find -exec basename {} vs find -exec echo $(basename {})

Question

I'm sure I'm missing something but I can't figure it out. Given:

$ find -type f
./hello.txt
./wow.txt
./yay.txt

how come the next two commands render different results?

$ find -type f -exec basename {} \;
hello.txt
wow.txt
yay.txt

$ find -type f -exec echo $(basename {}) \;
./hello.txt
./wow.txt
./yay.txt

Answer 1

$(basename {}) is evaluated before the command runs.

The result is {} so the command echo $(basename {}) becomes echo {} and basename is not run for each file.

Answer 2

A quick debug on that using the bash -x debugger demonstrated this,

[The example is my own, just for demonstration purposes]

bash -xc 'find -type f -name "*.sh" -exec echo $(basename {}) \;'
++ basename '{}'
+ find -type f -name '*.sh' -exec echo '{}' ';'
./1.sh
./abcd/another_file_1_not_ok.sh
./abcd/another_file_2_not_ok.sh
./abcd/another_file_3_not_ok.sh

And for just basename {}

bash -xc 'find -type f -name "*.sh" -exec basename {} \;'
+ find -type f -name '*.sh' -exec basename '{}' ';'
1.sh
another_file_1_not_ok.sh
another_file_2_not_ok.sh
another_file_3_not_ok.sh

As you can see in the first example, echo $(basename {}) gets resolved in two steps, basename {} is nothing but the basename on the actual file (which outputs the plain file name) which is then interpreted as echo {}. So it is nothing but mimic-ing the exact behaviour when you use find with exec and echo the files as

bash -xc 'find -type f -name "*.sh" -exec echo {} \;'
+ find -type f -name '*.sh' -exec echo '{}' ';'
./1.sh
./abcd/another_file_1_not_ok.sh
./abcd/another_file_2_not_ok.sh
./abcd/another_file_3_not_ok.sh