8 SC2015

Joachim Ansorg edited this page 2021-11-12 19:17:10 +01:00

Table of Contents

• Note that A && B || C is not if-then-else. C may run when A is true.
• Problematic code:
• Correct code:
• Rationale:
• Boring detail:
• Exceptions

Note that A && B || C is not if-then-else. C may run when A is true.

Problematic code:

[[ $dryrun ]] && echo "Would delete file" || rm file

Correct code:

if [[ $dryrun ]]
then
  echo "Would delete file"
else
  rm file
fi

Rationale:

It's common to use A && B to run B when A is true, and A || C to run C when A is false.

However, combining them into A && B || C is not the same as if A then B else C.

In this case, if A is true but B is false, C will run.

For the code sample above, if the script was run with stdout closed for any reason (such as explicitly running script --dryrun >&-), echo would fail and the file would be deleted, even though $dryrun was set!

If an if clause is used instead, this problem is avoided.

Boring detail:

We can think of the example above as

((([[ $dryrun ]]) && echo "Would delete file") || rm file)

expressing the left-associativity of the && || operators.

Whenever a command (strictly, a pipeline) succeeds or fails, the execution proceeds following the next && (for success) or || (for failure). (More strictly, the parentheses should be replaced with { command; } to avoid making a subshell, but that's ugly and boring.)

Exceptions

Ignore this warning when you actually do intend to run C when either A or B fails.

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