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Old 16th March 2018
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vanGrimoire vanGrimoire is offline
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Default Subshells

What is the difference between the various subshells?

$ $(firefox)
$ firefox &
[1] 69537
$ `firefox`
[1] + Done firefox
$ firefox &&
> echo hello world!
hello world!

Sorry if this is vague, but are there advantages and disadvantages or are they basically different means of skinning the same cat?
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Old 16th March 2018
Beastie Beastie is offline
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It's not a matter of there being any particular advantage or disadvantage to using one or another. These are different ways of executing commands and doing job control. Each has it's use.

Quote:
Originally Posted by vanGrimoire View Post
$ firefox &
The single ampersand executes the command in the background (in other words, it doesn't block the shell while the process is running: it runs the command, "detaches" it from the shell and returns to the shell right away).

Quote:
Originally Posted by vanGrimoire View Post
$ `firefox`
A pair of grave accents is used for command substitution. It's normally used within another command such as echo Hello, `id -un`! (the id(1) command within the echo(1) command).
In your case, since there's nothing else, it simply runs the command.

Quote:
Originally Posted by vanGrimoire View Post
$ firefox &&
> echo hello world!
The double ampersand is a logical operator (conditional AND).
The second command after the && runs only if the first command returns successfully. In your case you didn't specify that second command so the shell opened a subshell so that you can type and run it.
You could've just run firefox && echo hello world!


You're not using these features in a very "creative" way (no offense intended), that's why they feel similar. Their usefulness becomes more apparent when used within elaborate shell scripts.
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Old 17th March 2018
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IdOp IdOp is offline
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To comment on the other (first) example in the OP

$ $( command )

is just another syntax for

$ ` command `

Both do the same thing: command is run and its standard output is substituted in place of $( command ), in the cmd line where it occurred. Normally the command would be chosen to output some useful text that you want to insert in the original cmd line; firefox probably doesn't do this.

For more detail you should study the man page for your shell in the areas where these are described. There are all kinds of weird/cool things that can be done.
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Old 17th March 2018
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vanGrimoire vanGrimoire is offline
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Default Right on!

I was getting at that (grave accents vs parens), but my power blipped.

I did find,

Code:
#!/bin/ksh
firefox &
ff_pid=$!
which is embarrassingly easier than awk,

Last edited by vanGrimoire; 17th March 2018 at 02:21 AM. Reason: added shell
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Old 17th March 2018
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vanGrimoire vanGrimoire is offline
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Sorry the purpose was to start an application from the CLI, return the shell and not fill it up with debug logs, etc.

I think
Code:
supertux2 2> /dev/null &
is what I'm looking for.
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