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POBash parameter expansion
 

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  1. POBash parameter expansion
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    <p>I have a script which uses the following logic:</p> <pre><code>if [ ! -z "$1" ]; then # if any parameter is supplied ACTION= # clear $ACTION else ACTION=echo # otherwise, set it to 'echo' fi </code></pre> <p>This works fine, as-is. However, in reading the <a href="http://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion" rel="nofollow">Shell Parameter Expansion</a> section of the <code>bash</code> manual, it seems this should be able to be done in a single step. However, I can't quite wrap my head around how to do it.</p> <p>I've tried:</p> <pre><code>ACTION=${1:-echo} # ends up with $1 in $ACTION ACTION=${1:+} ACTION=${ACTION:-echo} # ends up always 'echo' </code></pre> <p>and a few ways of nesting them, but nesting seems to be disallowed as far as I can tell.</p> <p>I realize I've already got a working solution, but now I'm genuinely curious if this is possible. It's something that would be straightforward with a ternary operator, but I don't think bash has one.</p> <p>If this is possible, I'd like to see the logic to do this seeming two-step process, with no if/else constructs, but <strong>using only any combination of the <a href="http://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion" rel="nofollow">Shell Parameter Expansion</a> features</strong>.</p> <p>Thank you.</p> <hr> <p><strong>EDIT</strong> for elderarthis:</p> <p>The remainder of the script is just:</p> <pre><code>find . -name "*\?[NMSD]=[AD]" -exec ${ACTION} rm -f "{}" + </code></pre> <p>I just want <code>ACTION=echo</code> as a sanity check against myself, hence, passing any argument will actually do the deletion (by nullifying ${ACTION}, whereas passing no args leaves echo in there.</p> <p>And I know TIMTOWTDI; I'm looking to see if it can be done with just the stuff in the Shell Parameter Expansion section :-)</p> <hr> <p><strong>EDIT</strong> for Mikel:</p> <pre><code>$ cat honk.sh #!/bin/bash ACTION=${1-echo} echo $ACTION $ ./honk.sh echo $ ./honk.sh foo foo </code></pre> <p>The last needs to have <code>ACTION=''</code>, and thus return a blank line/null value.</p>
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    1. USSdaz MacSkibbons
    1. USSdaz MacSkibbons
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    1. COThe first one works for me with a pretty basic two-line script. Could you maybe provide the portion of the script where you execute `ACTION`?
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      1. USeldarerathis
    2. CO@elderarthis: Updated.
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      1. USSdaz MacSkibbons
    3. CO@Sdaz MacSkibbons: Oh, I see where the hangup is now. I actually misinterpreted what you were trying to do when I first read this.
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      1. USeldarerathis
 

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