echoing PROMPT_COMMAND from terminal and from bash script

Question

while trying to write a bash script i encountered a strange issue when i try to execute it. i want to echo the $PROMPT_COMMAND variable thus, i have the following piece of bash script

#!/bin/bash 
echo $PROMPT_COMMAND

executing this script echoes nothing. why is that? it does not make sense as when i do execute

echo $PROMPT_COMMAND

from the command prompt, i get the content that is stored inside the PROMPT_COMMAND

so... what am i doing wrong, what is missing?

a thing about the environment: tried this on ubuntu 20

Answer 1

The problem is based on how you are executing your script and what environment it is using.

After making the file executable, there are two ways of running it:

1. ./script.sh
2. . script.sh

The first way executes it in a new bash subshell, which doesn't have access to the $PROMPT_COMMAND variable since it is by definition, new.

The second way executes the script within the context of your current shell. This means it has access to the $PROMPT_COMMAND variable. This is the right way to proceed. Anytime you need to work on your prompt, you need to execute it this way.

Note: #2 is the same as running source script.sh. It is also referred to as "sourcing" your script. Just in case you run into these terms, you'll know what they mean.