Shell Programming and Scripting

<nvname>abc</nvname>
<nvvalue>1</nvvalue>
<nvname>def</nvname>
<nvvalue>2</nvvalue>

<nvname>abc</nvname>
<nvvalue>4</nvvalue>
<nvname>def</nvname>
<nvvalue>2</nvvalue>

awk '/>abc</{f=1}f&&!/>abc</{sub(/>.*</,4);f=0}1' inputfile

VAR="abc"

awk -v V="$VAR" '                                       # Assign V = "$VAR"
                {
                        if ( $0 ~ ">" V "<" )           # If pattern ">" V "<" (>abc<) in current record.
                                f = 1                   # If found, set f = 1
                        if ( f && $0 !~ ">" V "<" )     # If f == 1 and pattern ">" V "<" (>abc<) not in current record.
                        {
                                sub(/>.*</, ">4<")      # Substitute everything with in >.*< by ">4<"
                                f = 0                   # Reset f = 0
                        }
} 1 ' file

sed s/$original/$new/g

new=<a>123</a>\n<b>345</b>

<a>123</a><b>345</b>

$ awk '/>123</{sub(/></,">\n<")}1' file
<a>123</a>
<b>345</b>