test if shell is interactive

[tapczan]

Simple script a.sh:

#!/bin/bash
echo "PS1: $PS1"

Variable $PS1 is present:
# echo $PS1
\[\033[01;32m\]$USER@\h \[\033[00m\]\A \[\033[01;34m\]\w\[\033[00m\]\$

Variable $PS1 is environment:
# env | grep PS1
PS1=\[\033[01;32m\]$USER@\h \[\033[00m\]\A \[\033[01;34m\]\w\[\033[00m\]\$

Executing script:
# ./a.sh 
PS1:

Variable PS1 is empty in child process.

Whether the enviroment variable should not be copied to the child process?
Am I doing something wrong?


Popular methods to test if shell is interactive or not don't work for me:

1.
case "$-" in
    *i*)    echo This shell is interactive ;;
    *)    echo This shell is not interactive ;;
esac

2.
if [ -z "$PS1" ]; then
    echo This shell is not interactive
else
    echo This shell is interactive
fi
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