Using mv file{,.bak} renames "file" to "file.bak". My question is, can I use this syntax safely? I mean, it does seem to achieve what I want, but is it correct, so to say, give that {,.bak} can mean both nothing or ".bak"?

What bothers me in a way is that it does something useless before actually renaming it, namely it tries to rename "file" to "file", and then it achieves what I what.

If you are using the bash shell, then perhaps the following will illustrate what's happening.

This command:produces this output:This command:produces this output:So a list is expected, and the list you are giving it is nothing followed by .bak

As a result it is appending nothing to "file" and doing the rename, then appending .bak to file and doing the rename ( i.e. mv ).

If you wish to rename a single file you can simply do:

If instead you wish to rename multiple files, you can do something like this:

The above would rename all the files in the current directory, which have names that don't start with a dot. Naturally you can replace the asterisk with a different pattern that matches only selected file names.

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No, mv command sees only file and file.bak as arguments. The bash shell did the brace expansion before calling the mv command

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Thanks for the reply, rigor. Very helpful.

@keefaz
So what you're saying is that file{,.bak} means "file" and "file.bak", so that it is equal to "mv file file.bak", right?

Would the command, therefore, be fine to use it as it is? Am I taking any chances if I use it in a different context?

1. Yes
2. Yes it's fine to use as this as long as the shell has brace expansion (bash, sh, csh ..)

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Mmmmmmmmm, well you might want to keep in mind that a classical Bourne shell sh can do brace expansion somewhat differently than a fully "modern" bash shell. I believe that there's even a mention of that somewhere in the bash manual page. Also, often running something like /bin/sh on Linux will actually result in running a form of bash.

That all is why I tried to stress that what you suggest would work, if you are using the bash shell.

Breaking down a command into its individual elements, and testing them separately, can be a rather simple way to understand what the overall command will do. That's why I illustrated what the brace expansion would be, using echo commands. As keefaz mentioned, the result of the brace expansion is what's then passed to the mv command. You can do the same sort of thing with a variety of patterns/expansions/etc., using the echo command to see what the result of a pattern/expansion/etc will be, before using it in a more complete command. In that sense, the echo command tends to allow for harmless investigation of what would happen, if you used something in the command you ultimately wish to execute.

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Well actually I was interested more in the bash shell, of course, although I'm very much interested also if there are exceptions, such as the sh (although I know that in most modern operating system /bin/sh is simply a link to /bin/bash).

Very useful details, indeed, regarding the echo commands. I'll try it out in the future, too.