Bash & Linux

The following two formats of check file are the same:

if test -f "foo.txt"; then

fi

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

fi

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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:

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

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

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:

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

2. use double brackets:

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

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There are 3 main layers of Linux:

• user process
• linux kernel
• hardware

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