Bash script export/source strange behavior

Question

I have 2 scripts:

script: script1.sh

#!/bin/bash
[[ $0 = "$BASH_SOURCE" ]] && { echo "You must source me!"; exit; }
if [ -z ${I} ]; then
     echo
     echo -n "some string: "
     stty -echo
     read I
     #export I
     stty echo
     echo
else
     echo "ALREADY SET!!!"
fi
echo "--- $I"

script: script2.sh

#!/bin/bash
echo "--- $I"
if [ -z $I ]; then
        echo "VARIABLE NOT SET"
else
        echo "VARIABLE SET"
fi

And now the part that confused me... I first run script1 and then script2...

First scenario:

As you can see I can print $I variable (value: asd) inside script1. When I run script2 it returns "VARIABLE NOT SET", but when I try to echo $I I get "asd"

wolfy@VMtest:~$ . ./script1.sh

some string:
--- asd
wolfy@VMtest:~$ ./script2.sh
---
VARIABLE NOT SET
wolfy@VMtest:~$ echo $I
asd
wolfy@VMtest:~$

Second scenario:

Now I uncomment #export I and rerun both scripts as before (before rerun I created a new session so that all variables are reseted)

In this case I can read $I in script2 and echo it

wolfy@VMtest:~$ . ./script1.sh

some string:
--- asd
wolfy@VMtest:~$ ./script2.sh
--- asd
VARIABLE SET
wolfy@VMtest:~$ echo $I
asd

Can someone explain me why in my first scenario I can echo $I, but I can't use it in second script?

Ignacio Vazquez-Abrams · Accepted Answer · 2016-09-28 09:42:59Z

2

Because you didn't export it. Exporting a variable moves it from the current shell's variable list into the environment. Subprocesses only read existing variables from the environment.

answered Sep 28, 2016 at 9:42

Ignacio Vazquez-Abrams

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3 Comments

Wolfy Over a year ago

thanks for your explanation... but why can I echo it from prompt?

Ignacio Vazquez-Abrams Over a year ago

Because the "prompt" is the current shell.

William Pursell Over a year ago

Absurdly minor nit. It turns out 'export' does not necessarily move the variable into the environment. In bash it does, but in general it merely ensures that the variable will be in the environment of all subprocesses. Why any shell would choose to implement this behavior in any other way than simply putting the variable in the environment is baffling, but dash at least chooses not to do so. (Or did at the last version I checked, which was a few years ago.)

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