What is really happening with bash backticks?

Question

I've always thought of backticks in bash as "execute the output of 'some_command'." What is happening in the following... consider a script myscript.pl that prints the following text:

   export PS1="[STRING] $PS1"

If I copy the output from cat file.txt to the command line and execute, "[STRING] " gets added to the front of my prompt.

On the other hand, backtick-ing 'myscript.pl' makes my prompt "[STRING]. What is happening?

Answer 1

bash doesn't execute the output of a backtick command, only substitutes it. (The feature is called command substitution after all.)

If the substitution is not double-quoted, word splitting is done to the command's output, but quotes and other special characters are ignored; export PS1="[STRING] $PS1" is simply split by $IFS to:

• export
• PS1="[STRING
• $PS1"

---

If you want to execute a file's contents, source it:

. ./file.txt

or, non-POSIX:

source ./file.txt

If you want all standard parsing to be applied to a substitution -- for example, a variable -- you can use eval "$commands" or eval "$(foo)" or eval "`foo`". But be very careful with it.