Bash & Linux
The following two formats of check file are the same:
bashif test -f "foo.txt"; thenfi
bashif [ -f "foo.txt" ]; thenfi
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:
bashif [ $MSG == "0" ]; thenecho "do something.";fi
MSG is an empty string, bash will evaluate the code above as:
bashif [ == "0" ]; thenecho "do something.";fi
Therefore it will throws error.
To fix the code, there are two solutions:
MSGvariable with double quotes:bashif [ "$MSG" == "0" ]; then
use double brackets:bashif [[ $MSG == "0" ]; then
There are 3 main layers of Linux:
- user process
- linux kernel