Bash & Linux

The following two formats of check file are the same:

bash
if test -f "foo.txt"; then
fi

bash
if [ -f "foo.txt" ]; then
fi

It seems like [[ ]] is preferred over [ ] syntax.

[ ] will eval the expression literally so you need to be careful if the variable that you evaluate is empty string, which may cause syntax error if bash will ignore empty string.

[[ ]] is more specific syntax so you don't need to worry about that.

To illustrate, see the following example:

bash
if [ $MSG == "0" ]; then
    echo "do something.";
fi

If MSG is an empty string, bash will evaluate the code above as:

bash
if [  == "0" ]; then
    echo "do something.";
fi

Therefore it will throws error.

To fix the code, there are two solutions:

1. wrap the MSG variable with double quotes:

   bash
   if [ "$MSG" == "0" ]; then

2. use double brackets:

   bash
   if [[ $MSG == "0" ]; then

There are 3 main layers of Linux:

• user process
• linux kernel
• hardware