Is there a difference between “.” and “source” in bash, after all?

Question

I was looking for the difference between the "." and "source" builtin commands and a few sources (e.g., in this discussion, and the bash manpage) suggest that these are just the same.

However, following a problem with environment variables, I conducted a test. I created a file testenv.sh that contains:

#!/bin/bash
echo $MY_VAR

In the command prompt, I performed the following:

> chmod +x testenv.sh
> MY_VAR=12345
> ./testenv.sh

> source testenv.sh
12345
> MY_VAR=12345 ./testenv.sh
12345

[note that the 1st form returned an empty string]

So, this little experiment suggests that there is a difference after all, where for the "source" command, the child environment inherits all the variables from the parent one, where for the "." it does not.

Am I missing something, or is this is an undocumented/deprecated feature of bash?

[ GNU bash, version 4.1.5(1)-release (x86_64-pc-linux-gnu) ]

Answer 1

Short Answer

In your question, the second command uses neither the . shell built-in nor the source built-in. Instead, you are actually running the script in a separate shell, by invoking it by name (like any other executable file). This does give it a separate set of environment variables (though if you export an environment variable in its parent shell, it'll be included). If you change the / to a space, then that would run it with the . built-in, which is equivalent to source.

Extended Explanation

This is the syntax of the source shell built-in, which executes the contents of a script in the current shell (and thus with the current shell's environment variables):

source testenv.sh

This is the syntax of the . built-in, which does do the same thing as source:

. testenv.sh

However, this syntax runs the script as an executable file, launching a new shell to run it:

./testenv.sh

That is not using the . built-in. Rather, . is part of the path to the file you are executing. Generally speaking, you can run any executable file in a shell by invoking it with a name that contains at least one / character. To run a file in the current directory, preceding it by ./ is thus the easiest way. Unless the current directory is in your PATH, you cannot run the script with the command testenv.sh. This is to prevent people from accidentally executing files in the current directory when they intend to execute a system command or some other file that exists in some directory listed in the PATH environment variable.

Since running a file by name (rather than with source or .) runs it in a new shell, it will have its own set of environment variables. The environment variables do inherit from the environment variables of the calling process (which in this case is your interactive shell). However, for an environment variable to be passed to the new shell, one of the following must be the case:

• The environment variable has been exported. Use the export shell built-in for this. In your example, you can use export MY_VAR=12345 to set and export the variable in one step, or if it is already set you can simply use export MY_VAR.

• The environment variable is explicitly set and passed for the command you're running. This usually accomplishes that:

  MY_VAR=12345 ./testenv.sh
  

./ Syntax for Scripts Requires a Hashbang Line

By the way, please note that, when you invoke an executable by name as above (and not with the . or source shell built-ins), what shell program is used to run it is not determined by what shell you're running it from. Instead:

• For binary files, the kernel may be configured to run files of that particular type. This is how executable binaries are able to run (which is of course extremely important because a script can't run without a shell or other interpreter, which is an executable binary...plus many commands and applications are compiled binaries rather than scripts).

• For files that are supposed to run in a shell or other interpreted language, the first line looks like:

  #!/bin/sh
  

  /bin/sh may be replaced with whatever other shell or interpreter is intended to run the program. For example, a Python program might start with the line:

  #!/usr/bin/python
  

  These lines are called hashbang, shebang, and a number of other similar names. See this Wikipedia article for more information.

Answer 2

Yes, you're missing something. I think you're confusing the '.' that means current directory, as in ./testenv.sh and the '.' that means source, which would be . ./testenv.sh. Make sense?

So try this:

MY_VAR=12345 
. ./testenv.sh