Bash : automatically exit the script if a variable is not set

"set -u, an alias of set -o nounset, is used to exit the script everytime it's trying to call a not initialized variable. By putting this command at the top of your script, you can avoid to manually test your variables before each use.

Automatically test if a variable is set

#!/bin/bash

echo "No error before set: "$test;

if [ "$test" == "" ]; then
    echo "We can see that this variable is not initialized";
fi

# Starting here, the script will exit
# if a variable is not initialized.
set -u;

# Disable the automatic testing
set +u;
echo "This line will not provoke any error"$test;

# Enable the automatic testing
set -u;
echo "An error will be triggered : "$test;

echo "This line will not be displayed";

Result of this script
$ ./script.sh
No error before set:
We can see that this variable is not initialized
This line will not provoke any error
./script.sh: line 20: test : variable without link

Another use of set for testing return codes of commands.