Re: How to print both matched and non matched lines?

[Eric Blake]

On 11/21/2011 10:47 PM, Peng Yu wrote:
> Hi,
> 
> I use --color=auto, so that I can distinguish nonmatched lines from
> matched lines by looking at the colored text if the non matched lines
> is shown. But I don't see an option to allow me show all lines. Would
> you please let me know if there is such an option if I miss anything
> in the man page?

Use -C to expand the context sufficiently large to show all lines.  For
example,

grep --color=auto -C$(wc -l < file) pattern file

is guaranteed to show the entire file, as long as there is at least one
match for pattern.

Hmm, this points out a probable optimization bug in grep, that we should
consider fixing.  This test is done in grep.git:

$ time src/grep -C$((1<<20)) 'as is' README > /dev/null

real    0m0.007s
user    0m0.002s
sys     0m0.005s

$ time src/grep -C$((1<<25)) 'as is' README > /dev/null

real    0m0.081s
user    0m0.077s
sys     0m0.004s

$ time src/grep -C$((1<<30)) 'as is' README > /dev/null

real    0m0.759s
user    0m0.755s
sys     0m0.004s

so why does making the context at order of magnitude larger make the
computation take an order of magnitude more time in computing identical
results?

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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