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##Sed solution## #!/bin/bash

sed -nr '
    /^<[^<]*>$/ {
        N
        /^<([^<]*)>\n<\/\1>$/=
    }
' "$1" | awk '{print "Output: "$NF - 1" -> line number"}'

###Explanation:###

1. sed
   • /^<[^<]*>$/ if we are have one open tag in the line
   • N - append the next line of input into the pattern space.
   • /^<([^<]*)>\n<\/\1>$/ and check, does the next line have the equivalent closed tag.
   • if so, print this line number by = command. Bear in mind, that it is the closed tag line number. We should decrease it by one in further.
2. awk - decreases the line number and print it in the message string.

###Testing:### Input

<a>
</a>
<a>
<b></b>
<c></c>
<c>
</c>
</a>

Output

./empty_tag.sh input.txt 
Output: 1 -> line number
Output: 6 -> line number

##AWK solution## Usage: ./empty_tag.sh input.txt

#!/bin/bash

awk -F'[>/]' '
    line_num {
        if(NF == 3) {print "Output: " line_num " -> line number";}
        line_num = 0;
    }
    NF == 2 {line_num = NR;}
' "$1"

Sed solution

#!/bin/bash

sed -nr '
    /^<[^<]*>$/ {
        N
        /^<([^<]*)>\n<\/\1>$/=
    }
' "$1" | awk '{print "Output: "$NF - 1" -> line number"}'

Explanation:

1. sed
   • /^<[^<]*>$/ if we are have one open tag in the line
   • N - append the next line of input into the pattern space.
   • /^<([^<]*)>\n<\/\1>$/ and check, does the next line have the equivalent closed tag.
   • if so, print this line number by = command. Bear in mind, that it is the closed tag line number. We should decrease it by one in further.
2. awk - decreases the line number and print it in the message string.

Testing:

Input

<a>
</a>
<a>
<b></b>
<c></c>
<c>
</c>
</a>

Output

./empty_tag.sh input.txt 
Output: 1 -> line number
Output: 6 -> line number

AWK solution

Usage: ./empty_tag.sh input.txt

#!/bin/bash

awk -F'[>/]' '
    line_num {
        if(NF == 3) {print "Output: " line_num " -> line number";}
        line_num = 0;
    }
    NF == 2 {line_num = NR;}
' "$1"

##Sed solution## #!/bin/bash

sed -nr '
    /^<[^<]*>$/ {
        N
        /^<([^<]*)>\n<\/\1>$/=
    }
' "$1" | awk '{print "Output: "$NF - 1" -> line number"}'

###Explanation:###

1. sed
   • /^<[^<]*>$/ if we are have one open tag in the line
   • N - append the next line of input into the pattern space.
   • /^<([^<]*)>\n<\/\1>$/ and check, does the next line have the equivalent closed tag.
   • if so, print this line number by = command. Bear in mind, that it is the closed tag line number. We should decrease it by one in further.
2. awk - decreases the line number and print it in the message string.

###Testing:### Input

<a>
</a>
<a>
<b></b>
<c></c>
<c>
</c>
</a>

Output

./empty_tag.sh input.txt 
Output: 1 -> line number
Output: 6 -> line number

##AWK solution##

#!/bin/bash

awk -F'[>/]' '
    line_num {
        if(NF == 3) {print "Output: " line_num " -> line number"}
        line_num = 0;
    }
    NF == 2 {line_num = NR;}
' "$1"

##Sed solution## #!/bin/bash

sed -nr '
    /^<[^<]*>$/ {
        N
        /^<([^<]*)>\n<\/\1>$/=
    }
' "$1" | awk '{print "Output: "$NF - 1" -> line number"}'

###Explanation:###

1. sed
   • /^<[^<]*>$/ if we are have one open tag in the line
   • N - append the next line of input into the pattern space.
   • /^<([^<]*)>\n<\/\1>$/ and check, does the next line have the equivalent closed tag.
   • if so, print this line number by = command. Bear in mind, that it is the closed tag line number. We should decrease it by one in further.
2. awk - decreases the line number and print it in the message string.

###Testing:### Input

<a>
</a>
<a>
<b></b>
<c></c>
<c>
</c>
</a>

Output

./empty_tag.sh input.txt 
Output: 1 -> line number
Output: 6 -> line number

##AWK solution## Usage: ./empty_tag.sh input.txt

#!/bin/bash

awk -F'[>/]' '
    line_num {
        if(NF == 3) {print "Output: " line_num " -> line number";}
        line_num = 0;
    }
    NF == 2 {line_num = NR;}
' "$1"

#!/bin/bash

sed -nr '
    /^<[^<]*>$/ {
        N
        /^<([^<]*)>\n<\/\1>$/=
    }
' "$1" | awk '{print "Output: "$NF - 1" -> line number"}'

###Explanation:###

1. sed
   • /^<[^<]*>$/ if we are have one open tag in the line
   • N - append the next line of input into the pattern space.
   • /^<([^<]*)>\n<\/\1>$/ and check, does the next line have the equivalent closed tag.
   • if so, print this line number by = command. Bear in mind, that it is the closed tag line number. We should decrease it by one in further.
2. awk - decreases the line number and print it in the message string.

###Testing:### Input

<a>
</a>
<a>
<b></b>
<c></c>
<c>
</c>
</a>

Output

./empty_tag.sh input.txt 
Output: 1 -> line number
Output: 6 -> line number

##Sed solution## #!/bin/bash

sed -nr '
    /^<[^<]*>$/ {
        N
        /^<([^<]*)>\n<\/\1>$/=
    }
' "$1" | awk '{print "Output: "$NF - 1" -> line number"}'

###Explanation:###

1. sed
   • /^<[^<]*>$/ if we are have one open tag in the line
   • N - append the next line of input into the pattern space.
   • /^<([^<]*)>\n<\/\1>$/ and check, does the next line have the equivalent closed tag.
   • if so, print this line number by = command. Bear in mind, that it is the closed tag line number. We should decrease it by one in further.
2. awk - decreases the line number and print it in the message string.

###Testing:### Input

<a>
</a>
<a>
<b></b>
<c></c>
<c>
</c>
</a>

Output

./empty_tag.sh input.txt 
Output: 1 -> line number
Output: 6 -> line number

##AWK solution##

#!/bin/bash

awk -F'[>/]' '
    line_num {
        if(NF == 3) {print "Output: " line_num " -> line number"}
        line_num = 0;
    }
    NF == 2 {line_num = NR;}
' "$1"