Shortcutting in a loop

Question

Shortcutting: (expr1 && expr2), rather than an if ... fi statement. If expr1, expr2,... are to be generated by a loop, can you do it?

Here's a toy example:

$ for cond in {0,0,0}; do if [[ $cond = 0 ]]; then echo 0; else echo 1; break; fi; done && echo "true" || echo "false"
0
0
0
true

$ for cond in {0,1,0}; do if [[ $cond = 0 ]]; then echo 0; else echo 1; break; fi; done && echo "true" || echo "false"
0
1
true

How would I change the if...fi statement such that the first returns only true, and the second false? If I replace echo 1 by exit within, well, the terminal window shuts off.

You don't have all of the elements of the `for` loop inside the `for` loop; can you elaborate what you're actually trying to do? — Jeff Schaller, Jul 18 '19 at 00:03
Sorry for the miscommunication; my interpretation is that you want to summarize the `for` loop by testing each value for `0`; since the `if` statement inside the `for` loop only sees one value at a time, it cannot shortcut. Unless you want to exit the loop whenever you see a "false" value (such as `1` here)? — Jeff Schaller, Jul 18 '19 at 00:14
What it boils down to I think is how to I make the loop break and return status false when it encounters `$cond = 0`? Such that it can be chained with `&&` to the next command? I think I got it: `$ (for cond in {0,1,0}; do if [[ $cond = 0 ]]; then true; else false; exit; fi; done) && echo "true" || echo "false" false ` PS: from here: https://stackoverflow.com/questions/14059342/how-to-get-the-exit-status-a-loop-in-bash — Erwann, Jul 18 '19 at 00:46

score 1 · Answer 1 · answered Jul 18 '19 at 00:50

The exit status of the loop is that of the last command executed there, which is either echo or break, and both usually have a exit status of 0. So you need to preserve and check the status of the condition. I'd suggest using a function:

loopcheck() {
for cond in {0,1,0}; do
  if [[ $cond = 0 ]]; then
    echo 0; 
  else
    ret=$?;   # preserve exit status of test
    echo 1; 
    return $ret;  # pass it on
  fi;
done
}

loopcheck && echo "true" || echo "false"

Another option is saving $ret like above and checking its value outside the loop.

LL3 · Answer 2 · 2019-07-23T22:48:37.563

The for name in ... loop relays the exit status of the last command executed.

When you break a loop, the break command itself has an exit status of 0 by default, which is therefore relayed by the for, thus making your && test after the done be equal to true.

One way to work around that is as you said in your self-answer. That is a nice and clean one, and works well with any POSIX compliant shell.

But it has one drawback in that, by being a sub-shell, it doesn't fully share its Execution Environment¹ with its "outer" shell. It does inherit it on beginning execution, but does not "propagate" it back to its "outer" parent on finishing execution, meaning for instance that any variable set or modified within the loop won't be seen after outside the loop.

Speaking for Bash, there is an alternative: you can avoid the sub-shell and still use break while forcing it to fail by passing it a -1 argument, because -1 is always an invalid number of loops to quit. Bash will display an error message for that, but you can mute it by redirecting to 2>/dev/null.

However note also that this behavior is not properly POSIX compliant, because break is considered a special built-in, and a "special built-in" incurring into error should exit a non-interactive shell (e.g. a script).

^{¹[...]. Changes made to the subshell environment cannot affect the shell’s execution environment. [...]}

score 0 · Answer 3 · answered Jul 21 '19 at 12:47

0

I take this over the other answer (however useful) because it is closer to the requirement implicit in the question:

$ (for cond in {0,1,0}; do if [[ $cond = 0 ]]; then true; else false; exit; fi; done) && echo "true" || echo "false" false

answered Jul 21 '19 at 12:47

Erwann

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Shortcutting in a loop

3 Answers3