D1="$(dirname $0)"
echo $D1
D0="(dirname $0)"
echo $D0

What I type on the command line:

$ ./test_script.sh


(dirname ./test_script.sh)

What significance does the first dollar sign in D1's value have s.t. the command gets executed while D0's value is interpreted as a string?


A trivial search for $( in the shell man page gives the answer.

As $name causes parameter expansion $(command) causes command substitution i.e. it is replaced by the output of the command (with trailing newline(s) removed). $(command) causes word splitting after the expansion, "$(command)" avoids it (like $name vs. "$name").

"(dirname)" on the other hand is just a literal string to the shell.

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