Grouping commands in curly braces and piping does not preserve variable

Question

Say I have a file myfile in my current working directory. I want to set a variable if a command executes normally, but also use its result.

$ ls         


        

Answer 1

You can use BASH_SUBSHELL variable to verify whether you're in subshell or not.

# BASH_SUBSHELL will print level of subshell from top due to pipe
{ unset v && ls file && v=3 && echo "$BASH_SUBSHELL - $v"; } | nl
     1  file
     2  1 - 3

# outside BASH_SUBSHELL will print 0
echo "$BASH_SUBSHELL - $v";
0 -

You can use for piped command it prints 1 meaning it is in a subshell hence value of v isn't available outside (evident from 2nd output)

Answer 2

It's not the curly braces that are causing a subshell to be created, it's the pipe.

To prove it:

$ { ls && v=3; } > tmp
$ echo "$v"
3

To quote Greg:

In most shells, each command of a pipeline is executed in a separate SubShell.