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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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.
Jonathan Leffler
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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
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@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
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Matt Ball
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3The `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