Bash - Command Substitution, Subshell and Codeblock

Quick and simple differences between using $(..), (..) and {..;}.

• $(command) or `command` - Command substitution (the output of a command replaces the command name) and is executed in a subshell. Please give preference to $(..) for better readability
• ( command ) - Command list is executed in a subshell
• { command ;} - Command list is executed in the current shell

Executing code

$ ( echo 123 )
123

$ $(echo 123) # this executes the output of the command
123: command not found

$ { echo 123 ;}
123

Fiding subshell level with built-in BASH_SUBSHELL variable

$ ( echo $BASH_SUBSHELL )
1
$ ( ( echo $BASH_SUBSHELL ) )
2

$ $($BASH_SUBSHELL)
1: command not found

$ { echo $BASH_SUBSHELL ;}
0

Passing variables to subshell

$ var1=1

$ ( echo $var1 )
1

$ $(echo $var1)
1: command not found

$ { echo $var1 ;}
1

Changing and getting variables values in subshell

$ var=1

$ ( var=2 ) ; echo $var
1

$ $(var=2) ; echo $var
1

# Because we did not create a subshell, 
# variable is available to original shell
$ { var=2 ;} ; echo $var 
2

Other usages

Example 1:

Multiple commands and multiple lines. This can be used with (..) and {..}.

$ { echo 1 ; echo 2 ; echo 3; }
1
2
3

$ { 
> echo 1
> echo 2
> echo 3;
> }
1
2
3

Example 2:

Changing the environment for subshell with separate environment. Here, set -u is only set in the subshell, so only the first echo will error out.

unset var1 var2
(
  set -u
  echo $var2
) 
echo $var1