to add handling for the top and bottom section like this:

sed -E -e '1i #Sector top' -e 'N;P;/^(.*)\n\1/D;s/.*(\n.*)(row<)([0-9bt])*>/#Sector \3\1\2\3>/;s/#Sector b/&ottom/;P;D'

Explanation in detail:

• option -E is to make the output more readable. Instead, you could also add eight backslashes for the (…) sections
• 1i #Sector top simply adds the first sector header
• Now we always append the Next line to always process two lines together and Print the first one
• /^(.*)\n\1/ is an expression that the first line gets repeated, so there is no reason to insert a section header, so we Delete the first line to continue with the second
• Now some regular expression magic: s/.*(\n.*)(row<)([0-9bt])*>/#Sector \3\1\2\3>/ replaces the first line (already printed!) with the section header and the row number extracted in the third () pair
• Finally simply change the b to bottom with s/#Sector b/&ottom/
• and close the cycle by Printing the first line and Deleting it to continue with the second

Answer 3

Using GNU sed

$ sed -Ez 's/(\S+row<([^>]*)>)(.*\1)?/#Sector \2\n&/g;s/\<t\>/Top/;s/\<b\>/Bottom/' input_file
#Sector Top
acb/xyz/row<t>
acb/xyz/row<t>
#Sector Bottom
abc/xyz/row<b>
abc/xyz/row<b>
#Sector 0
abc/xyz/row<0>
abc/xyz/row<0>
#Sector 1
abc/xyz/row<1>
abc/xyz/row<1>
#Sector 2
abc/xyz/row<2>
abc/xyz/row<2>
#Sector 3
abc/xyz/row<3>
abc/xyz/row<3>
#Sector 4
abc/xyz/row<4>
abc/xyz/row<4>

Answer 4

You can do this using awk :

awk -F'[<>]' '!seen[$2]++{print "#Sector " $2} {print}' file

and if you need to replace t with Top and b with Bottom you can add some condition :

awk -F'[<>]' '{
    if(!seen[$2]++){
        if($2=="t"){
            print "#Sector Top"
        }else if($2=="b"){
            print "#Sector Bottom"
        }else{
            print "#Sector " $2
        }
    }
    print
}' file

In both case this suppose you don't have more of these < > in your line.

This also suppose each sector are sorted together and that you only have the lines you showed in your data file. If you have an unsorted file it won't insert a new line after the first match.

Answer 5

The below Script will work in a bash shell

#!/bin/bash
awk 'BEGIN{ FS="/" }
{a=$3;
    if(a != b)
        print "Sector " substr(a,5,1)"\n" a
    else
        print a;
    b=$3}' myfile.txt