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I saw the command like ". test.sh" in some shell source code, but I don't know what it does. So, I have tried this. And, that .sh file is executed.

However, I don't understand how "." works. Can you explain?

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marked as duplicate by Daniel Beck Mar 23 '13 at 18:43

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2 Answers 2

The . command (which has a synonym, source, in bash, but not in most other Bourne shell derivatives) reads the named file as part of the current shell.

Consider:

$ cat test.sh
export ENVVAR="Welcome"
echo $ENVVAR
$ echo $ENVVAR

$ test.sh
Welcome
$ echo $ENVVAR

$ . test.sh
Welcome
$ echo $ENVVAR
Welcome
$

NB: Cheat 1: I assume test.sh is executable. Cheat 2: I assume test.sh is in a directory on $PATH.

It means that environment variables set in test.sh affect the current shell. By contrast, executing a command without the . does not affect the environment of the current shell. The . mechanism is used when .profile and related files are read, for example.

Note that . looks for simple names (like test.sh with no slash in it) on PATH, but the file only has to be readable; it does not have to be executable.

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Note that when bash is not in POSIX mode, ./source also checks the current directory if no file is found using PATH. –  chepner Mar 23 '13 at 15:31
    
@chepner: That's entirely plausible. I haven't bothered to experiment with whether the current directory is searched first or last. For the most part, it is not important to this question, which is about the high-level difference between sourcing and executing a script. I agree, though, that it does matter if you're going to make a bullet-proof system depending on such a script. –  Jonathan Leffler Mar 23 '13 at 18:30
    
Thank you guys. I understand. –  sinapps Mar 24 '13 at 18:03

It's a shorthand for this, nothing more:

source test.sh

http://ss64.com/bash/period.html

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3  
The dot command pre-dates the source command by years. It was in the original Bourne shell. The C shell had the source command for the same job; bash imported that as a synonym for . some time later. –  Jonathan Leffler Mar 23 '13 at 15:16

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