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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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2 Answers 2

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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, 2013 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. Mar 23, 2013 at 18:30
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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. Mar 23, 2013 at 15:16

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