Operation of ` with echo when trying to awk

Question

How come

 echo `echo "foo"`

works but

 echo `awk '{ print "foo" }'`

doesn't?

Answer 1

The awk program will wait for input and, for each line of input, print the word foo. That is what the awk program { print "foo" } does.

In contrast, echo, in the first command substitution, does not wait for input.

Would you want an awk program to just print something, without any input, do the output in a BEGIN block:

awk 'BEGIN { print "foo" }'

The BEGIN block is executed before reading the first line of input, and since there are no other blocks in the script, and no input file, it will then exit.

Also, never write code like echo $( ... ) or echo ` ... `, just use the code inside the command substitution instead.