ternary construction not equivalent with if then else

问题

The following if/then test (in bash):

if [ 1 ]; then ls /undef_dummy > /dev/null 2>&1; else echo "else stmt"; fi

seems not equivalent to its ternary version:

(test 1) && ls /undef_dummy > /dev/null 2>&1 || echo "else stmt"

The first one will print nothing but the second one will print "else stmt".

This is because the || operator in the ternary version relates to the return status of either the test or the command executed if the test passes. But usually, we want the else statement to only relate to the test, so it's not safe to use the ternary version here.

Am I right?

回答1:

This command:

(test 1) && ls /undef_dummy > /dev/null 2>&1 || echo "else stmt"

|| is being evaluated because ls is returning non-zero return code to shell

If you use this command:

test 1 && date || echo "else stmt"

Then it will print date value instead of else stmt

With proper grouping your command:

test 1 && { ls /undef_dummy > /dev/null 2>&1 || true; } || echo "else stmt"

will not print anything

回答2:

What you refer to as the ternary version is actually equivalent to:

if (test 1); then
  if ! ls /undef_dummy > /dev/null 2>&1; then
    echo "else stmt"
  fi
fi

and is pretty different from saying:

if [ 1 ]; then ls /undef_dummy > /dev/null 2>&1; else echo "else stmt"; fi

来源：https://stackoverflow.com/questions/19857792/ternary-construction-not-equivalent-with-if-then-else

标签

bash

shell

unix

if-statement