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Finding text and what surrounds it - contextual grep

grep is a very useful tool for finding all the lines in a file (or series of files) that contain a particular string or match some other pattern / criteria. For example, I have a file that contains data for 52 staff members and I want to find who knows Lua ...

[trainee@melksham Download]$ grep Lua requests.xyz
lisa PHP Python Lua ASP Perl Java
graham Lua Perl Tcl/Tk
[trainee@melksham Download]$


But what if I want to see the context of my matches - to be given back not only the matched lines, but a few lines before and after the match to see what it relates too - "other nearby employees" in my example, but more commonly "the rest of the paragraph / example" in the case of text files. On Linux's grep, you can use the -A and -B options to specify the number of lines to display before and after each match, and the -C option if you want to show the same number before and after. Let's see that:

[trainee@melksham Download]$ grep -A2 -B1 Lua requests.xyz
kerry Perl Tcl/Tk Ruby MySQL
lisa PHP Python Lua ASP Perl Java
margaret XML Perl Ruby MySQL Tcl/Tk
nina Tcl/Tk Perl ASP Ruby
--
fred MySQL Perl Java XML
graham Lua Perl Tcl/Tk
harry PHP Python Java
ivan Ruby Java Perl Tcl/Tk MySQL
[trainee@melksham Download]$ grep -C3 Lua requests.xyz
iris Perl MySQL Java Tcl/Tk
jenny XML Perl Ruby ASP
kerry Perl Tcl/Tk Ruby MySQL
lisa PHP Python Lua ASP Perl Java
margaret XML Perl Ruby MySQL Tcl/Tk
nina Tcl/Tk Perl ASP Ruby
olivia MySQL Python ASP PHP
--
david Perl Tcl/Tk Java
ed Ruby Perl Java PHP
fred MySQL Perl Java XML
graham Lua Perl Tcl/Tk
harry PHP Python Java
ivan Ruby Java Perl Tcl/Tk MySQL
john PHP XML Java Perl
[trainee@melksham Download]$


If you're on a system that's got a version of grep that doesn't support these options, what can you do? Well - you could do worse that write a Perl script - in fact we've used the requirement for a contextual grep for an example on our Perl Programming course.
(written 2009-10-30)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
A166 - Web Application Deployment - Linux Utilities
  [4682] One line scripts - Awk, Perl and Ruby - (2016-05-20)
  [4586] Extending your bash shell with aliases, functions and extra commands - (2015-11-28)
  [3902] Shell - Grep - Sed - Awk - Perl - Python - which to use when? - (2012-10-22)
  [3764] Shell, Awk, Perl of Python? - (2012-06-14)
  [3446] Awk v Perl - (2011-09-18)
  [2638] Finding what has changed - Linux / Unix - (2010-02-17)
  [2320] Helping new arrivals find out about source code examples - (2009-08-03)
  [2145] Using the internet to remotely check for power failure at home (PHP) - (2009-04-29)
  [1690] Conversion of c/r line ends to l/f line ends - (2008-06-28)
  [1366] awk - a powerful data extraction and manipulation tool - (2007-09-25)
  [1361] Korn shell course - (2007-09-22)
  [71] Comparators in Linux and Unix - (2004-10-03)
  [63] Almost like old times - (2004-09-26)

P208 - Perl - Lists
  [4609] Mapping an array / list without a loop - how to do it in Perl 6 - (2016-01-03)
  [3939] Lots of ways of doing the same thing in Perl - list iteration - (2012-12-03)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)
  [3870] Writing more maintainable Perl - naming fields from your data records - (2012-09-25)
  [3669] Stepping through a list (or an array) in reverse order - (2012-03-23)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3400] $ is atomic and % and @ are molecular - Perl - (2011-08-20)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2813] Iterating over a Perl list and changing all items - (2010-06-15)
  [2295] The dog is not in trouble - (2009-07-17)
  [2226] Revision / Summary of lists - Perl - (2009-06-10)
  [2067] Perl - lists do so much more than arrays - (2009-03-05)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [1917] Out of memory during array extend - Perl - (2008-12-02)
  [1828] Perl - map to process every member of a list (array) - (2008-10-09)
  [1703] Perl ... adding to a list - end, middle, start - (2008-07-09)
  [1316] Filtering and altering Perl lists with grep and map - (2007-08-23)
  [1304] Last elements in a Perl or Python list - (2007-08-16)
  [968] Perl - a list or a hash? - (2006-12-06)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [773] Breaking bread - (2006-06-22)
  [762] Huge data files - what happened earlier? - (2006-06-15)
  [622] Queues and barrel rolls in Perl - (2006-02-24)
  [560] The fencepost problem - (2006-01-10)
  [463] Splitting the difference - (2005-10-13)
  [355] Context in Perl - (2005-06-22)
  [240] Conventional restraints removed - (2005-03-09)
  [230] Course sizes - beware of marketing statistics - (2005-02-27)
  [140] Comparison Chart for Perl programmers - list functions - (2004-12-04)
  [28] Perl for breakfast - (2004-08-25)


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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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