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    <p>Note that the first example is dependent on the shell. The pipe is an IPC (inter-process communications) operator and the shell can implement that by creating a subshell on either side of the pipe. (Technically I suppose some shell could even evaluate both sides in separate sub-processes).</p> <p>The <code>read</code> command is a built-in (must, inherently be so). So, in shells such as <code>bash</code> and the classic Bourne shell derivatives the suprocess (subshell) is on the right of the pipe (reading from the current shell) and that process ends after its <code>read</code> (at the semicolon in this example). Korn shell (at least as far back as '93) and <code>zsh</code> put their subshell on the other side of the pipe and are reading data from those into the current process.</p> <p>That's the point of the interview question. </p> <p>The point of my question here is to look for some consensus or metric for how highly to rate this level of question. It's not a matter of trivia because it does affect real world scripts and portability for shell scripting and it relies upon fundamental understanding of the underlying UNIX and shell semantics (IPC, pipes, and subprocesses/subshells).</p> <p>The second example is similar but more subtle. I will point out that the following change "works" (the <code>ssh</code> will execute on each of the hosts in the file):</p> <pre><code>cat $SOME_FILE | while read host; do ssh "$host" "$some_cmd" &lt; /dev/null; done </code></pre> <p>Here the issue is that the <code>ssh</code> command buffers up input even if the command on the remote never reads from its <em>stdin</em>. Because the shell/subshell (reading from the pipe) and the <code>ssh</code> are sharing the same input stream, the <code>ssh</code> is "stealing" most of the input from the pipeline, leaving only the occasional line for the <code>read</code> command.</p> <p>This is not an artificial question. I actually encountered it in my work and had to figure it out. I know from experience that understanding this second example is at least a notch or two above the first. I also know, also from years of experience, that fewer than 10% of the candidates (for sysadmin and programming positions) that I've interviewed can get the first question right away).</p> <p>I've never used the second question in a live interview and I've been discouraged from doing so.</p>
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    1. POWhat level of expertise would this interview item suggest? unset foo; echo bar | read foo; echo $foo
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    1. USJim Dennis
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    1. POWhat level of expertise would this interview item suggest? unset foo; echo bar | read foo; echo $foo
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    1. COHow much time did it take you to figure out ssh was eating input? If more than a couple minutes, it's probably a bad question, because then you know a candidate who hasn't seen it yet would have to grok shell better than you to work it out in an interview setting.
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      1. USGraham
    2. COUnless of course they've learned of it through a posting such as this, by me or someone else. :)
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      1. USJim Dennis
    3. COBy the way, I usually care more for the way in which a candidate works on the answer and how they talk through the possibilities then I do about whether they answer the question correctly. If they can immediately rattle off the correct answer then I've learned very little about them. If they work through a process of considering hypotheses and eliminating them (possibly by asking clarifying question or proposing possible answers) then what I learn has a good deal more predictive value. I care how they will handle harder problems and the unknown far more than whether they know trivia.
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      1. USJim Dennis
 

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