CODEWITHSUNDEEP
shell
Sam
Sam

Reputation: 127

Whether to redirect stderr to stdout OR redirect both to the same file?

Which is better?

Is there even a difference?

I know two reasons to choose the first one: It does also work with > instead of >>. It is more popular, therefore someone knowing shell-scripts would except it right away.

But, I still feel like the second one is better readable, and works without having to know the [n]>&[n] syntax, which IMHO is kinda confusing.

Upvotes: 3

Views: 111

Answers (1)

Scott McPeak
Scott McPeak

Reputation: 12923

What is the difference?

Let's examine what each of these commands means. I will assume that the POSIX shell specification applies since the question doesn't ask about anything more specific.

The first command is cmd >>file 2>&1. This runs cmd after setting up the specified redirections.

The redirection >>file opens the named file with O_APPEND. As explained in the specification of open, this creates a new Open File Description, which notably contains the current file offset, and arranges for File Descriptor 1 to refer to that description. The meaning of O_APPEND is "the file offset shall be set to the end of the file prior to each write".

The redirection 2>&1 says that file descriptor 2 "shall be made to be a copy" of file descriptor 1. That specification is a little vague, but I think the only sensible interpretation (and what shells actually do) is it means to call dup2(1, 2), which "shall cause the file descriptor [2] to refer to the same open file description as the file descriptor [1]". Crucially, we get another file descriptor, but continue to use the same file description, meaning they both have the same file offset.

The second command is cmd 1>>file 2>>file. Based on the specifications cited above, this creates two separate file descriptions for file, each with their own offset.

Now, if the only thing that cmd does to file descriptors 1 and 2 is to call write, then these two situations are equivalent, because every call to write will atomically update the offset to point to the end of the file before performing the write, and therefore the existence of two separate offsets in the second command will not have any observable effect.

However, if cmd performs some other operation, for example lseek, then the two cases are not equivalent because that will reveal that the first command has one shared offset while the second command has two independent offsets.

Additionally, the above assumes the POSIX-specified semantics of O_APPEND. But real computer systems do not always implement that; for example, NFS does not have atomic append. Without atomic append, the second command may behave differently (most likely corrupting the output) even when only write is performed.

Which is better?

As the two commands do not mean the same thing, which is better presumably depends on which meaning is closer to what you intend. I speculate that, in almost all cases, the intent is to append to file both the standard output and standard error from cmd, which is presumed to only write to these descriptors. That is precisely the meaning of the first command (cmd >>file 2>&1), and hence is the better choice.

While the second command does use fewer shell features, and hence might be easier to understand for some people, it would probably seem odd to those who do have greater familiarity with redirection syntax, and might even behave differently than intended in some circumstances. Therefore I would advise against it, and if I found it in some code I was maintaining, would be inclined to change it to the first form.

Of course, if you truly want separate file descriptions, and hence separate file offsets, then the second command makes sense, so long as you put a comment nearby explaining the rationale for the unusual construction.

Upvotes: 3

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