Why is "source <(cmd)" working but not "$(cmd)"

Question

I want to have a script to call with params which adds some aliases to my current shell. The aliases have to be builded in respect to the calling parameters and the current directory from where I start it. If possible I would like to NOT have to source the script, just run it like any other script.

This is what I have done until now:

I wrote a small shell script to generate some aliases, like

echo 'alias foo="$1"'

and save it as file "myscript" with +x rights. Then I execute it via console to set the alias

$(myscipt hello)

that works fine

When I change my shell script like

echo 'alias foo="bar $1 -param"'

and execute again like above, the result is

bash: alias hello" not found
bash: alias -param" not found

???. I escaped already the blanks between bar and -param but can't get it work.

when I execute

source <(myscript hello)

everything is fine like expected (That is my solution by now but not sure if it is the best).

Some ideas how to get a better solution?

Answer 1

You're using the source command wrong. I'm assuming you have a file called myscript that contains these commands. Instead of having stuff to generate the right commands in the file, like this

echo 'alias foo="bar -param"'

you should just have the commands, like this

alias foo="bar -param"

Then you pass your script directly to the source command, which runs it in the current shell so that the changes made in the script are reflected in your current shell session:

source myscript

Also, the . command is equivalent:

. myscript

(Note that there's a space between the . and myscript.)

---

Update

In response to your comment: you should still just use a file. Since the name of an alias can be set using a variable value, you can just add the logic in the file to do all the dynamic aliases. Here's a super simple example:

$ foo=hello
$ alias $foo='echo hi'
$ hello
hi

You can use conditions, loops, etc to build up much more complex logic for your dynamic aliases.

Answer 2

set -x turns out to be informative:

: $; set -x
: $; $(./alias-builder)
++ ./alias-builder
+ alias 'foo="bar' '$1' '-param"'
bash: alias: $1: not found
bash: alias: -param": not found
: $; 

So, what's going on here is that when bash is treating the output of alias-builder (which is the name I gave to a script containing your echo command) as something to substitute inline as arguments, it's separating the output on words, as might normally happen, but treating the quotes as part of the words, rather than what you're expecting, which is to parse the output the way it would as if it was typed.

I think ultimately your answer is to use the source option.

Now, if that's inconvenient to type, you could do something like this:

metamyscript() {
    source <(myscript "$@")
}

And then you should be able to do, simply:

: $; metamyscript hello

and it will run myscript hello, and source the output of that.

(Of course, you could switch the names around to your liking.)