mjhjmtu

Source file example:
<HDR></HDR><b></b><c></c>

(XML file created in a single line)

OR

Source file example:
<HDR>
</HDR>
<b>
</b>
<c>
</c>

I need to remove all the content of the file before <b> in both of the source format.
I tried using the below method

sed 's/^.*b/b/'

But this is not replacing it. Please let me know if there is an alternative way.

Assuming your XML document is well formed, like

<document>
<HDR>
</HDR>
<b>
</b>
<c>
</c>
</document>

Then you may use XMLStarlet to remove all HDR tags like so:

xmlstarlet ed -d '//HDR' file.xml >newfile.xml

To only remove the HDR tags that are immediately followed by a b tag:

xmlstarlet ed -d '//HDR[following-sibling::*[1][name() = "b"]]' file.xml >newfile.xml

XMLStarlet may also be used to modify the contents of tags:

$ xmlstarlet ed -u '//HDR[following-sibling::*[1][name() = "b"]]' -v 'New header value' file.xml
<?xml version="1.0"?>
<document>
 <HDR>New header value</HDR>
 <b/>
 <c/>
</document>

$ xmlstarlet ed -i '//HDR[following-sibling::*[1][name() = "b"]]' -t attr -n 'new_attribute' -v 'hello' file.xml
<?xml version="1.0"?>
<document>
 <HDR new_attribute="hello"/>
 <b/>
 <c/>
</document>

Type 1:

 echo "<HDR></HDR><b></b><c></c>" | sed 's/^.*<b>/<b>/' 
 <b></b><c></c>

• will replace everything up to <b> with <b>
  

Type 2:

sed -n '/<b>/,$p' file
<b>
</b>
<c>
</c>

• will print the first occurrence of <b> to end of the file ($).

Question:

remove all contents of the file before <b>

Answer:

perl -0777 -lape 's/^.*<b>/<b>/s'

Test run:

==> in1.txt <==
<HDR></HDR><b></b><c></c>

==> in2.txt <==
<HDR>
</HDR>
<b>
</b>
<c>
</c>

$ perl -i -0777 -lape 's/^.*<b>/<b>/s' in1,2.txt

==> in1.txt <==
<b></b><c></c>

==> in2.txt <==
<b>
</b>
<c>
</c>