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    <blockquote> <p>What should an if statement do when the condition is a command substitution where the command produces no output?</p> </blockquote> <p>Output doesn't matter. What matters is the <em>exit code</em>:</p> <pre class="lang-none prettyprint-override"><code> if list; then list; [ elif list; then list; ] ... [ else list; ] fi The if list is executed. If its exit status is zero, the then list is executed. Otherwise, each elif list is executed in turn, and if its exit status is zero, the corresponding then list is executed and the command completes. Otherwise, the else list is executed, if present. The exit status is the exit status of the last command executed, or zero if no condition tested true. </code></pre> <p>If you replace your <code>$(echo true)</code> and <code>$(echo false)</code> with something else you will probably see what is going on:</p> <pre><code>$ if $(echo false) ; then echo yes ; else echo no ; fi no $ if $(echo command-does-not-exist) ; then echo yes ; else echo no ; fi command-does-not-exist: command not found no $ </code></pre> <p>The <code>$(..)</code> runs the command and then <code>if</code> <em>executes the results</em> (in this case, just <code>true</code> or <code>false</code> or <code>does-not-exist</code>). An empty <code>$()</code> starts a subshell which successfully runs to completion and returns an exit code of <code>0</code>:</p> <pre><code>$ if $() ; then echo yes ; else echo no ; fi yes </code></pre> <p>Aaron raised some interesting points:</p> <pre><code>$ if $(echo true; false) ; then echo yes ; else echo no ; fi yes $ if $(echo 'true ; false') ; then echo yes ; else echo no ; fi yes $ if true ; false ; then echo yes ; else echo no ; fi no $ if $(echo false ; true) ; then echo yes ; else echo no ; fi no $ if $(echo 'false ; true') ; then echo yes ; else echo no ; fi no $ if false ; true ; then echo yes ; else echo no ; fi yes $ </code></pre> <p>It appears that when executing a command constructed via the <code>$()</code> subshell, the first command's exit status is what matters.</p>
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    1. POWhy does `if $(true) ; then ... fi` succeed?
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    1. USsarnold
    1. USsarnold
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    1. COThe exit code of the command inside the `$( )` doesn't matter if the output is non-empty. `if $(echo true; false) ; then echo yes ; else echo no ; fi` gives `yes`. So the `false` is ignored because the *outputed* command ("true") is executed instead.
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      1. USAaron McDaid
    2. CO@Aaron, that's a cool example. I had to play with it a bit further and now I'm more confused than when I started. :)
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      1. USsarnold
    3. CO*It appears that when executing a command constructed via the $() subshell, the first command's exit status is what matters.* If that were the case then the following should produce `no`: `if $(false;echo 'true ; false') ; then echo yes ; else echo no ; fi`
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      1. USSiegeX
 

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