Why AND && logic is used in chained commands in most shells, not OR || logic

Question

I am curious about the logic behind && in continuous commands execution in shell.

Lets see an example: command_a && command_b && command_c

In shell, this means that once a command fails, the consequent commands will not execute and the chain should stop.

If we replace each command by its exit code, the chain becomes, for example this expression 0 && 1 && 0. (command_a succeed, command_b failed, command_c succeed)

If we try evaluating this expression, the evaluation should stop right after the first command, 0 value.

If && logic is replaced by ||, the expression would be more fit to the meaning of original chained command. 0 || 1 || 0.

Expression's evaluation stops after command_b execution

Answer 1

There's a difference between the semantics of a successful command and the representation of success via the numeric value of the exit code. If you consider the abstract "is a command successful", && makes more sense, since the "success" abstract boolean is true. That's why && is used. You need A to run, AND you need B to run.

But, due to the fact that there's usually only one status for success, but many different types of errors, the value 0 has been defined long ago to be used for exit status to indicate success.

So, the exit status of a command can simply not replace the command itself in such an expression. The semantics and the representation are just different.

Answer 2

Check out this post.

"The right side of && will only be evaluated if the exit status of the left side is zero. || is the opposite: it will evaluate the right side only if the left side exit status is nonzero..."

$ false && echo howdy!

$ true && echo howdy!
howdy!
$ true || echo howdy!

$ false || echo howdy!
howdy!

Answer 3

I have seen || used in some shell script too. I think it depends on the occasions. You may use command-a && command-b when you want command-b to be executed only after command-a success.

Likewise, you may use command-a || command-b to deal with the situation when command-a fails.

Answer 4

Update: After reading your question three times I now understand what puzzles you: That 0 represents success, and/or (sic) that logical operators treat it as true. Yes, that can be confusing, coming from C. The other Peter's answer explains that well. I let my original answer stand anyway because it is not "wrong".

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It is what logic dictates if commands are impossible to perform or don't make sense unless their predecessors succeeded. That happens quite often: "Retrieve a HTML document from the web, verify the time stamp, parse a value out of it, and write that value as an updated data somewhere." Each step depends on the success of all preceding steps.

If, instead, you have something "retrieve a document from the web, or from the cache, or from disk, or take a default here document, in this order of preference", then the appropriate way to write that is indeed with logical ORs. This happens but is less common.

ORing is a common idiom though for error handling, because subsequent commands are exactly performed if the pervious failed. Consider Perl's idiomatic cmd() || die();. (You have more lives. Phew.)