How to use sed to extract substring

Question

I have a file containing the following lines      lt parameter name  PortMappingEnabled  access  readWrite  type  xsd boolean  gt  lt  parameter gt     lt parameter name  PortMappingLeaseDuration  access  readWrite  activeNotify  canDeny  type  xsd unsignedInt  gt  lt  parameter gt     lt parameter name  RemoteHost  access  readWrite  gt  lt  parameter gt     lt parameter name  ExternalPort  access  readWrite  type  xsd unsignedInt  gt  lt  parameter gt     lt parameter name  ExternalPortEndRange  access  readWrite  type  xsd unsignedInt  gt  lt  parameter gt     lt parameter name  InternalPort  access  readWrite  type  xsd unsignedInt  gt  lt  parameter gt     lt parameter name  PortMappingProtocol  access  readWrite  gt  lt  parameter gt     lt parameter name  InternalClient  access  readWrite  gt  lt  parameter gt     lt parameter name  PortMappingDescription  access  readWrite  gt  lt  parameter gt    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

User · Answer

You should not parse XML using tools like sed, or awk. It's error-prone.

If input changes, and before name parameter you will get new-line character instead of space it will fail some day producing unexpected results.

If you are really sure, that your input will be always formated this way, you can use cut. It's faster than sed and awk:

cut -d'"' -f2 < input.txt

It will be better to first parse it, and extract only parameter name attribute:

xpath -q -e //@name input.txt | cut -d'"' -f2

To learn more about xpath, see this tutorial: http://www.w3schools.com/xpath/

User · Answer

Explaining how you can use cut   cat yourxmlfile   cut -d    -f2   It will  cut  all the lines in the file based on   delimiter  and will take the 2nd field   which is what you wanted

User · Answer

grep was born to extract things   grep -Po  name   K         test with your data   kent   echo   lt parameter name  PortMappingEnabled  access  readWrite  type  xsd boolean  gt  lt  parameter gt     lt parameter name  PortMappingLeaseDuration  access  readWrite  activeNotify  canDeny  type  xsd unsignedInt  gt  lt  parameter gt     lt parameter name  RemoteHost  access  readWrite  gt  lt  parameter gt     lt parameter name  ExternalPort  access  readWrite  type  xsd unsignedInt  gt  lt  parameter gt     lt parameter name  ExternalPortEndRange  access  readWrite  type  xsd unsignedInt  gt  lt  parameter gt     lt parameter name  InternalPort  access  readWrite  type  xsd unsignedInt  gt  lt  parameter gt     lt parameter name  PortMappingProtocol  access  readWrite  gt  lt  parameter gt     lt parameter name  InternalClient  access  readWrite  gt  lt  parameter gt     lt parameter name  PortMappingDescription  access  readWrite  gt  lt  parameter gt    grep -Po  name   K       PortMappingEnabled PortMappingLeaseDuration RemoteHost ExternalPort ExternalPortEndRange InternalPort PortMappingProtocol InternalClient PortMappingDescription

User · Answer

You want awk    This would be a quick and dirty hack   awk -F        print  2    tmp file txt  PortMappingEnabled PortMappingLeaseDuration RemoteHost ExternalPort ExternalPortEndRange InternalPort PortMappingProtocol InternalClient PortMappingDescription

User · Answer

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

[linux] How to use sed to extract substring

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