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Looking at correcting an issue in /etc/init.d/hostapd on Debian. However, I have no clue what this line of code does nor how it works
[ -n "$DAEMON_CONF" ] || exit 0
In searching online for bash tutorials, I've never seen anyone do this
When I run the code, my shell window closes (because $DAEMON_CONF is not set to anything). If I change the code to
[ -n "not empty" ] || exit 0
my console window does not close.
so, -n evaluates to true, and or'ed with exit 0, is what?
622
Exit Status Each shell command returns an exit code when it terminates, either successfully or unsuccessfully. By convention, an exit code of zero indicates that the command completed successfully, and non-zero means that an error was encountered.
For the bash shell's purposes, a command which exits with a zero (0) exit status has succeeded. A non-zero (1-255) exit status indicates failure. If a command is not found, the child process created to execute it returns a status of 127.
exit exits the calling shell or shell script with the exit status specified by n . If you omit n , the exit status is that of the last command executed (an end-of-file exits the shell). return exits a function with the return value specified by n . If you omit n , the return status is that of the last command executed.
It checks whether $variable evaluates to true . There are a couple of normal values that evaluate to true , see the PHP type comparison tables. if ( ) can contain any expression that ultimately evaluates to true or false .
If the expression in [] returns false, do the thing after the or || (and exit 0). Otherwise, it will short circuit and the next statement will be evaluated.
198
[ is and alternate name for the command test. You can learn about the parameters/flags whatnot by looking at test's manpage:
man test
You'll see for -n:
-n STRING
the length of STRING is nonzero
Furthemore || means OR. So if the test command returns False then the stuff after the || will be executed. If test returns true, then it won't be executed.
Written out your command says: "If the variable $DAEMON_CONF lacks a value, then exit with return code 0"
The longhand version would be something like:
if test ! -n "$DAEMON_CONF"; then
exit 0
fi
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