Linux difference between when to use parentheses

Question

Why do I get extra empty line when running 2). To me 1 is like 2. So why the extra line in 2)?

1)

export p1=$(cd $(dirname $0) && pwd)
#                                  ^
echo p1

2)

export p2=$(cd $(dirname $0)) && pwd
#                           ^
echo p2 

Answer 1

p1 captures the output of cd (empty) and pwd .

p2 only captures the output of cd , and then runs pwd without redirection.

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echo p1 prints a literal p1 (with a newline). I guess you didn't actually copy-paste from your terminal, but instead typed in some thing else.

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peter@tesla:~$ export p2=$(true) && pwd
/home/peter
peter@tesla:~$ echo "x${p2}x"
xx

cd in a subshell doesn't affect the parent shell's pwd, so I just substituted the true command to make it more readable.

Answer 2

$echo $0
/bin/bash
$ echo $(cd $(dirname $0) && pwd)
/bin
$ echo $(cd $(dirname $0)) && pwd

/home/user
$

In the 1st expression it becomes echo $(cd /bin && pwd) . Therefore the inner 2 commands execute in a subshell and return back the pwd value which is then echoed.

In the 2nd expression it gets reduced to echo $(cd /bin) && pwd . Therefore only the cd command executes in a subshell and returns nothing to echo (hence by default echo just prints an empty line). Since echo ran successfully(exit code=0) && results in true and pwd cmd is run in current shell and pwd gets printed