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I recently learned, that

. ./.a.a and ./.a.a is the same.

However trying source source .a.a gives an error. IMO, . being Bash alias for source shouldn't behave differently, so what am I missing?

Bonus, why is . . OK while source source is not?

LAFK says Reinstate Monica
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    You could have chosen a clearer name for your example :). In any case, whatever gave you the idea that `. script` and `./script` are the same thing? [They most certainly are not](http://unix.stackexchange.com/q/43882/22222). Also, `.` is not an alias to `source`. It's the other way around. `.` is the standard and `source` is an alias of `.`. – terdon Sep 14 '16 at 09:27
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    After spending some time looking at shell scripts, you get used to the `.` command, but I mean `source` is just such a better command, when you see `source somefile` in a script you immediately know what that statement does, but `. somefile` not only is not obvious what it does, you can easily miss the `.` character altogether – the_velour_fog Sep 14 '16 at 09:47
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    _Bonus, why is . . OK while source source is not?_ - in how far the error message `bash: .: .: is a directory` on `. .` is ok needs to be explained. – countermode Sep 14 '16 at 10:25
  • @terdon - absolutely right. The original example was `. ./.a.a .` to be honest. :-D A quiz of sorts, if you will. :-) Linked question is awesome, many thanks. What gave me that idea: the fact that I had seen no difference in executing files this way and the fact I read somewhere `.` is an alias for `source`. :-) – LAFK says Reinstate Monica Sep 14 '16 at 16:14

2 Answers2

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You can't just replace . with source everywhere; if

. ./.a.a

works, you can replace the first . (at least in Bash):

source ./.a.a

The second . represents the current directory, you can't replace that with source (especially not ./ with source as you've done).

source source

would be OK if you had a file called source in the current directory, containing something meaningful for your current shell. I can't see how . . would be OK...

Also, . ./.a.a and ./.a.a aren't the same, the second form runs .a.a in a separate shell. See What is the difference between sourcing ('.' or 'source') and executing a file in bash? for details.

Community
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Stephen Kitt
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13

source is a shell keyword that is supposed to be used like this: sourcefile where file contains valid shell commands. These shell commands will be executed in the current shell as if typed from the command line. Now, .file does exactly the same.

Beyond that . alone means "the current working directory" as in ./xyz ("xyz in this directory") or a/b/./c/./d (which is identical to a/b/c/d).

Beyond that . in a filename has a meaning only by convention as in .foobar which indicates a "hidden" file (not really...) or as in foobar.pdf, which indicates a file format by the suffix (here .pdf).

These different meanings cannot be interchanged.

countermode
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    `.` is in no way outdated. It, unlike `source` is the portable, [POSIX-defined](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#dot) way of sourcing a file. The `source` alias is shell-specific and can be either absent (for example in `dash`) or behave differently from the standard `.`. – terdon Sep 14 '16 at 16:25
  • Thanks @terdon, [Wiki](https://en.wikipedia.org/wiki/Source_\(command\)) says that the source as a dot (.) is not acceptable in C shell, where the command first appeared. – Noam Manos Mar 15 '18 at 10:07
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    @NoamManos perhaps, but the C-shell is not POSIX, so it can (and does) behave very differently. – terdon Mar 15 '18 at 10:10