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Message-ID: <4D6ABCC2.3050109@wonkology.org>
Date: Sun, 27 Feb 2011 22:06:10 +0100
From: Alex Schuster <wonko@wonkology.org>
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To: gentoo-user@lists.gentoo.org
Subject: Re: [gentoo-user] OT: cut replacement with bash builtins
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Florian Philipp writes:

> I'm currently streamlining some of my shell scripts to avoid unnecessary
> process calls where bash itself is powerful enough.
> 
> At the moment, I want to replace stuff like this:
> string='foo:bar:foo'
> second_field=$(echo $string | cut -d : -f 2) # should read "bar"
> 
> My current solution is using two string operations:
> string='foo:bar:foo'
> # remove everything up to and including first ':'
> second_and_following=${string#*:}
> # remove everything from the first ':' following
> second_field=${second_and_following%%:*}

That's how I do these things, too.

> Of course, I normally do this in a single line with a subshell but it

Hmm, I don't get this. Subshell?

> still looks cumbersome. Is there a way to do it in a single operation
> without a temporary variable? The following does not work:
> string='foo:bar:foo'
> second_field=${string#:%%:*}

I don't think so. But you can write a shell function for this:

getfield()
{
	local str=${1#*:}
	echo "${str%%:*}
}

string='foo:bar:foo'
second_field=$( getfield "$string" )

But if you need to do this very often in a loop, sometimes going back to
cut can speed things up, when you place it outside:

See

for string in $( < inputfile )
do
	second_field=$( getfield "$string" )
	do_something_with $second_field
done

vs.

secondfields=( $( cut -d : -f 2 inputfile ) )
for secondfield in ${secondfields[@]}
do
	do_something_with $second_field
done	

So, in th e2nd example, cut is called only once, but processes all input
lines. The result is put into an array.

Of course, all stuff should be put into double quotes in case there is
whitescape involved. Setting IFS to $'\n' may be necessary for this when
creating the array.

	Wonko