Multiple output with bash command-CodePudding

I want to find the 2 version of python. If I write:

{python2,python3}" --version; "

I am expecting it to run:

python2 --version; python3 --version;

and provide me the versions like if I would have written the above line directly :

Python 2.7.18 
Python 3.8.10

but instead I get:

python2 --version;: command not found

even though if I test the output with:

echo {python2,python3}" --version"

I do get the expected:

python2 --version; python3 --version;

Trying

`echo {python2,python3}" --version"`

didn't work either. Why? How do I make it work?

CodePudding user response：

Because that's not what it does. You are creating two quoted strings "python2 --version; " and "python3 --version; " neither of which is a valid command. You could get around that with eval but ... that way lies madness.

# XXX DON'T
eval python{2,3}" --version;"
2.7.18
Python 3.8.10

The (not so) simple but straightforward solution would be something like

for v in 2 3; do
    python$v --version
done

or if you are hellbent on using a wildcard, run a subshell:

echo python{2,3}" --version;" | bash

Perhaps notice the (lack of) difference between

echo python2" --version; "

and

echo "python2 --version; "

The quotes simply tell the shell to keep things together as a single string. (For what it's worth, the shell removes the quotes as part of its parsing process, so the argument to echo does not contain any actual quotes, just the parsed string from between the quotes.)

It's possible to have command names with literal spaces in them, but incredibly cumbersome; so you don't see this much in practice.

For fun, maybe try

# Have a directory in your PATH which is in your home directory
# If you don't, maybe create $HOME/bin and add it to your PATH
bash$ echo "$PATH" | tr ':' '\n' | grep "$HOME"
/users/you/bin

bash$ cat >"$HOME/bin/python2 --version"
#!/bin/sh
exec python2 --version
^D

bash$ chmod  x "$HOME/bin/python2 --version"

bash$ "python2 --version"
2.7.18

(Maybe delete the script when you're done experimenting.)

CodePudding user response：

Your first attempt can work if you pipe it through xargs with the delimiter option set to a newline, and then pass it to bash -c:

$ echo {python2,python3}" --version;" | xargs -d $'\n' bash -c
Python 2.7.17
Python 3.6.9
$