How to use sed to extract substring

Question

I have a file containing the following lines:

  <parameter name="PortMappingEnabled" access="readWrite" type="xsd:boolean"></parameter>   <parameter name="PortMappingLeaseDuration" access="readWrite" activeNotify="canDeny" type="xsd:unsignedInt"></parameter>   <parameter name="RemoteHost" access="readWrite"></parameter>   <parameter name="ExternalPort" access="readWrite" type="xsd:unsignedInt"></parameter>   <parameter name="ExternalPortEndRange" access="readWrite" type="xsd:unsignedInt"></parameter>   <parameter name="InternalPort" access="readWrite" type="xsd:unsignedInt"></parameter>   <parameter name="PortMappingProtocol" access="readWrite"></parameter>   <parameter name="InternalClient" access="readWrite"></parameter>   <parameter name="PortMappingDescription" access="readWrite"></parameter> 

I want to execute command on this file to extract only the parameter names as displayed in the following output:

$sedcommand file.txt PortMappingEnabled PortMappingLeaseDuration RemoteHost ExternalPort ExternalPortEndRange InternalPort PortMappingProtocol InternalClient PortMappingDescription 

What could be this command?

Answer 1

grep was born to extract things:

grep -Po 'name="\K[^"]*' 

test with your data:

kent$  echo '<parameter name="PortMappingEnabled" access="readWrite" type="xsd:boolean"></parameter>   <parameter name="PortMappingLeaseDuration" access="readWrite" activeNotify="canDeny" type="xsd:unsignedInt"></parameter>   <parameter name="RemoteHost" access="readWrite"></parameter>   <parameter name="ExternalPort" access="readWrite" type="xsd:unsignedInt"></parameter>   <parameter name="ExternalPortEndRange" access="readWrite" type="xsd:unsignedInt"></parameter>   <parameter name="InternalPort" access="readWrite" type="xsd:unsignedInt"></parameter>   <parameter name="PortMappingProtocol" access="readWrite"></parameter>   <parameter name="InternalClient" access="readWrite"></parameter>   <parameter name="PortMappingDescription" access="readWrite"></parameter> '|grep -Po 'name="\K[^"]*' PortMappingEnabled PortMappingLeaseDuration RemoteHost ExternalPort ExternalPortEndRange InternalPort PortMappingProtocol InternalClient PortMappingDescription 

Answer 2

sed 's/[^"]*"\([^"]*\).*/\1/'

does the job.

explanation of the part inside ' '

• s - tells sed to substitute
• / - start of regex string to search for
• [^"]* - any character that is not ", any number of times. (matching parameter name=)
• " - just a ".
• ([^"]*) - anything inside () will be saved for reference to use later. The \ are there so the brackets are not considered as characters to search for. [^"]* means the same as above. (matching RemoteHost for example)
• .* - any character, any number of times. (matching " access="readWrite"> /parameter)
• / - end of the search regex, and start of the substitute string.
• \1 - reference to that string we found in the brackets above.
• / end of the substitute string.

basically s/search for this/replace with this/ but we're telling him to replace the whole line with just a piece of it we found earlier.